Thermo Scientific 9512BNWP Instruction Manual

TABLE OF CONTENTS

GENERAL INFORMATION 1

Introduction 1
Required Equipment 1
Required Solutions 2

USING THE ELECTRODE 3

Set Up 3

Electrode Assembly 3
Electrode Preparation 4
Checking Electrode Operation (Slope) 7

Before Analysis 8

Units of Measurement 8
Sample Requirements 8
Measuring Hints 8
Sample Storage 9

Analytical Procedures 10

Analytical Techniques 10
Direct Calibration 11
Low Level Measurements By Direct Calibration 14
Known Addition 16

Electrode Storage 19

TROUBLESHOOTING 22

Troubleshooting Checklist 22
Troubleshooting Guide 26
Assistance 28

ELECTRODE CHARACTERISTICS 29

Electrode Response 29
Reproducibility 30
Temperature Effects 30
Interferences 30
pH Effects 31
Complexation 31
Effect of Dissolved Species 31
Electrode Life 31
Theory of Operation 32

ORDERING INFORMATION 37

SPECIFICATIONS 38

1

GENERAL INFORMATION

Introduction

The Orion 95-12 Ammonia Electrode allows fast, simple,
economical, and accurate measurements of dissolved ammonia in
aqueous solutions. This gas-sensing electrode can also be used to
measure the ammonium ion after conversion to ammonia, or
organic nitrogen after Kjeldahl digestion of the sample. Sample
color and turbidity do not affect the measurement, and samples
need not be distilled. Almost all anions, cations, and dissolved
species, other than volatile amines, do not interfere.

All apparatus and solutions required for measurement, electrode
characteristics and electrode theory are discussed in this manual.
General analytical procedures, low-level procedures and a method
for measuring ammonia in solutions that wet the membrane
are given.

The Orion 95-12 comes with the following:

• forty loose membranes

• reusable membrane cap

• tweezers for handling membranes

• dispensing cap

• electrode filling solution

• electrode instruction manual.

Operator instructions for Orion Meters are outlined in the individual
meter’s instruction manual. Our Technical Service Chemists can
be consulted for assistance and troubleshooting advice. Please
refer to Troubleshooting Section for information on contacting
Technical Services.

2

Required Equipment

Meter

The easiest type of meters to use is direct concentration readout
specific ion meter, such as Orion EA940, 920A, 920Aplus, 720A,
720Aplus, 710A, 710Aplus, 290A, 290Aplus or 370. If unavailable, a
pH/mV meter with readability to 0.1 mV could be used.

Magnetic Stirrer

Highly recommended for laboratory measurements.

Graph Paper

4-cycle semilogarithmic paper for preparing calibration curves
(for use with digital pH/mV laboratory meters)

Required Solutions

Distilled or Deionized Water

Water MUST be ammonia-free. Pass distilled or deionized water
through an ion-exchange column containing a strong acidic cation
exchange resin, such as Dowex 50W-X8.

Standard Solutions Orion

Ionic Strength Adjustor (ISA) 951211
Electrode Filling Solution 951202

0.1 M Ammonium Chloride Standard 951006
1000 ppm as Nitrogen Standard 951007

Solutions Prepared by Customer

For inner body check:

To check the operation of the electrode inner body.

Solution 1 — pH 4.01 Buffer with 0.1M NH

4

Cl (or 0.1 M NaCl)

Take 200 mL of the pH 4.01 buffer solution, Orion 910104, add 1.07 g
reagent-grade NH

4

Cl (or 1.16g reagent-grade NaCl), stir to mix and
label bottle as Solution 1. Store the buffer for repeated use.
Discard buffer if turbidity develops.

Solution 2 — pH 7.00 Buffer with 0.1M NH

4

Cl (0.1 M NaCl)

Take 200 mL of pH 7.00 buffer solution, Orion 910107, add 1.07 g
reagent-grade NH

4

Cl (or 1.16g reagent-grade NaCl), stir to mix and
label bottle as Solution 2. Store the buffer for repeated use.
Discard bottle if turbidity develops.

3

USING THE ELECTRODE

Set Up

Electrode Assembly

NOTE: Soak inner body in electrode filling solution for at
least two hours before assembling new electrode. For best
results, soak inner body overnight.

Assemble electrode according to instructions below. The
electrode is shipped dry with loose membranes and a loose
membrane cap. Avoid excessive handling of the membrane during
assembly; this may affect the membrane’s hydrophobic properties,
causing shortened membrane life. Use the tweezers provided.
A membrane will last from one week to several months depending
on usage.

NOTE: Membrane failure is characterized by a shift in
electrode potential, drift, and poor response. See
Troubleshooting section. Membrane failure may be apparent
on visual inspection as dark spots or discoloration of the
membrane.

4

Electrode Preparation

NOTE: Soak inner body in electrode filling solution for at
least two hours before assembling electrode. For best
results, soak inner body overnight.

1. While holding the probe vertically, unscrew the top cap (a)

and carefully remove the glass electrode inner body (b) from
the electrode outer body (c) (See Fig. 1). Set cap with inner
body aside carefully. Pour out remaining electrode filling
solution from outer body if applicable, new probe will not have
any filling solution in the body.

2. Unscrew membrane cap (d) from outer body (See Fig. 2).

Remove old membrane, if applicable.

3. Wearing gloves, use tweezers to carefully grasp corner of

white membrane from between paper separators (See Fig. 3).
Do not to touch center of membrane.

(a)

(b)

(c)

(d)

Fig. 1

(d)

Fig. 2

Fig. 3

4. With free hand, grasp electrode outer body with threads

oriented to the top. Align straight edge of membrane against
threaded shoulder and hold with thumb (See Fig. 4). With other
hand, gently stretch the membrane upwards (See Fig. 5), then
across the opening (See Fig. 6), then down to align other edge
with the opposite shoulder (See Fig. 7). While holding each
edge on both sides, gently stretch each serrated side of the
membrane out and down over threads making sure that
membrane surface is smooth and without wrinkles (See Fig. 8).
Smooth any loose material taking care not to touch center of
membrane (See Fig. 9).

5

Fig. 4

Fig. 7

Fig. 8

Fig. 5

Fig. 6

Fig. 9

6

NOTE: Do not overstretch the membrane. The membrane is
considered overstretched if the black body material can be
seen through the membrane. Discard membrane if
overstretched and replace with new membrane. Continue
from Step 3.

5. Replace membrane cap being careful not to touch membrane
center (See Fig. 10). Screw down half way and tuck any loose
membrane material under cap while twisting. Make sure cap
is fully screwed on.

6. Fill the outer body with 2.5 ml filling solution, or approximately
50 drops from the bottle (See Fig. 11).

7. Place inner body into outer body containing filling solution and
screw on upper cap (See Fig. 12).

8. Carefully shake fully assembled electrode as if it were a
clinical thermometer to remove bubbles. Gently pull spring
loaded cable back and slowly release to allow filling solution
to migrate between membrane and glass electrode inner body
(See Fig. 13).

9. Rinse the electrode well and wipe dry.

Fig. 10

Fig. 11

Fig. 12 Fig. 13

Checking Electrode Operation (Slope)

These are general instructions, which can be used with most
meters to check electrode operation. See individual meter
instruction manuals for more specific information.

This procedure measures electrode slope. Slope is defined as the
change in millivolts observed with every tenfold change in
concentration. Obtaining the slope value provides the best means
for checking electrode operation.

1. If electrodes have been stored dry, prepare the electrodes as
described under the section entitled Electrode Preparation.

2. Connect electrodes to the meter. For an electrode with a BNC
connector, turn the connector clockwise to attach to the
meter. For an electrode with a U.S. Standard connector, insert
reference pin-tip connector and the sensing electrode
connector into appropriate jacks on the meter. Non-Orion
meters may require special adaptors. Consult your meter
instruction manual.

3. Place 100 mL distilled water into a 150 mL beaker. Add 2 mL
pH-adjusting ISA, Orion 951211. Stir thoroughly. Set the meter
to the mV mode.

4. Rinse electrode with distilled water and place in the solution
prepared in step 3 above. To prevent air entrapment on the
membrane surface, be sure to use an electrode holder that
keeps the electrode at a 20˚ angle. If bubbles appear on the
sensing membrane, tap electrode gently to remove.

5. Select either 0.1 M or 1000 ppm ammonium chloride standard.
Pipet 1 mL of the standard into the beaker, stir thoroughly.
When a stable reading is displayed, record the electrode
potential in millivolts.

6. Pipet 10 mL of the same standard into the same beaker. Stir
thoroughly. When a stable reading is displayed, record the
electrode potential in millivolts.

7. Take the first potential reading and subtract it from the
second one. The difference should be in the range of

-54 to -60 mV/decade when the solution temperature is
between 20-25 ˚C. If the potential is not within this range,
refer to Troubleshooting.

7

Before Analysis

Units of Measurement

Ammonia can be measured in units of moles per liter, parts per
million as ammonia, parts per million as nitrogen, or any other
convenient unit (see Table 1).

Table 1
Concentration Unit Conversion Factors

Moles/Liter ppm as N ppm as NH

3

10

-4

1.40 1.70

10

-3

14.0 17.0

10

-2

140 170

10

-1

1400 1700

Sample Requirements

Samples must be aqueous; they must not contain organic solvents.
Consult Our Technical Service Chemists for use of the electrode in
unusual applications. Samples and standards should be at the
same temperature. A 1 ˚C difference in temperature will give rise
to about 2% measurement error. In all analytical procedures, pH­adjusting ISA must be added to all samples and standards
immediately before measurement. After addition of the ISA all
solutions should fall within a pH 11 to 14 range (solution should be
blue in this range) and have a total level of dissolved species
below 1 M. If the total level of dissolved species is above 1 M, see
section entitled Effect of Dissolved Species.

Measuring Hints

Minimize NH3loss from sample by following the recommendations
below:

• Store samples according to procedure in Sample Storage.

• Use beakers that minimize the ratio of surface area to volume.

• Keep beakers containing standards and samples covered
between measurements.

8

• Add 2 mL of pH-adjusting ISA to 100 mL of sample or standard
immediately before making measurements, and make sure the
blue color is observed.

• Between measurements, rinse the electrode with distilled
water.

• Be sure samples, standards, and electrodes are at the same
temperature.

• Samples and standards should be stirred using a magnetic
stirrer. Some magnetic stirrers generate enough heat to
change solution temperature. Placing a piece of insulating
material such as cork or styrofoam between the beaker and
the stirring plate can minimize this effect.

• Verify calibration every two hours by placing electrodes in a
fresh aliquot of the first standard solution used for calibration.
If value has changed, recalibrate the electrode.

• Always use fresh standards for calibration.

• After immersion in solution, check electrode for any air
bubbles on membrane surface and remove by shaking the
electrode as you would with a thermometer or lightly tapping
of the electrode.

• If electrode response is slow, the membrane may contain a
surface layer of contaminant. Restore performance by
soaking electrode in distilled water for about 5 minutes, then
rinse and soak in a standard solution for about 1 hour
before use.

Sample Storage

If possible, alkaline samples should be measured at once. The rate
of ammonia loss at room temperature from a stirred 100 mL basic
solution in a 100 mL beaker is about 50% in six hours. If samples
must be stored, make them slightly acidic (pH 6) by adding 0.5 mL
of 1 M HCl to each liter of sample, and place them in tightly capped
vessels. Make stored samples basic with pH-adjusting ISA
immediately before measurement.

9

Analytical Procedures

Analytical Techniques

A variety of analytical techniques are available to the analyst. The
following is a description of these techniques.

Direct Calibration is a simple procedure for measuring a large
number of samples. Only one meter reading is required for each
sample. Calibration is performed in a series of standards. The
concentration of the samples is determined by comparison to the
standards. ISA is added to all solutions to ensure that samples and
standards have similar ionic strength.

Incremental Techniques provide a useful method for measuring
samples, since calibration is not required. As in direct calibration,
any convenient concentration unit can be used. The different
incremental techniques are described below. They can be used to
measure the total concentration of a specific ion in the presence of
a large (50-100 times) excess of complexing agents.

• Known Addition is an alternate method useful for measuring
dilute samples, checking the results of direct calibration
(when no complexing agents are present), or measuring the
total concentration of an ion in the presence of an excess
complexing agent. The electrodes are immersed in the
sample solution and an aliquot of a standard solution
containing the measured species is added to the sample.
From the change in potential before and after the addition, the
original sample concentration is determined.

• Known Subtraction is useful as a quick version of a titration,
or for measuring species for which stable standards do not
exist. It is necessary to know the stoichiometric ratio
between standard and sample. For known subtraction, an
electrode sensing the sample species is used. Stable
standards of a species reacting completely with the sample in
a reaction of known stoichiometry are necessary.

• Analate Addition is often used to measure soluble solid
samples, viscous samples, small or very concentrated
samples, to diminish the effects of complex sample matrices,
or to diminish the effects of varying sample temperatures.
This method is not suitable for dilute or low concentration
samples. Total concentration is measured even in the
presence of complexing agents. The electrodes are immersed
in a standard solution containing the ion to be measured and
an aliquot of the sample is added to the standard. The original
sample concentration is determined from the change in
potential before and after the addition.

10

• Analate Subtraction is used in the measurement of ions for
which no ion-selective electrode exists. The electrodes are
immersed in a reagent solution which contains a species that
the electrode senses, and that reacts with the sample. It is
useful when sample size is small, or samples for which a
stable standard is difficult to prepare, and for viscous or very
concentrated samples. The method is not suited for very
dilute samples. It is also necessary to know the
stoichiometric ratio between standard and sample. Specific
instructions for known subtraction, analate addition and
analate subtraction, can be found in the Orion meter
instruction manuals. Titrations are quantitative analytical
techniques for measuring the concentration of a species by
incremental addition of a reagent (titrant) that reacts with the
sample species. Sensing electrodes can be used for
determination of the titration end point. Ion-selective
electrodes are useful as end point detectors, because they
are unaffected by sample color or turbidity. Titrations are
approximately 10 times more precise than direct calibration,
but are more time-consuming.

Direct Calibration

Set Up

1. Prepare the electrode as described in Electrode Assembly
and Electrode Preparation.

2. Connect electrode to meter.

3. Prepare two standards which bracket the expected sample
range and differ in concentration by a factor of ten. Standards
can be prepared in any concentration unit to suit the
particular analysis requirement. All standards should be at
the same temperature as the samples. (For details on
temperature effects on electrode performance, refer to
Temperature Effects.)

11

If Using Meter with Direct Concentration Readout Capability:

NOTE: See individual meter instruction manuals for more
specific information.

1. Measure 100 mL of the more dilute standard into a 150 mL
beaker. Add 2 mL of ISA. Stir thoroughly.

2. Rinse electrodes with distilled water, blot dry and place into
the beaker. Wait for a stable reading then adjust the meter to
display the value of the standard as described in the meter
instruction manual.

3. Measure 100 mL of the more concentrated standard into a
second 150 mL beaker. Add 2 mL of ISA. Stir thoroughly.

4. Rinse electrodes with distilled water, blot dry and place into
the beaker with more concentrated standard. Wait for a
stable reading then adjust the meter to display the value of the
second standard, as described in the meter instruction
manual.

5. Measure 100 mL of the sample into a 150 mL beaker. Add
2 mL of ISA. Stir thoroughly.

6. Rinse electrodes with distilled water, blot dry and place into
sample. The concentration will be displayed on the meter. In
the direct calibration procedure, a calibration curve is
constructed on semilogarithmic paper. Electrode potentials of
standard solutions are measured and plotted on the linear axis
against their concentrations on the log axis. In the linear
regions of the curves, only two standards are needed to
determine a calibration curve. In non-linear regions, more
points must be taken. The direct measurement procedures in
the manual are given for concentrations in the region of linear
electrode response. Low level measurement procedures are
given for measurements in the non-linear region.

12

If Using Meter with Millivolt Readout Only:

13

1. Adjust the meter to measure mV.

2. Measure 100 mL of the more dilute standard into a 150 mL
beaker. Add 2 mL of ISA. Stir thoroughly.

3. Rinse electrodes with distilled water, blot dry and place into
the beaker. When a stable reading is displayed, record the
mV value and corresponding standard concentration.

4. Measure 100 mL of the more concentrated standard into a
second 150 mL beaker. Add 2 mL ISA. Stir thoroughly.

5. Rinse electrodes with distilled water, blot dry and place into
the second beaker. When a stable reading is displayed,
record the mV value and corresponding standard
concentration.

6. Using semilogarithmic graph paper, prepare a calibration
curve by plotting the millivolt values on the linear axis and the
standard concentration values on the logarithmic axis.

7. Measure 100 mL of the sample into a 150 mL beaker. Add
2 mL of ISA. Stir thoroughly.

8. Rinse electrodes with distilled water, blot dry and place into
the beaker. When a stable reading is displayed, record the
mV value.

9. Using the calibration curve prepared in step 6, determine the
unknown concentration.

350

change in
electrode

300

potential (mV)

250

200

150

100

50

0.01 0.1 1 10 100

-7 -6

10

10

-5

10

-4

10

-3

10

NH 3 concentration (M)

Figure 14: Typical Ammonia Electrode Calibration Curve

(ppm)

1000

10-210

-1

Low-Level Measurements By Direct Calibration

These procedures are for solutions with an ammonia concentration
of less than 4 x 10

-6

M (0.07 ppm NH3). For solutions low in
ammonia but high in total ionic strength, perform the same
procedure with one change: prepare a calibration solution with a
composition similar to the sample. Accurate measurement
requires that the following conditions be met:

• Adequate time must be allowed for electrode stabilization.

Longer response time will be needed at low-level
measurements

• Remember to stir all standards and samples at a uniform rate

Electrode response is relatively slow at low levels of ammonia and
is faster going in the direction of increasing concentration. Diluting
the electrode filling solution with distilled water 1:10 can improve
response at low levels. To speed up the measurement of a sample
containing less than 4 x 10

-6

M ammonia (0.07 ppm NH3or

0.06 ppm as N), the electrode is first placed in an ammonia-free
pH 4 buffer, then into the sample. When measuring low levels of
ammonia, keep samples and standards covered and work with
large solution volumes to minimize surface-area-to-volume ratio.
This avoids ammonia absorption from air. Time response is slow at
low levels. Allow 5-10 minutes for a stable reading in the pH 4
buffer or a low-level solution.

Set Up

1. Prepare electrode as described under Electrode Assembly

and Electrode Preparation.

2. Place electrode in a pH 4 buffer solution for several minutes.

Stir all solutions throughout the procedure.

3. Prepare a 10

-3

M or 10 ppm standard by serial dilution of the

0.1 M or 1000 ppm standard.

4. Connect electrode to the meter.

14

Measurement

1. Measure 100 mL of ammonia-free distilled water in an

Erlenmeyer flask and add 2 mL of pH-adjusting ISA.

2. Rinse the electrode with distilled water, and place into flask.

Stir thoroughly.

3. Add increments of the 10

-3

M or 10 ppm standard to the
calibration solution using the steps outlined in Table 2.
Measure the electrode potential after each increment and plot
on semilogarithmic paper the concentration (log axis) against
the potential (linear axis).

4. Rinse the electrode and place in pH 4 buffer for several
minutes.

5. Measure 100 mL of sample into a beaker.

6. Rinse the electrode and place in sample solution. Add 2 mL of
pH-adjusting ISA to each 100 mL sample. Stir thoroughly.
Wait for a stable reading and record data. Determine the
concentration from the low-level calibration curve.

7. Prepare a new low-level calibration curve daily by using fresh
solutions.

Table 2
Preparation of Calibration Curve For Low Level Measurements

Additions of 10

-3

M or 10 ppm standard to 100 mL distilled water,
plus 2 mL pH-adjusting ISA. “A” is a 1 mL graduated pipet. “B” is
a 2 mL pipet.

Concentration

Step Pipet Size Added Volume ppm Molarity

1A 0.10 mL 0.01 9.8 x 10

-7

2A 0.10 mL 0.02 2.0 x 10

-6

3A 0.20 mL 0.04 3.9 x 10

-6

4 A 0.20 mL 0.06 5.8 x 10

-6

5 A 0.40 mL 0.10 9.7 x 10

-6

6B 2.00 mL 0.29 2.9 x 10

-5

7 B 2.00 mL 0.47 4.7 x 10

-5

15

Known Addition

Known Addition is a convenient technique for measuring samples
because no calibration curve is needed. It can be used to verify
the results of a direct calibration or to measure the total
concentration of an ion in the presence of a large excess of a
complexing agent. The sample potential is measured before and
after addition of a standard solution. Accurate measurement
requires that the following conditions be met:

• Concentration should approximately double as a result of

the addition.

• Sample concentration should be known to within a factor

of three.

• In general, either no complexing agent or a large excess of

the complexing agent may be present.

• Ratio of the uncomplexed ion to complexed ion must not be

changed by addition of the standard.

• All samples and standards should be at the same temperature.

Set Up

1. Prepare the electrode as described under Electrode Assembly

and Electrode Preparation.

2. Connect electrodes to the meter.

3. Upon addition to the sample, prepare a standard solution

which will cause concentration of the ammonia to double.
Refer to Table 3 as a guideline.

4. Determine the slope of the electrode by performing the

procedure under Checking Electrode Operation (Slope).

5. Rinse electrode between solutions with distilled water.

16

If Using Meter with Direct Known Addition Readout Capability:

NOTE: See individual meter instruction manuals for more
specific information.

1. Set up meter to measure in the known addition mode.

2. Measure 100 mL of the sample into a beaker.

3. Rinse electrodes with distilled water, then place in sample

solution. Add 2 mL of ISA. Stir thoroughly.

4. When a stable reading is displayed, calibrate the meter as

described in the meter instruction manual.

5. Pipet the appropriate amount of the standard solution into the

beaker. Stir thoroughly.

6. When a stable reading is displayed, record the sample

concentration.

If Using Meter with Millivolt Readout Only:

Use this procedure when no instructions for known addition are
available in the meter instruction manual.

1. Set the meter to relative millivolt mode.

2. Measure 100 mL of the sample into a 150 mL beaker. Add

2 mL of ISA. Stir thoroughly.

3. Rinse electrodes with distilled water, blot dry and place into

beaker. When a stable reading is displayed, set the reading to

000.0 by turning the calibration control. If the reading cannot
be set to 000.0, record the mV value.

4. Pipet the appropriate amount of standard solution into the

beaker. Stir thoroughly.

5. When a stable reading is displayed, record the mV value. If

the meter could not be zeroed in step 3, subtract the first
reading from the second to find ∆E.

6. From Table 4, find the value, Q, that corresponds to the

change in potential, ∆E. To determine the original sample
concentration, multiply Q by the concentration of the added
standard:

17

C

sam

=QC

std

where:

C

std

= standard concentration

C

sam

= sample concentration

Q=reading from known addition table

The table of Q values is calculated for a 10% volume change for
electrodes with slope in the range of -57.2 to -60.2 mV. The
equation for the calculation of Q for different slopes and volume
changes is given below:

Q=

p

(1 + p)10

∆E/S

- 1

where:

Q=reading from known addition table

∆E=E

2

- E

1

S=slope of the electrode

p=

volume of standard
volume of sample

Table 3

Volume of Addition Concentration of Standard

1 mL 100 x sample concentration
5 mL 20 x sample concentration
10 mL* 10 x sample concentration

* Most convenient volume to use

18

Electrode Storage

Between measurements, keep the electrode tip immersed in a
10

-3

M or 10 ppm standard with added ISA. For low-level
measurements, keep the electrode in a pH 4 buffer between
measurements. Do not store overnight in a pH 4 buffer.

For overnight or week-long storage, the electrode tip should be
immersed in 0.1 M or 1000 ppm standard without added ISA.

For storage over one week or if the electrode is stored indefinitely,
disassemble completely and rinse the inner body, out body, and
bottom cap with distilled water. Dry and reassemble electrode
without filling solution or membrane. When rebuilding, follow
procedure in Electrode Assembly and Electrode Preparation.

If the electrode is accidentally left in the air and erratic results are
obtained, the space between the inside of the membrane and the
sensing element may be dry. To make the electrode usable again,
pull back the cable slightly to allow solution flow between the
sensor and the membrane.

19

20

Table 4

Known Addition Table for an added volume one-tenth the sample
volume. Slopes (in the column headings) are in units of mV/decade.

∆E Q, Concentration Ratio
Monovalent (-57.2) (-58.2) (-59.2) (-60.1)

5.0 0.2894 0.2933 0.2972 0.3011

5.2 0.2806 0.2844 0.2883 0.2921

5.4 0.2722 0.2760 0.2798 0.2835

5.6 0.2642 0.2680 0.2717 0.2754

5.8 0.2567 0.2604 0.2640 0.2677

6.0 0.2495 0.2531 0.2567 0.2603

6.2 0.2436 0.2462 0.2498 0.2533

6.4 0.2361 0.2396 0.2431 0.2466

6.6 0.2298 0.2333 0.2368 0.2402

6.8 0.2239 0.2273 0.2307 0.2341

7.0 0.2181 0.2215 0.2249 0.2282

7.2 0.2127 0.2160 0.2193 0.2226

7.4 0.2074 0.2107 0.2140 0.2172

7.6 0.2024 02.056 0.2088 0.2120

7.8 0.1975 0.2007 0.2039 0.2023

8.0 0.1929 0.1961 0.1992 0.2023

8.2 0.1884 0.1915 0.1946 0.1977

8.4 0.1841 0.1872 0.1902 0.1933

8.6 0.1800 0.1830 0.1860 0.1890

8.8 0.1760 0.1790 0.1820 0.1849

9.0 0.1722 0.1751 0.1780 0.1809

9.2 0.1685 0.1714 0.1742 0.1771

9.4 0.1649 0.1677 0.1706 0.1734

9.6 0.1614 0.1642 0.1671 0.1698

9.8 0.1581 0.1609 0.1636 0.1664

10.0 0.1548 0.1576 0.1603 0.1631

10.2 0.1517 0.1544 0.1571 0.1598

10.4 0.1487 0.1514 0.1540 0.1567

10.6 0.1458 0.1484 0.1510 0.1537

10.8 0.1429 0.1455 0.1481 0.1507

11.0 0.1402 0.1427 0.1453 0.1479

11.2 0.1375 0.1400 0.1426 0.1451

11.4 0.1349 0.1374 0.1399 0.1424

11.6 0.1324 0.1349 0.1373 0.1398

11.8 0.1299 0.1324 0.1348 0.1373

12.0 0.1276 0.1300 0.1324 0.1348

12.2 0.1253 0.1277 0.1301 0.1324

12.4 0.1230 0.1254 0.1278 0.1301

12.6 0.1208 0.1232 0.1255 0.1278

12.8 0.1187 0.1210 0.1233 0.1256

13.0 0.1167 0.1189 0.1212 0.1235

13.2 0.1146 0.1169 0.1192 0.1214

13.4 0.1127 0.1149 0.1172 0.1194

13.6 0.1108 0.1130 0.1152 0.1174

13.8 0.1089 0.1111 0.1133 0.1155

14.0 0.1071 0.1093 0.1114 0.1136

14.2 0.1053 0.1075 0.1096 0.1118

14.4 0.1036 0.1057 0.1079 0.1100

14.6 0.1019 0.1040 0.1061 0.1082

14.8 0.1003 0.1024 0.1045 0.1065

15.0 0.0987 0.1008 0.1028 0.1048

15.5 0.0949 0.0969 0.0989 0.1009

16.0 0.0913 0.0932 0.0951 0.0971

16.5 0.0878 0.0897 0.0916 0.0935

17.0 0.0846 0.0865 0.0883 0.0901

∆E Q, Concentration Ratio
Monovalent (-57.2) (-58.2) (-59.2) (-60.1)

17.5 0.0815 0.0833 0.0852 0.0870

18.0 0.0786 0.0804 0.0822 0.0839

18.5 0.0759 0.0776 0.0793 0.0810

19.0 0.0733 0.0749 0.0766 0.0783

19.5 0.0708 0.0724 0.0740 0.0757

20.0 0.0684 0.0700 0.0716 0.0732

20.5 0.0661 0.0677 0.0693 0.0708

21.0 0.0640 0.0655 0.0670 0.0686

21.5 0.0619 0.0634 0.0649 0.0664

22.0 0.0599 0.0614 0.0629 0.0643

22.5 0.0580 0.0595 0.0609 0.0624

23.0 0.0562 0.0576 0.0590 0.0605

23.5 0.0545 0.0559 0.0573 0.0586

24.0 0.0528 0.0542 0.0555 0.0569

24.5 0.0512 0.0526 0.0539 0.055

25.0 0.0497 0.0510 0.0523 0.0536

25.5 0.0482 0.0495 0.0508 0.0521

26.0 0.0468 0.0481 0.0493 0.0506

26.5 0.0455 0.0467 0.0479 0.0491

27.0 0.0442 0.0454 0.0466 0.0478

27.5 0.0429 0.0441 0.0453 0.0464

28.0 0.0417 0.0428 0.0440 0.0452

28.5 0.0405 0.0417 0.0428 0.0439

29.0 0.0394 0.0405 0.0416 0.0427

29.5 0.0383 0.0394 0.0405 0.0416

30.0 0.0373 0.0383 0.0394 0.0405

31.0 0.0353 0.0363 0.0373 0.0384

32.0 0.0334 0.0344 0.0354 0.0364

33.0 0.0317 0.0326 0.0336 0.0346

34.0 0.0300 0.0310 0.0319 0.0328

35.0 0.0285 0.0294 0.0303 0.0312

36.0 0.0271 0.0280 0.0288 0.0297

37.0 0.0257 0.0266 0.0274 0.0283

38.0 0.0245 0.0253 0.0261 0.0269

39.0 0.0233 0.0241 0.0249 0.0257

40.0 0.0222 0.0229 0.0237 0.0245

41.0 0.0211 0.0218 0.0226 0.0233

42.0 0.0201 0.0208 0.0215 0.0223

43.0 0.0192 0.0199 0.0205 0.0212

44.0 0.0183 0.0189 0.0196 0.0203

45.0 0.0174 0.0181 0.0187 0.0194

46.0 0.0166 0.0172 0.0179 0.0185

47.0 0.0159 0.0165 0.0171 0.0177

48.0 0.0151 0.0157 0.0163 0.0169

49.0 0.0145 0.0150 0.0156 0.0162

50.0 0.0138 0.0144 0.0149 0.0155|

51.0 0.0132 0.0137 0.0143 0.0148

52.0 0.0126 0.0131 0.0136 0.0142

53.0 0.0120 0.0125 0.0131 0.0136

54.0 0.0115 0.0120 0.0125 0.0130

55.0 0.0110 0.0115 0.0120 0.0124

56.0 0.0105 0.0110 0.0115 0.0119

57.0 0.0101 0.0105 0.0110 0.0114

58.0 0.0096 0.0101 0.0105 0.0109

59.0 0.0092 0.0096 0.0101 0.0105

60.0 0.0088 0.0092 0.0096 0.0101

21

TROUBLESHOOTING

Troubleshooting Checklist

Symptom Possible Causes

Off-scale or Over-range Defective meter
reading

Defective inner body

Electrodes not plugged in properly

Electrode filling solution not added

Air bubble on membrane

Electrodes not in solution

Noisy or unstable readings Insufficient electrode filling solution
(erratic-rapidly changing)

Defective meter

Bottom cap loose

Defective inner body

ISA not used

Drift (Reading slowly Meter or stirrer improperly
changing in one direction) grounded

Electrode filling solution leakage

Incorrect electrode filling solution

Total level of dissolved species
above 1 M

Electrode in sample too long;

Membrane failure (wet, perforation,
discoloration)

22

Next Step

Perform meter checkout procedure (see meter instruction manual)

Refer to Troubleshooting Guide (check inner body operation)

Unplug electrodes and reseat

Fill outer body of electrode with proper amount of Electrode filling
solution

Remove bubble by redipping electrode in solution

Put electrodes in solution

Fill outer body of electrode with proper amount of Electrode filling
solution

Perform meter checkout procedure (see meter instruction manual)

Ensure that bottom cap is screwed on tight enough to close gap
between bottom cap and body

Check inner body operation

Use recommended ISA, Orion No. 951211

Check meter and stirrer for grounding

Ensure that membrane is installed properly

Refill outer body of electrode using filling solution shipped with
electrode

Dilute solution

Reduce surface-area-to-volume loss ratio, slow rate of stirring,
avoid high temperatures

Replace membrane

23

Troubleshooting Checklist (cont.)

Symptom Possible Causes

Solutions not at constant
temperature

Heat generated by magnetic stirrer

Defective inner body

Electrode exposed to air for
extended period

Samples and standards at different
temperatures

Low slope or No slope Standards contaminated or

incorrectly made

ISA not used

Standard used as ISA

Electrode exposed to air for
extended period

Membrane failure (wet, perforation,
discoloration)

Defective inner body

“Wrong Answer” (But Incorrect scaling of semilog paper
calibration curve is OK)

Incorrect sign

Incorrect standards

Wrong units used

Complexing agents in sample

ISA added to standards and not
samples

24

Next Steps

Allow solutions to come to room temperature before use

Place insulating material between stirrer and beaker

Check inner body operation

Hold electrode by outer body and pull up on electrode cable.
Electrode filling solution will flow under membrane and restore
electrode response

Allow solutions to come to room temperature before measurement

Prepare fresh standards

Use recommended ISA, Orion No. 951211

Use ISA!

Hold electrode by outer body and pull up on electrode cable.
Electrode filling solution will flow under membrane and restore
electrode response

Replace membrane

Check inner body operation

Plot millivolts on the linear axis. On the log axis, be sure
concentration numbers within each decade are increasing with
increasing concentration

Be sure to note sign of millivolt value correctly

Prepare fresh standards

Apply correct conversion factor:
10

-3

M = 17 ppm as NH3= 14 ppm as N

Use known addition or titration techniques, or a decomplexing
procedure

Add some proportion of ISA to standards and samples

25

Troubleshooting Guide

The most important principle in troubleshooting is to isolate the
components of the system and check each in turn. The
components of the system are: 1) Meter, 2) Electrodes,

3) Standard, 4) Sample, and 5) Technique.

Meter

The meter is the easiest component to eliminate as a possible
cause of error. Orion meters are provided with an instrument
checkout procedure in the instruction manual and a shorting strap
for convenience in troubleshooting. Consult the manual for
complete instructions and verify that the instrument operates as
indicated and is stable in all steps.

Electrodes

1. Rinse electrode thoroughly with distilled water.

2. Check electrode operation (slope).

3. If electrode fails this procedure, soak ammonia electrode

again as directed in Electrode Preparation and Electrode
Assembly.

4. Repeat step 2, Checking Electrode Operation.

5. If the electrode still does not perform as described, determine

whether the ammonia inner body is working properly.

Checking inner body:

NOTE: This is a troubleshooting procedure. If electrode
slope is found to be low during operation, disassemble
electrode and check inner body.

Disassemble ammonia electrode. If the electrode is dry, first soak
the glass tip of the inner body in filling solution for at least two
hours. Rinse the inner body with distilled water and immerse in
pH 7 buffer solution with 0.1 M NH

4

Cl added. Make sure that the
reference wire is covered (see Required Solutions for
preparation). Stir throughout the procedure. Record the millivolt
reading after two minutes.

NOTE: Use caution so that the buffer solution comes into
contact with the reference wire, but not the protective rubber
sleeve. If contact is made with the sleeve, be sure to
thoroughly rinse electrode with deionized water.

26

Rinse the inner body in distilled water and place in the pH 4 buffer
solution with 0.1 M NH

4

Cl added (See Required Solutions for
preparation). Watch the change in meter reading carefully. The
reading should change 100 mV in less than 30 seconds after
immersion in the pH 4 buffer solution. The mV difference between
pH 7 and pH 4 should be greater than 150 mV if the inner body
sensing elements are operating correctly after three minutes.

6. If the stability and slope check out properly, but measurement

problems still persist, the sample may contain interferences or
complexing agents, or the technique may be in error. See
Standard, Sample, and Technique sections.

7. Before replacing a “faulty” electrode, review the instruction

manual and be sure to:

• Clean the electrode thoroughly

• Prepare the electrode properly

• Use proper filling solutions, ISA and standards

• Measure correctly

• Review Troubleshooting Checklist

Standard

The quality of results depends greatly upon the quality of the
standards. ALWAYS prepare fresh standards when problems arise

- it could save hours of frustrating troubleshooting! Error may
result from contamination of prepared standards, accuracy of
dilution, quality of distilled water, or a mathematical error in
calculating the concentrations. The best method for preparation of
standards is by serial dilution. This means that an initial standard
is diluted, using volumetric glassware, to prepare a second
standard solution. The second is similarly diluted to prepare a third
standard, and so on, until the desired range of standards has been
prepared.

Sample

If the electrode works properly in standards but not in the sample,
look for possible interferences, complexing agents, or substances
which could affect response or physically damage the sensing
electrode or the reference electrode. If possible, determine the
composition of the samples and check for problems. See Sample
Requirements, Interferences, and pH Requirements.

27

Technique

Check the method of analysis for compatibility with your sample.
Direct measurement may not always be the method of choice. If a
large amount of complexing agents are present, known addition
may be best. If the sample is viscous, analate addition may solve
the problem. If working at low levels, be sure to follow the low
level measurement technique. Also, be sure that the expected
concentration of the ion of interest is within the electrode’s limit of
detection. If problems persist, review operational procedures and
instruction manuals to be sure that proper technique has been
followed. Reread Measuring Hints and Analytical Procedures.

Assistance

For the most current warranty information, visit www.thermo.com.

After troubleshooting all components of your measurement system,
contact The Technical Edge

SM

for Orion products. Within the United

States call 1.800.225.1480, outside the United States call

978.232.6000 or fax [phone]. In Europe, the Middle East and
Africa, contact your local authorized dealer. For the most current
contact information, visit www.thermo.com

.

28

ELECTRODE CHARACTERISTICS

29

Electrode Response

The electrode exhibits good time response (95% response in one
minute or less) for ammonia concentrations above 4 x 10
(0.07 ppm NH

or 0.06 ppm N). Below this value, response times

3

-6

M

are longer and ammonia absorption from the air may become a
source of error. Above 1.0 M, ammonia is rapidly lost to the air.
Samples above 1.0 M ammonia concentration can be diluted
before measurement.

For solutions with low concentration in ammonia but high in total
ionic strength, perform the same procedure with one change:
prepare a calibration solution with a composition similar to the
sample. Accurate measurement requires that the following
conditions be met.

• Adequate time must be allowed for electrode stabilization.

Longer response time will be needed at low level
measurements.

• Remember to stir all standards and samples at a uniform rate.

When plotted on semilogarithmic paper, electrode potential
response as a function of ammonia concentration is a straight
line with a slope of about -58 mV per decade.

electrode
potential
(mV)

-100

-50

0

50

100

150

10-3 to 10

0 to 10

10-3 to 10

10-3 to 10

10-3 to 10

-2

M NH

3

-3

M NH

3

-4

M NH

3

-5

M NH

3

-6

M NH

3

time(minutes)

1

2

3

Figure 15: To Step Changes in Ammonia Concentration

Typical Electrode Response

4

Reproducibility

Reproducibility is limited by factors such as temperature
fluctuations, drift and noise. Within the operating range of the
electrode, reproducibility is independent of concentration.
Reproducibility can be obtained to ± 2% when calibration is
performed every hour.

Temperature Effects

A change in temperature will cause electrode response to shift and
change slope. Table 5 lists the variation of theoretical response
with temperature. At 10

-3

M, a 1 ˚C temperature change gives rise
to a 2% error. Samples and standards should be at the same
temperature. Note that the higher the temperature, the faster the
ammonia loss from solution.

Table 5

Temperature (˚C) Slope (mV)

0 -54.20
5 -55.20
10 -56.18
15 -57.17
20 -58.16
25 -59.16
30 -60.15
35 -61.14
40 -62.13

Interferences

Volatile amines interfere with electrode measurements. Most
gases do not interfere as they are converted to the ionic form in
basic solution. Ionic species cannot cross the gas-permeable
membrane and are not direct electrode interferences. However,
the level of ions in solution can change the solubility of ammonia.
Standards and samples should have about the same level of ions in
the solution and about the same level of dissolved species. Also,
some metallic ions complex ammonia, causing falsely low results
in direct measurements.

30

pH Effects

See Theory of Operation.

Complexation

Ammonia forms metal complexes with a number of metal ions:
mercury, silver, copper, gold, nickel, cobalt, cadmium, and zinc.
Most of these metals are removed in the form of hydroxide
complexes or precipitates in basic solution. When hydroxide is
present at the 0.1 M level and the ammonia concentration is below
10

-3

M, only mercury will appreciably complex ammonia. The total
ammonia level of the sample will be measured if the mercury in the
sample is preferentially bound to some other species. Iodide is
recommended for this purpose, since it forms a soluble mercury
complex at all pH levels. Use of Orion Ammonia pH-adjusting ISA
inhibits the formation of some these common metal complexes
in the sample, because it contains a high concentration of
hydroxide ion.

Effect of Dissolved Species

Water vapor is a potential electrode “interference.” Water can
move across the membrane as water vapor, changing the
concentration of the electrode filling solution under the membrane.
Such changes will be seen as electrode drift. Water vapor transport
across the membrane is not a problem if the total level of dissolved
species in solution (osmotic strength) is below 1 M or the electrode
and sample temperatures are the same. Addition of pH-adjusting
ISA to samples of low osmotic strength automatically adjusts them
to the correct level. Samples with osmotic strengths above 1 M
should be diluted before measurement. Dilution should not reduce
the ammonia level below 10

-5

M. Samples with high osmotic

strengths (above 1 M) and low ammonium levels (below 10

-5

M) can
be measured without dilution if the osmotic strength of the electrode
filling solution is adjusted. To adjust the electrode filling solution,
add 4.25 g solid NaNO3to each 100 mL electrode filling solution.

Electrode Life

A membrane will last from one week to several months depending
on usage (membrane failure is characterized by a shift in electrode
potential, drift or poor response). See Troubleshooting.
Membrane failure may be apparent on visual inspection as dark
spots or discoloration of the membrane.

31

Theory of Operation

The ammonia electrode uses a hydrophobic gas-permeable
membrane to separate the sample solution from the electrode
filling solution. Dissolved ammonia is the sample solution diffuses
through the membrane until the partial pressure of ammonia is the
same on both sides of the membrane. In any
given sample the partial pressure of ammonia will be proportional
to its concentration.

Ammonia diffusing through the membrane dissolves in the filling
solution and, to a small extent, reacts reversibly with water in the
filling solution.

NH

3

+ H2O  NH

4

+

+OH

-

The relationship between ammonia, ammonium ion and hydroxide
is given by the following equation:

[NH

4

] [OH-]

= constant

[NH

3

]

The electrode filling solution contains ammonium chloride at a
sufficiently high level so that the ammonium ion concentration can
be considered fixed. Thus:

[OH

-

] = [NH3] • constant

The potential of the electrode sensing element with respect to the
internal reference element is described by the Nernst equation:

E=E

O

- S log [OH-]

where:

E=measured electrode potential
E

O

= reference potential
OH-= hydroxide concentration in solution
S=electrode slope (-59.2 mV/decade)

Since the hydroxide concentration is proportional to ammonia
concentration, electrode response to ammonia is also Nernstian.

E=E

O

- S log [NH3]

The reference potential, E

O

is partly determined by the internal
reference element which responds to the fixed level of chloride in
the filling solution.

32

Ammonium Ion

33

When ammonia is dissolved in water it reacts with hydrogen ion to
form ammonium ion:

+ H3O+ NH

NH

3

+

4

+ H2O

The relative amount of ammonia and ammonium ion is determined
by the solution pH (see Figure 16). In acid solution, where hydrogen
ion is readily available, virtually all the ammonia is converted to
ammonium ion. At a pH of about 9.3, half the ammonia will be in
the form of ammonium ion.

Theoretically, it is possible to calculate the ratio of ammonia to
ammonium ion, if the pH is known. The equilibrium constant for the
reaction is:

[NH

+

]

4

=

[NH

+

]

4

= K10

-9.3

[H3O+][NH3] 10-pH[NH3]

at 25 ˚C µ= 0.1
and pK = 9.3. The ratio of ammonium to ammonia is given by:

+

]

[NH

4

= K 10

-pH

= 10

9.3-pH

[NH3]

Martell, A.; Smith, R., Critical Stability Constants, Plenum Press.
New York, NY, 1974.

The exact value of K will vary with both temperature and ionic
strength. For example, while the pK, at 25 ˚C and µ = 0.1, is given
as 9.3 (as in the discussion above) an increase in ionic strength to
µ= 1.0 yields a pK of 9.4.

Partial Pressure of Ammonia

34

As discussed in Theory of Operation, the ammonia electrode
responds to the partial pressure of dissolved ammonia gas. The
partial pressure of dissolved ammonia gas is related to the
ammonia concentration by Henry’s Law:

] aqueous

[NH

3

=

K

h

P

NH

= 56 moles/liter — atm. (25 ˚C)

3

The Henry’s Law constant, Kh, varies both with temperature and
the level of dissolved species. For example, the constant is about
20% lower in 1 M NaCl that in distilled water.

To keep the Henry’s Law constant close to the same value,
standards and samples should contain the same level of dissolved
species and be about the same temperature.

% of
species

ammonia

100

80

60

40

20

ammonium

0

6 7 8 9 10 11 12 13 14

solution pH

Figure 16: Percent of Ammonia and Ammonium as a Function of pH

Measuring Ammonia

In Solutions That Wet The Membrane

The membrane of the ammonia electrode is gas-permeable and
hydrophobic - liquid water does not wet it and does not penetrate
the holes. If a sample solution is nonaqueous, or if it contains a
surfactant which wets the membrane, the liquid does penetrate the
membrane. This causes difficulties in samples such as sewage,
which contains surfactants, and samples which are nonaqueous,
such as latex paint or nylon. To measure ammonia in such
samples, the electrode should be suspended above the sample.

If the ammonia electrode is placed in a closed system saturated
with water vapor, it reacts to ammonia in the gas phase.
Measurements of solutions above 10

-3

M (14 ppm) ammonia are

possible under these conditions.

To measure ammonia in samples containing surfactants, or
nonaqueous solutions, adjust the sample pH to 11-13 with ISA.
Transfer the solution to a 125 mL Erlenmeyer flask containing a
magnetic stirring bar. The neck of the flask is fitting with a rubber
stopper with a hole large enough to hold the electrode snugly. The
closed flask forms an air-tight system whose gas phase is
saturated with water vapor and has a partial pressure of ammonia
in equilibrium with the solution.

Normal analytical techniques may be used with the electrode in
the gas phase. Calibrate the electrode in a closed flask of
standards,
or make a standard addition to the closed flask of sample. The
electrode in the gas phase has a longer response time than if it
were actually in a surfactant-free aqueous solution. A minimum
air space between solution and electrode is necessary for best
time response.

Organic Nitrogen

For information concerning the measurement of organic nitrogen,
contact Our Technical Service Chemists and request Orion’s Guide
to Water and Wastewater Analysis.

353637

ORDERING INFORMATION

Orion Description

9512BN Ammonia Electrode, BNC Connector

951211 Ionic Strength Adjustor (ISA), 475 mL

951202 Electrode Filling Solution, 60 mL bottle

951204 Membranes, box of 20

951006 Standard Solution, 0.1M NH

4

Cl

951007 Standardizing Solution, NH

4

Cl, 1000 ppm as N

951205 Bonded Membrane Caps, 3 per pack

38

SPECIFICATIONS

Concentration Range

1 M to 5 x 10-7M
(0.01 to 17,000 ppm NH3or 0.01 to 14,000 ppm N)

pH Range

Samples and standards must be adjusted to above pH 11

Temperature Range

0 to 50 ˚C

Electrode Resistance

Less than 5,000 megohms

Reproducibility

±2%

Minimum Sample Size

2.5 mL in a 30 mL beaker

Size

Electrode Length: 14.9 cm
Body Diameter: 12 mm
Cap Diameter: 16 mm
Cable Length: 110 cm

Specifications subject to change without notice.