Instruction Manual Cadmium Electrode
1
EUTECH INSTRUMENTS PTE LTD.
INSTRUCTION MANUAL
CADMIUM ION ELECTRODE
Cadmium Electrode Instruction Manual
2
TABLE OF CONTENTS
GENERAL INSTRUCTIONS ..........................................................................................................3
Introduction.................................................................................................................................................3
Required Equipment...................................................................................................................................3
Required Solutions......................................................................................................................................3
GENERAL PREPARATION............................................................................................................4
Electrode Preparation..................................................................................................................................4
Electrode Slope Check (with a pH/mV meter)........................................................................................... 4
Electrode Slope Check (with ion meter).....................................................................................................4
MEASUREMENT..............................................................................................................................5
Measuring Hints..........................................................................................................................................5
Sample Requirements.................................................................................................................................5
Units of Measurement................................................................................................................................. 6
MEASUREMENT PROCEDURE ...................................................................................................6
Direct Measurement.................................................................................................................................... 6
Direct Measurement of Cadmium (using a pH/mV meter).........................................................................6
Direct Measurement of Cadmium (using an ion meter)..............................................................................7
Low Level Cadmium Determination (using a pH/mV meter)....................................................................8
Low Level Cadmium Determination (using an ion meter).........................................................................9
Titration ......................................................................................................................................................9
Titration of Cadmium ...............................................................................................................................10
ELECTRODE CHARACTERISTICS........................................................................................... 11
Reproducibility.........................................................................................................................................11
Interferences .............................................................................................................................................11
Precipitation and Complexation................................................................................................................11
Temperature Influences ............................................................................................................................12
Electrode Response...................................................................................................................................12
Limits of Detection...................................................................................................................................13
pH Effects.................................................................................................................................................13
Electrode Life ...........................................................................................................................................14
Electrode Storage......................................................................................................................................14
ELECTRODE THEORY................................................................................................................14
Electrode Operation..................................................................................................................................14
TROUBLESHOOTING GUIDE....................................................................................................15
Meter.........................................................................................................................................................16
Glass-ware ................................................................................................................................................16
Electrodes .................................................................................................................................................16
Standards and Reagents............................................................................................................................16
Sample ......................................................................................................................................................17
Technique .................................................................................................................................................17
TROUBLESHOOTING HINTS.....................................................................................................18
SPECIFICATIONS..........................................................................................................................20
ORDERING INFORMATION.......................................................................................................20
Instruction Manual Cadmium Electrode
3
EUTECH INSTRUMENTS PTE LTD
CADMIUM ION ELECTRODE
INSTRUCTION MANUAL
GENERAL INSTRUCTIONS
Introduction
Eutech Instruments Cadmium Ion Electrode is used to measure cadmium ions in aqueous solutions
quickly, simply, accurately, and economically.
Required Equipment
1. A pH/mV meter or an ion meter, either line operated or portable.
2. Semi-logarithmic 4-cycle graph paper for preparing calibration curves when using the
meter in the mV mode.
3. A magnetic stirrer.
4. Eutech Cadmium Ion Combination Epoxy-body Electrode, Code. no. EC-CD-03.
5. Polishing Paper, Code no. EC-MIS-PP, to polish dirty or etched electrode membranes.
Required Solutions
1. Deionized or distilled water for solution and standard preparation.
2. Eutech Ionic Strength Adjuster (ISA), 5M NaNO
3
, Code. no. EC-ISA-CD1-BT. To prepare
this solution from your own laboratory stock, half fill a one liter volumetric flask with
distilled water and add 425 grams reagent-grade sodium nitrate, NaNO
3
. Swirl the flask
gently to dissolve the solid. Fill to the mark with distilled water, cap, and upend several
times to mix the solution.
3. Cadmium Nitrate Standard Solution, 0.1M. To prepare this solution from your own
laboratory stock, half fill a one liter volumetric flask with distilled water and add 30.85
grams of reagent-grade Cd(NO3)
2
.
4H2O. Swirl the flask gently to dissolve the solid. Fill to
the mark with distilled water, cap, and upend several times to mix the solution.
4. Cadmium Nitrate Standard Solution, 1,000 ppm. To prepare this solution from your own
laboratory stock, half fill a one liter volumetric flask with distilled water and add 2.74
grams of reagent-grade Cd(NO3)
2
.
4H2O. Swirl the flask gently to dissolve the solid. Fill to
the mark with distilled water, cap, and upend several times to mix the solution.
5. EDTA Titrant 1M stock solution for the titration of cadmium. To prepare this solution
from your own laboratory stock, add 37.2 grams of reagent-grade Na2EDTA to a 100 ml
volumetric flask about three-fourths full of distilled water. Swirl the flask gently to
Cadmium Electrode Instruction Manual
4
dissolve the solid. Fill to the mark with distilled water, cap, and upend several times to
mix the solution.
GENERAL PREPARATION
Electrode Preparation
Remove the rubber caps covering the electrode tips and the rubber insert covering the filling hole of
the cadmium combination electrode or the reference electrode. Fill the reference electrode or the
combination electrode with the filling solution shipped with the electrode to a level just below the
fill hole. No preparation is required with a sealed reference electrode. Connect the electrodes to the
proper terminals as recommended by the meter manufacturer.
Electrode Slope Check (with a pH/mV meter)
(Check electrodes each day)
1. To a 150 ml beaker, add 100 ml of distilled water and 2 ml of ISA. Place the beaker on the
magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is in the
millivolt mode, lower the electrode tips into the solution.
2. Using a pipet, add 1 ml of either the 0.1M or 1,000 ppm cadmium standard to the beaker.
When the reading has stabilized, record the mV reading.
3. Using a pipet, add 10 ml of the standard used above to the beaker. When the reading has
stabilized, record the mV reading.
4. Determine the difference between the two readings. The electrode is operating correctly if
the mV potential has changed 27±2 mV, assuming the temperature is between 20o and
25oC. See the TROUBLESHOOTING sections if the potential change is not within this range.
Slope is defined as the change in potential observed when the concentration changes by a factor of
10.
Electrode Slope Check (with ion meter)
(Check electrodes each day)
1. Prepare standard cadmium solutions whose concentrations vary be tenfold. Use either the
0.1M or 1,000 ppm standard and the serial dilution method for this preparation.
2. To a 150 ml beaker, add 100 ml of the lower value standard and 2 ml of ISA. Place the
beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips
into the solution. Assure that the meter is in the concentration mode.
3. Adjust the meter to the concentration of the standard and fix the value in the memory
according to the meter manufacturer's instructions.
4. Rinse the electrodes with distilled water and blot dry.
Instruction Manual Cadmium Electrode
5
5. To a 150 ml beaker, add 100 ml of the higher value standard and 2 ml of ISA. Place the
beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips
into the solution.
6. Adjust the meter to the concentration of the standard and fix the value in the memory.
7. Read the electrode slope according to the meter manufacturer's instructions. Correct
electrode operation is indicated by a slope of 90-100%. If the slope is not within this range,
see the TROUBLESHOOTING sections if the slope is not within this range.
MEASUREMENT
Measuring Hints
All samples and standards should be at the same temperature for precise measurement. A difference
of 1oC in temperature will result in about a 2% measurement error.
Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can
generate sufficient heat to change the solution temperature. To counteract this effect, place a piece
of insulating material, such as a styrofoam sheet, between the stirrer and beaker.
Always rinse the electrode tips with distilled water and blot dry. Use a clean, dry tissue to prevent
cross-contamination.
For samples with high ionic strength, prepare standards whose composition is similar to the sample.
Use fresh standards for calibration.
Use 2 ml of ISA for each 100 ml of sample or standard.
Always check to see that the membrane is free from air bubbles after immersion into the standard or
sample. Dilute concentrated samples (>0.1M) before measurement.
Sample Requirements
All samples must be aqueous and not contain organics which can dissolve the epoxy electrode body
and/or the cement bonding the sensing crystal to the electrode body. Infrequent measurements in
solutions containing methanol, benzene, or acetonitrile are permitted. Highly polar solvents slowly
attack the electrode. Please check with Eutech Instruments Pte Ltd. before using these electrodes in
other organic solvents.
The temperature of the sample solutions and of the standard solutions should be the same and below
80oC. About a 4% error in the slope will occur for each 1oC difference in temperature. Interferences
should be absent. If they are present, use the procedure found in the
Interference and Electrode
Response
sections to remove them.
Adjust sample pH with 1M HNO3 to below pH 7 to avoid precipitation of cadmium hydroxide,
Cd(OH)2.
Cadmium Electrode Instruction Manual
6
Units of Measurement
Cadmium ion concentrations are measured in units of parts per million, moles per liter, equivalents
per liter, or any other convenient concentration unit. Table 1 indicates some of these concentration
units.
TABLE 1
: Concentration Unit Conversion Factors
ppm Cd+2 moles/liter
1124.0 1.0X10
-2
112.4 1.0X10
-3
11.2 1.0X10-4
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with ISA. The temperature of both sample solutions and of
standard solutions should be the same.
Direct Measurement of Cadmium (using a pH/mV meter)
1. By serial dilution prepare three standard solutions from the 0.1M, 1,000 ppm or 100 ppm
standard. The resultant concentrations should be 10-2, 10-3, and 10-4M or 1,000, 100 and 10
ppm standards. Add 2 ml of ISA to each 100 ml of standard. Prepare standards with a
composition similar to the samples if the samples have an ionic strength above 0.1M.
2. Place 100 ml of the most dilute solution (1.0X10-4M or 10 ppm) in a 150 ml beaker on the
magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is in the
mV mode lower the electrode tips into the solution. After the reading has stabilized, record
the mV reading.
3. Place 100 ml of the mid-range solution (1.0X10
-3
M or 100 ppm) in a 150 ml beaker on the
magnetic stirrer and begin stirring at a constant rate. After rinsing the electrodes with
distilled water, blot dry and immerse the electrode tips in the solution. After the reading
has stabilized, record the mV reading.
4. Place 100 ml of the most concentrated solution (1.0X10
-2
M or 1,000 ppm) in a 150 ml
beaker on the magnetic stirrer and begin stirring at a constant rate. After rinsing the
electrodes with distilled water, blot dry and immerse the electrode tips in the solution.
After the reading has stabilized, record the mV reading.
5. Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against
concentration (log axis). Extrapolate the curve down to about 1.0X10-5 or 1 ppm. A typical
calibration curve can be found in Figure 1.
Instruction Manual Cadmium Electrode
7
A calibration curve is constructed on semi-logarithmic paper when using
a pH/mV meter in the millivolt mode. The measured electrode potential
in mV (linear axis) is plotted against the standard concentration (log
axis). In the linear region of the curve, only three standards are necessary
to determine a calibration curve. In the non-linear region, additional
points must be measured. The direct measurement procedures given are
for the linear portion of the curve. The non-linear portion of the curve
requires the use of low level procedures.
6. To a clean, dry 150 ml beaker, add 100 ml of the sample and 2 ml of ISA. Place the beaker
on the magnetic stirrer and begin stirring at a constant rate. Rinse the electrode tips with
distilled water, blot dry and lower the electrode tips into the solution. After the reading has
stabilized, record the mV reading. Using the calibration curve, determine the sample
concentration.
7. The calibration should be checked every two hours. Assuming no change in ambient
temperature, immerse the electrode tips in the mid-range standard. After the reading has
stabilized, compare it to the original reading recorded in Step 3 above. A reading differing
by more than ± 0.5 mV, or a change in the ambient temperature will necessitate the
repetition of Steps 2-5 above. A new calibration curve should be prepared daily.
Direct Measurement of Cadmium (using an ion meter)
1. By serial dilution of the 0.1M or 1,000 ppm cadmium standard, prepare two cadmium
standards whose concentration is near the expected sample concentration. Measure out 100
ml of each standard into individual 150 ml beakers and add 2 ml of ISA to each.
2. Place the more dilute solution on the magnetic stirrer and begin stirring at a constant rate.
Assure that the meter is in the concentration mode. Lower the electrode tips into the
solution.
Cadmium Electrode Instruction Manual
8
3. Adjust the meter to the concentration of the cadmium standard and fix the value in the
memory according to the meter manufacturer's instructions after stabilization of the
reading.
4. Rinse the electrodes with distilled water and blot dry.
5. Place the more concentrated solution on the magnetic stirrer and begin stirring at a
constant rate. Lower the electrode tips into the solution
6. Adjust the meter to the concentration of the cadmium standard and fix the value in the
memory according to the meter manufacturer's instructions after stabilization of the
reading.
7. Place 100 ml of the sample and 2 ml of ISA into a 150 ml beaker. Place the beaker on the
magnetic stirrer and begin stirring at a constant rate.
8. Immerse the rinsed, dried electrode tips in the solution and wait for the reading to stabilize.
Read the concentration directly from the meter display.
9. The calibration should be checked every two hours. Assuming no change in ambient
temperature, immerse the electrode tips in the first cadmium standard. After the reading
has stabilized, compare it to the original reading recorded in Step 3 above. A reading
differing by more than ± 0.5 mV, or a change in ambient temperature will necessitate the
repetition of Steps 2-6 above. The meter should be re-calibrated daily.
Low Level Cadmium Determination (using a pH/mV meter)
This procedure is recommended for solutions with cadmium concentrations of less than 1.0X10-5M
(1.1 ppm). If the solution is high in ionic strength, but low in cadmium ion, use the same procedure,
but prepare a calibration solution with a composition similar to the sample.
1. Using 20 ml of standard ISA, dilute to 100 ml with distilled water. This low level ISA
(1.0M NaNO3) is added at the rate of 1 ml low level ISA to each 100 ml of solution. The
background ionic strength will be 1.0X10-2M.
2. Dilute 1ml of 0.1M standard to one liter to prepare a 1.0X10-4M solution for
measurements in moles per liter. Prepare a 10 ppm standard solution by diluting 1ml of the
1,000 ppm standard to 100 ml of solution for measurements in ppm. Standards should be
prepared fresh daily.
3. To a 150 ml beaker add 100 ml of distilled water and 1 ml of low level ISA. Place the
beaker on the magnetic stirrer and begin stirring at a constant rate.
4. Place the electrode tips in the solution. Assure that the meter in the mV mode.
5. Add increments of the 1.0X10-4M or 10 ppm standard as given in Table 2 below.
6. After the reading has stabilized, record the mV reading after each addition.
TABLE 2: Step-wise Calibration for Low Level Cadmium Measurements
Instruction Manual Cadmium Electrode
9
Added Concentration
Step Pipet Volume (ml) ppm M
1 A 0.1 0.01 1.0X10-7
2 A 0.1 0.02 2.0X10-7
3 A 0.2 0.04 4.0X10-7
4 A 0.2 0.06 6.0X10-7
5 A 0.4 0.10 9.9X10-7
6 B 2.0 0.29 2.9X10-6
7 B 2.0 0.48 4.8X10
-6
Pipet A = 1 ml graduated pipet
Pipet B = 2 ml pipet
Solutions: additions of 10 ppm or 1.0X10
-4
M standard to 100 ml of solution prepared in
Step 3 above.
7. On a semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis) as in Figure 1.
8. Rinse the electrodes with distilled water and blot dry.
9. Measure out 100 ml of the sample into a 150 ml beaker and add 1ml of low level ISA.
Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the
electrode tips into the solution. After the reading has stabilized, record the mV reading and
determine the concentration from the low level calibration curve.
10. Prepare a new low level calibration curve daily. Check the calibration curve every two
hours by repeating Steps 3 - 7 above.
Low Level Cadmium Determination (using an ion meter)
1. Using 20 ml of standard ISA, dilute to 100 ml with distilled water. This low level ISA
(1.0M NaNO
3
) is added at the rate of 1ml low level ISA to each 100 ml of solution. The
background ionic strength will be 1.0X10
-2
M.
2. Follow the steps given in Direct Measurement of Cadmium (using an ion meter) to
the end of Step 6.
3. To a 150 ml beaker, add 100 ml of distilled water and 1 ml of low level ISA. Place the
beaker on the magnetic stirrer and begin stirring at a constant rate.
4. Lower the electrode tips into the solution. When the reading has stabilized, fix the blank
value in the meter according to the meter manufacturer's instructions.
5. Continue with Steps 7-9 in Direct Measurement of Cadmium (using an ion meter),
remembering to use the low level ISA described in Step 1 above.
Titration
Titration is a very accurate determination of total cadmium ion concentration. This method makes
use of the electrode as an endpoint detector when EDTA is used as a titrant.
Cadmium Electrode Instruction Manual
10
Titration of Cadmium
The method outlined in this section makes use of the cadmium ion electrode as a highly sensitive
endpoint detector for cadmium-containing samples. The titrant used is EDTA.
EDTA complexes cadmium as well as other cations. The sample pH can be adjusted to pH 10 by
adding ammonia to eliminate unwanted ion complexes. Masking agents may be added in some
cases.
1. Prepare the stock EDTA titrant as given in the section
Required Solutions. Dilute the
EDTA to 10 to 20 times as concentrated as the suspected sample concentration. The
sample should contain at least 1.0X10-4M cadmium for a good detection of the endpoint.
2. Fill a 50 ml burette with the EDTA solution. Pipet 100 ml of the sample into a 150 ml
beaker, place the beaker on the magnetic stirrer and begin stirring at a constant rate. Adjust
the sample to pH 10 by adding ammonia.
3. Position the burette tip in the beaker, slightly above the liquid level in the beaker and
slightly off center. Position the electrode tips in the solution about halfway between the
center of the beaker and the beaker wall.
4. Begin adding the EDTA in 0.5 ml to 1.0 ml increments and about 0.1 ml to 0.2 ml
increments as the potential begins to change more rapidly. Record the mV potential after
each addition. Continue the additions several milliliters past the endpoint until little
change is noted in the mV reading even when adding 0.5 - 1.0 ml increments.
5. Plot the milliliters of EDTA added against the mV potential on standard coordinate graph
paper. (See Figure 2.) The point of greatest potential change is the endpoint.
6. The cadmium ion concentration of the sample is calculated as follows:
VtMt
Instruction Manual Cadmium Electrode
11
M
Cd
+2
= ————
V
Cd
+2
where:
M
Cd
+2
= concentration of cadmium ion in the sample (moles/liter)
Vt = volume of EDTA added at endpoint
Mt = EDTA concentration (moles/liter)
Cd
+2
= volume of unknown sample (100 ml)
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every
hour. Factors like temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is
independent of concentration within the electrode's operating range.
Interferences
A surface layer of silver metal may be formed by strongly reducing solutions. A layer of silver salt
may be deposited on the membrane if high levels of ions forming very insoluble salts are present in
the sample. Proper performance can be restored by polishing. See the section entitled Electrode
Response
for proper polishing procedure.
The cadmium ion electrodes do not respond to anions or most cations. The electrode membrane is
poisoned by solutions containing copper, mercury, and silver. These ions must be absent from the
solution.
If the level of ferric ion or lead ion exceeds the level of cadmium ion in the sample, the
measurement will be affected. Sodium fluoride added to the sample will eliminate ferric ion.
Precipitation and Complexation
Sulfide, carbonate, oxalate, phosphate, hydroxide, and other ions precipitate insoluble cadmium
salts. The level of cadmium ion, the level of the precipitating ion, and the pH of the sample
determine formation of a precipitate.
A wide variety of species, including acetate, ammonia, bromide, chloride, citrate, cyanide, and
EDTA, form complexes with cadmium. The total cadmium concentration, the concentration of the
complexing species, the solution pH, and the ionic strength all determine the extent of
complexation. Complexation reduces the free cadmium ion concentration and, since the electrode
responds only to free cadmium ions, a false reading results.
Cadmium Electrode Instruction Manual
12
Temperature Influences
Samples and standards should be at the same temperature, since the electrode potentials are
influenced by changes in temperature. A 1
o
C difference in temperature results in a 4% error at the
1.0X10-3M cadmium concentration. Because of the solubility equilibrium on which the electrode
depends, the absolute potential of the reference electrode changes slowly with temperature. The
slope of the electrode, as indicated by the factor "S" in the Nernst equation, also varies with
temperature. Table 3 gives values for the "S" factor in the Nernst equation for the cadmium ion.
TABLE 3: Temperature vs. Values for the Electrode Slope
Temp.(oC) "S"
0 27.10
10 28.10
20 29.08
25 29.58
30 30.07
40 31.07
50 32.06
If changes in temperature occur, the electrodes should be re-calibrated. The temperature range for
the Eutech Cadmium Ion Electrode is 0o-80oC, provided that temperature equilibrium has occurred.
If the temperature varies substantially from room temperature, equilibrium times up to one hour are
recommended.
Electrode Response
Plotting the mV potential against the cadmium concentration on semi-logarithmic graph paper
results in a straight line with a slope of about 27 mV per decade. (Refer to Figure 1.)
The time needed to reach 99% of the stable electrode potential reading, the electrode response time,
varies from several seconds in highly concentrated solutions to several minutes near concentrations
of 1.0X10
-6
M cadmium. Below 10-6M, considerably longer response time can be expected. (Refer
to Figure 3.)
Instruction Manual Cadmium Electrode
13
A drifting potential reading or a decrease in electrode slope may mean that the electrode membrane
needs polishing.
To polish the membrane:
1. If using polishing paper, cut off a 1-2" piece and place it face up on the lab bench.
2. Put a few drops of distilled or deionized water in the center of the paper.
3. Holding the paper (cotton) steady with one hand, bring the membrane of the electrode
down perpendicular to the paper and, with a slight swirling motion, gently polish the tip of
the electrode against the surface of the polishing paper (cotton) for a few seconds.
4. Rinse the electrode surface with distilled or deionized water and soak the electrode tip in
standard solution for about five minutes before use.
5. If using jeweller's rouge, place a cotton ball on the table top and flatten it using the bottom
of a beaker.
6. Put 1-2 drops of distilled or deionized water in the center of the cotton pad.
7. Add a small amount of jeweller's rouge to the damp cotton.
8. Continue with Steps 3 and 4 above.
Limits of Detection
The upper limit of detection in pure cadmium nitrate solutions is 0.1M. In the presence of other
ions, the upper limit of detection is above 1.0x10-2M cadmium, but the possibility of a liquid
junction potential developing at the reference electrode and the "salt extraction effect" are two
limiting factors. Some salts may infuse into the electrode membrane at high salt concentrations
causing deviation from the theoretical response. Either dilute samples between 0.1M and 1.0x10-2M
or calibrate the electrode at 4 or 5 intermediate points.
The lower limit of detection is influenced by the slight water solubility of the electrode pellet. Refer
to Figure 1 for a comparison of the theoretical response to the actual response at low levels of
cadmium. Neutral solutions containing free cadmium ions can be measured down to 1.0X10-7M
(0.01 ppm). Extreme care must be taken with measurements below 1.0X10-5M (1.1 ppm) to avoid
contamination of samples.
pH Effects
Figure 4 shows how the electrode responds to cadmium ion in solution at various pH levels.
Hydrogen ion interferes with low cadmium ion measurements. The minimum pH at which cadmium
ion concentrations can be measured without interference is given by the solid line "limit of
detection" above the shaded area in Figure 4.
At a high pH, free cadmium ion precipitates with hydroxide ion, thereby reducing the cadmium ion
concentration. The maximum pH at which cadmium concentration can be measured without
Cadmium Electrode Instruction Manual
14
interference from hydroxide is given by a dashed line to the left of the dotted area on Figure 4.
Within this dotted area, cadmium combines with hydroxide to form Cd(OH)2. Since only free
cadmium concentration can be measured with the cadmium ion electrodes, a false reading results.
Electrode Life
A cadmium electrode will last six months in normal laboratory use. On-line measurements might
shorten operational lifetime to several months. In time, the response time will increase and the
calibration slope decreases to the point calibration is difficult and electrode replacement is required.
Electrode Storage
The cadmium electrode may be stored for short periods of time in 1.0x10
-2
M cadmium solution. For
longer storage (longer than two weeks), rinse and dry the sensing pellet and cover the membrane tip
with any protective cap shipped with the electrode. The reference portion of the combination
electrode (or the outer chamber of the reference electrode) should be drained of filling solution, if
refillable, and the rubber insert placed over the filling hole.
ELECTRODE THEORY
Electrode Operation
The electrode consists of sulfides of cadmium and other metals bonded into an epoxy or glass body.
When an electrode potential develops across the membrane, the membrane is in contact with a
solution containing cadmium ions. The electrode potential is measured against a constant reference
potential, using a standard pH/mV meter. The level of cadmium ions, corresponding to the
measured potential, is described by the Nernst equation:
E = Eo + S log X
where: E
= measured electrode potential
Eo = reference potential (a constant)
S = electrode slope (∼27 mV/decade)
X = level of cadmium ions in solution
The activity, X, represents the effective concentration of free cadmium ions in solution. Both
bound, Cb, and free, Cf, cadmium ions are included in the total cadmium ion concentration, Ct. The
cadmium ion electrode will only respond to free cadmium ions, the concentration of which is:
Cf = Ct + Cb
The activity, X, represents the effective concentration of the ions in solution. The activity is related
to the free cadmium ion by the activity coefficient, γ , by:
X = γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½ Σ CxZ
x
2
Instruction Manual Cadmium Electrode
15
where:
Cx = concentration of ion X
Zx = charge of ion X
Σ = sum of all the types of ions in the solution
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient, γ, is constant and the activity, X, is directly proportional to the concentration.
To adjust the background ionic strength to a high and constant value, ionic strength adjuster is
added to samples and standards. The recommended ISA solution for the cadmium electrodes is
sodium nitrate, NaNO3. Solutions other than this may be used as ionic strength adjusters as long as
ions that they contain do not interfere with the electrode's response to cadmium ions. Samples with
high ionic strength (greater than 0.1M) do not need ISA added and standards for these solutions
should be prepared with a composition similar to the samples.
The reference electrode must also be considered. When two solutions of different composition are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur
from the inter-diffusion of ions in the two solutions. Electrode charge will be carried unequally
across the solution boundary resulting in a potential difference between the two solutions, since
ions diffuse at different rates. When making measurements, it is important to remember that this
potential be the same when the reference is in the standardizing solution as well as in the sample
solution or the change in liquid junction potential will appear as an error in the measured electrode
potential.
The composition of the liquid junction filling solution in the reference electrode is most important.
The speed with which the positive and negative ions in the filling solution diffuse into the sample
should be as nearly equal as possible, that is, the filling solution should be equitransferent. No
junction potential can result if the rate at which positive and negative charge carried into the sample
is equal.
Strongly acidic (pH = 0-2) and strongly basic (pH = 12-14) solutions are particularly troublesome to
measure. The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask
their effect on the junction potential with any concentration of an equitransferent salt. One must
either calibrate the electrode in the same pH range as the sample or use a known increment method
for ion measurement.
Cadmium Electrode Instruction Manual
16
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through checking each of the system
components in turn: the meter, the glassware, the electrodes, the standards and reagents, the sample,
and the technique.
Meter
The meter may be checked by following the check-out procedure in instrument instruction manual.
Glass-ware
Clean glass-ware is essential for good measurement. Be sure to wash the glass-ware well with a
mild detergent and rinse very well with distilled or deionized water. Clean glass-ware will drain
without leaving water droplets behind.
Electrodes
The electrode may be checked by using the procedure found in the sections entitled Electrode
Slope Check.
1. Be sure to use distilled or deionized water when following the procedures given in
Electrode Slope Check.
2. If the electrode fails to respond as expected, see the sections on Measuring Hints and
Electrode Response. Repeat the slope check.
3. If the electrode still fails to respond as expected, substitute another cadmium ion electrode
that is known to be in good working order for the questionable electrode. If the problem
persists and you are using an electrode pair, try the same routine with a working reference
electrode.
4. If the problem persists, the reagent may be of poor quality, interferences in the sample may
be present or the technique may be faulty. (See Standards and Reagents, Sample, and
Technique sections below.)
5. If another electrode is not available for test purposes, or if the electrode in use is suspect,
review the instruction manual and be sure to:
- Clean and rinse the electrodes thoroughly.
- Prepare the electrodes properly.
- Use the proper filling solution.
- Adjust the pH and the ionic strength of the solution by the use of the proper ISA.
- Measure correctly and accurately.
- Review
TROUBLESHOOTING HINTS.
Standards and Reagents
Whenever problems arise with the measuring procedure that has been used successfully in the past,
be sure to check the standard and reagent solutions. If in doubt about the credibility of any of the
solutions prepare them again. Errors may result from contamination of the ISA, incorrect dilution,
poor quality distilled/deionized water, or a simple mathematical miscalculation.
Instruction Manual Cadmium Electrode
17
Sample
Look for possible interferences, complexing agents, or substances which could affect the response
or physically damage the sensing electrode (or reference electrode) if the electrodes work perfectly
in the standard, but not in the sample.
Try to determine the composition of the samples prior to testing to eliminate a problem before it
starts. (See
Measuring Hints, Sample Requirements, and Interferences.)
Technique
Be sure that the electrode's limit of detection has not been exceeded. Be sure that the analysis
method is clearly understood and is compatible with the sample.
Refer to the instruction manual again. Reread sections GENERAL PREPARATION and ELECTRODE
CHARACTERISTICS.
If trouble still persists, call Eutech Instruments Pte Ltd. at (65) 6778-6876 and ask for the Customer
Services Department.
Cadmium Electrode Instruction Manual
18
TROUBLESHOOTING HINTS
Symptom Possible Causes Next Step
Out of Range defective meter check meter with
Reading shorting strap
(see meter instruction manual)
defective electrode check electrode operation
electrodes not unplug electrodes
plugged in properly and reset
reference electrode be sure reference
not filled electrode is filled
air bubble on remove bubble by
membrane redipping electrode
electrodes not put electrodes
in solution in solution
Noisy or Unstable defective meter check meter with
Readings (readings shorting strap
continuously or
rapidly changing) air bubble on remove bubble by
membrane re-dipping electrode
electrode exposed soak electrode in
to interferences cadmium standard
defective electrode replace electrode
ISA not used use recommended ISA
meter or stirrer ground meter or
not grounded stirrer
Drift (reading samples and standards allow solutions to come
slowly changing at different to room temperature
in one direction) temperatures before measurement
electrode exposed check section
to complexing agents entitled Precipitation
and Complexation
Instruction Manual Cadmium Electrode
19
incorrect reference use recommended
filling solution filling solution
Low Slope or standards contaminated prepare fresh
No Slope or incorrectly made standards
ISA not used use recommended ISA
standard used as ISA use ISA
electrode exposed check section
to complexing agents entitled Precipitation
and Complexation
air bubble on remove bubble by
membrane redipping probe
"Incorrect Answer" incorrect scaling plot millivolts on
(but calibration of semi-log paper the linear axis. On
curve is good) the log axis, be sure
concentration numbers
within each decade are
increasing with
increasing concentration.
incorrect sign be sure to note sign of
millivolt number correctly
incorrect standards prepare fresh standards
wrong units used apply correct
conversion factor:
10-3M = 112 ppm as Cd+2
complexing agents check section entitled
in sample Precipitation and
Complexation;
use titration
Cadmium Electrode Instruction Manual
20
SPECIFICATIONS
Concentration Range: 1.0X10-1M to 1.0X10-7M (11,200 to 0.01 ppm)
pH Range: 2 to 12
Temperature Range: 0o-80oC
Resistance: <1 Mohm
Reproducibility: +/- 2%
Samples: Aqueous solutions only; no organic solvents
Size: 110 mm length; 12 mm diameter; 1 m cable length
Storage: Store in cadmium solution
ORDERING INFORMATION
CODE/NO. DESCRIPTION
EC-CD-03 Cadmium Ion Combination Electrode, epoxy body
EC-ISA-CD1-BT Ionic Strength Adjuster (ISA), 5M NaNO3
EC-MIS-PP Polishing Paper for the Cadmium Ion Electrodes
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