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CALCIUM EPOXY
User Manual
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Eutech Instruments CALCIUM EPOXY User Manual

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Instruction Manual Calcium Electrode
EUTECH INSTRUMENTS PTE LTD.
INSTRUCTION MANUAL
CALCIUM ION ELECTRODE
1
Calcium Electrode Instruction Manual
TABLE OF CONTENTS
Introduction..............................................................................................................................................3
Required Equipment.................................................................................................................................3
Required Solutions...................................................................................................................................3
Electrode Preparation...............................................................................................................................4
Electrode Slope Check (with pH/mV meter) ...........................................................................................4
Electrode Slope Check (with ion meter) ..................................................................................................5
Measuring Hints.......................................................................................................................................5
Sample Requirements...............................................................................................................................6
Units of Measurement..............................................................................................................................6
MEASUREMENT PROCEDURE ...................................................................................................6
Direct Measurement.................................................................................................................................6
Direct Measurement of Calcium (using a pH/mV meter)........................................................................6
Direct Measurement of Calcium (using an ion meter).............................................................................8
Low Level Calcium Determination (using a pH/mV meter)....................................................................8
Low Level Calcium Determination (using an ion meter).........................................................................9
Titration....................................................................................................................................................9
Titration of Calcium...............................................................................................................................10
ELECTRODE CHARACTERISTICS...........................................................................................11
Reproducibility.......................................................................................................................................11
Interferences...........................................................................................................................................11
Complexation.........................................................................................................................................12
Temperature Influences..........................................................................................................................12
Electrode Response................................................................................................................................13
Limits of Detection.................................................................................................................................13
Electrode Life.........................................................................................................................................13
Electrode Storage ...................................................................................................................................14
Electrode Operation ...............................................................................................................................14
TROUBLESHOOTING GUIDE....................................................................................................15
Meter......................................................................................................................................................15
Glass-ware..............................................................................................................................................15
Electrodes...............................................................................................................................................15
Standards and Reagents..........................................................................................................................16
Sample....................................................................................................................................................16
Technique...............................................................................................................................................16
TROUBLESHOOTING HINTS.....................................................................................................17
ORDERING INFORMATION.......................................................................................................19
2
Instruction Manual Calcium Electrode
EUTECH INSTRUMENTS PTE LTD.
CALCIUM ELECTRODE
INSTRUCTION MANUAL
GENERAL INSTRUCTIONS
Introduction
Eutech Instruments Calcium Ion Electrode is used to measure calcium 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 Calcium Epoxy-body Combination Electrode, Code. no. EC-CAL-03.
Required Solutions
1. Deionized or distilled water for solution and standard preparation.
2. Eutech Calcium Standard, 0.1M CaCl2, Code. no. EC-SCS-CA1-BT. To prepare this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 14.7 grams of reagent-grade calcium chloride (CaCl2.2H2O). 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. Eutech Calcium Standard, 1,000 ppm Ca solution from you own laboratory stock, half fill a one liter volumetric flask with distilled water and add 3.67 grams of reagent-grade calcium chloride (CaCl gently to dissolve the solid. Fill to the mark with distilled water, cap, and upend several times to mix the solution.
4. Eutech Calcium Standard, 100 ppm Ca+2 as CaCO3, Code. no. EC-SCS-CA3-BT. To prepare this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 0.15 grams of reagent-grade calcium chloride (CaCl2.2H2O). 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. Eutech Ionic Strength Adjuster (ISA), 4 M KCl, Code. no. EC-ISA-CA1-BT. To prepare this solution from your own laboratory stock, half fill a 1,000 ml volumetric flask with distilled water and add 298 grams of reagent-grade potassium chloride (KCl). Swirl the
+2
, Code. no. EC-SCS-CA2-BT. To prepare this
.2H2O). Swirl the flask
2
3
Calcium Electrode Instruction Manual
flask gently to dissolve the solid. Fill to the mark with distilled water, cap, and upend several times to mix the solution.
6. EDTA titrant, 1M stock solution, for the titration of calcium. To prepare this titrant, add
37.2 grams of reagent grade Na2EDTA.2H2O, ethylenediaminetetraacetic acid dihydrate, disodium salt, to a 100 ml volumetric flask, add about 75 ml of distilled water, and swirl the flask gently to 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 reference electrode. Fill the combination electrode or the reference 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. Gently shake the electrode downward in the same manner as a clinical thermometer to remove any air bubbles which might be trapped behind the calcium membrane. Prior to first usage, or after long-term storage, immerse the calcium membrane in calcium standard for thirty minutes. The electrode is now ready for use.
Connect the electrodes to the proper terminals as recommended by the meter manufacturer.
Electrode Slope Check (with pH/mV meter)
(Check electrodes each day)
1. To a 150 ml beaker, add 100 ml of distilled water. Place the beaker on a 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. If drifting or instability is observed, see the
TROUBLESHOOTING section.
2. Using a pipet, add 1 ml of 0.1M, 1,000 ppm, or 100 ppm (as calcium carbonate) standard and 2 ml of ISA to the beaker. When the reading is stable, record the mV reading.
3. Using a pipet, add 10 ml of the same calcium 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 by 27±2 mV, assuming the solution temperature is between 20o and 25oC. See the TROUBLESHOOTING section 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.
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Instruction Manual Calcium Electrode
Electrode Slope Check (with ion meter)
(Check electrodes each day)
1. Prepare standard calcium solutions whose concentrations vary by tenfold. Use either the
0.1M Ca+2, 1,000 ppm Ca+2, or the 100 ppm Ca+2 (as calcium carbonate) standard stock solutions. Use 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.
5. To another 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%. 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
o
C in temperature will result in a 4% measurement error.
of 1
The sensing membrane is normally subject to water uptake and might appear milky. This has no effect on performance.
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 with compositions similar to that of the sample. Always check to see that the membrane is free from air bubbles after immersion into standard or sample.
A slow responding electrode may be caused by interferences to the electrode. To restore proper performance, soak the electrode in distilled water for about 5 minutes to clean the membrane, rinse, and soak in diluted standard solution for about 5 minutes.
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Calcium Electrode Instruction Manual
Dilute concentrated samples (over 0.1M) before measurement. Recalibrate every few hours for routine measurement.
Sample Requirements
All samples and standards must be aqueous and not contain organics which can dissolve in the membrane or extract out the liquid ion exchanger.
The temperature of the standard solutions and of the sample solutions should be the same and below 40oC. About a 2% error will be introduced for a 1oC difference in temperature.
The pH range for the calcium ion electrode is 3-10. Neutralize samples outside this range with NaOH or HCl to bring them in range.
Interferences should be absent. If they are present, use the procedures found in the Interferences section to remove them.
Units of Measurement
Calcium concentrations are measured in units of ppm as calcium, ppm as CaCO3, moles per liter, or any other convenient concentration unit. Table 1 indicates some of the concentration units.
TABLE 1: Concentration Unit Conversion Factors
ppm Ca+2 ppm CaCO3 moles/liter
4.01 10.0 1.0X10-4
10.00 24.9 2.5X10-4
40.10 100.1 1.0X10-3
400.80 1,000.9 1.0X10
-2
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 temperature of both sample solution and of standard solutions should be the same.
Direct Measurement of Calcium (using a pH/mV meter)
1. By serial dilution prepare three standard solutions from the 0.1M, 1,000 ppm, or 100 ppm
-2
standard. The resultant concentrations should be 10
, 10-3, and 10-4M or 100, 10 and 1
ppm standards. Add 2 ml of ISA to each 100 ml of standard.
2. Place the most dilute solution (1.0X10-4M or 1 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.
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Instruction Manual Calcium Electrode
3. Place the mid-range solution (1.0X10-3M or 10 ppm) in a 150 ml beaker on the magnetic stirrer and begin stirring. After rinsing the electrodes with distilled water, blot dry, and immerse the electrode tips in the solution. When the reading has stabilized, record the mV value.
4. Place the most concentrated solution (1.0X10-2M or 100 ppm) in a 150 ml beaker on the magnetic stirrer and begin stirring. After rinsing the electrodes in distilled water, blot dry and immerse the electrode tips in the solution. When the reading has stabilized, record the mV reading.
5. Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the concentration (log axis). A typical calibration curve can be found in Figure 1.
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 non­linear portion of the curve requires the use of low level procedures.
6. To a clean, dry 150 ml beaker, add 100 ml of 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 in the solution. When the reading has stabilized, record the mV reading. Using the calibration curve, determine the sample concentration.
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Calcium Electrode Instruction Manual
7. The calibration should be checked every two hours. Assuming no change in ambient temperature, place 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 Calcium (using an ion meter)
1. By serial dilution of the 0.1M, 1,000 ppm, or 100 ppm calcium standard, prepare two calcium 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.
3. Adjust the meter to the concentration of the calcium standard and fix the value in the memory according to the meter manufacturer's instructions after stabilization of the reading. Rinse the electrode tips with distilled water and blot dry.
4. Place the more concentrated solution on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips into the solution.
5. Adjust the meter to the concentration of the calcium standard and fix the value in the memory according to the meter manufacturer's instructions after stabilization of the reading.
6. For low level measurements, place the rinsed, dried electrodes into a solution containing 100 ml of distilled water and 2 ml of ISA. After stabilization, fix the blank value in the meter according to the meter manufacturer's instructions.
7. Place 100 ml of the sample and 2 ml of ISA in a 150 ml beaker. Place the beaker on the magnetic stirrer and begin stirring.
8. Immerse the 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, place the electrode tips in the first calcium standard. After the reading has stabilized, compare it to the original reading 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 Calcium Determination (using a pH/mV meter)
-2
This procedure is recommended for solutions with ionic strengths less than 1.0X10
M. If the solution is high in ionic strength, but low in calcium, use the same procedure, but prepare a calibration solution with a composition similar to the sample.
8
Instruction Manual Calcium Electrode
1. Dilute 10 ml of the 0.1M standard to 1,000 ml to prepare a 1.0x10-3M standard solution for measurements in moles per liter. Dilute 10 ml of the 1,000 ppm or 1 ml of the 100 ppm standard to 1,000 ml to prepare a 10 ppm standard solution for measurements in ppm.
2. Soak the calcium electrode for at least 1 hour in 1.0x10-3M or 100 ppm calcium standard solution.
3. To a 150 ml beaker, add 100 ml of distilled water. 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 is in the mV mode.
5. Add increments of the 1.0X10-3M 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 Calcium Measurements
Added Concentration Step Pipet Volume (ml) M ppm 1 A 0.1 1.0X10-6 1.0X10-2 2 A 0.1 2.0X10-6 2.0X10-2 3 A 0.2 4.0X10-6 4.0X10-2 4 A 0.2 6.0X10-6 6.0X10
-2
5 A 0.4 9.9X10-6 1.0X10-1 6 B 2 2.9X10-5 2.9X10 7 B 2 4.8X10-5 4.8X10
-1
-1
Pipet A = 1 ml graduated pipet Pipet B = 2 ml pipet Solutions: additions of 1.0x10-3M or 10 ppm standard to 100 ml of distilled water.
7. On semi-logarithmic graph paper, plot mV reading (linear axis) against the concentration (log axis) as in Figure 1.
8. Rinse the electrodes in distilled water and blot dry.
9. Measure out 100 ml of the sample into a 150 ml beaker and place the beaker on the magnetic stirrer and begin stirring. 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 1-2 hours by repeating Steps 2-7 above.
Low Level Calcium Determination (using an ion meter)
Follow the procedure given for normal calcium determinations using an ion meter and the blank correction procedure.
Titration
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Calcium Electrode Instruction Manual
The progressive and quantitative addition of a reagent to a measured sample until neither active species (reagent or sample) is in excess. Ion selective electrodes are excellent endpoint detectors since they are not influenced by solution color or turbidity. Though titration is more time consuming than direct measurement, it is about 10 times more accurate.
Titration of Calcium
The method outlined in this section makes use of the calcium ion electrode as a highly sensitive endpoint detector for calcium-containing samples. The titrant used is EDTA.
EDTA complexes calcium as well as other cations. The sample pH can be adjusted to pH 10 by adding ammonia to eliminate unwanted ion complexes. Masking agents can be added in some cases.
1. Soak the calcium ion electrode tip in 10-3M or 100 ppm calcium standard solution for a minimum of one hour prior to use.
2. 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 1.0X10-3M calcium for a good detection of the endpoint.
3. Fill a 50 ml buret 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.
4. Position the buret tip in the beaker, slightly above the liquid level in the beaker and slightly off center. Position the electrode tips in the solution about half way between the center of the beaker and the beaker wall.
5. 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.
6. 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.
7. The calcium ion concentration from the unknown is calculated as follows.
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Instruction Manual Calcium Electrode
VtMt M V
+2
= ———
Ca
Ca
+2
where M
+2
= concentration of calcium ion in the unknown (moles/liter)
Ca
Vt = volume of EDTA added at endpoint Mt = EDTA concentration (moles/liter) V
+2
= volume of unknown sample
Ca
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to ±4% can be obtained if the electrode is calibrated every hour. Factors such as temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is independent of concentration within the electrode's operating range.
Interferences
Table 3 lists some common cations that, if present in high enough levels, will cause electrode interferences and measurement errors or electrode drift when using the calcium electrodes. Electrode drift and slow response could indicate the presence of high interferences from the ions listed. Soak the electrode in distilled water for five minutes, then for five minutes in calcium standard solution to restore proper response.
TABLE 3: Concentration of Possible Interferences Causing a 10% Error at Various Levels of Calcium.
Interferences (moles/liter) 10-2M 10-3M 10-4M
+2
Mg
1.0X10+1 1.0X10 Zn+2 1.0X10+1 1.0X10 Ba+2 7.0X10 K
+1
4.0X10
0
7.0X10-1 7.0X10
0
4.0X10-1 4.0X10-2
Na+1 2.0X100 2.0X10-1 2.0X10
0
1.0X10
0
1.0X10
-1
-1
-2
-2
Ni+2 5.0X10-1 5.0X10-2 5.0X10-3 Cu+2 4.0X10-1 4.0X10-2 4.0X10 Fe+2 2.0X10
-2
2.0X10-3 2.0X10-4
-3
Sr+2 6.0X10-2 6.0X10-3 6.0X10-4 H+1 4.0X10-2 4.0X10-3 4.0X10 Hg+2 4.0X10-2 4.0X10-3 4.0X10
-4
-4
Pb+2 1.0X10-4 1.0X10-5 1.0X10-6
11
Calcium Electrode Instruction Manual
Interferences (ppm) 1,000 ppm CaCO3 100 ppm CaCO3 10 ppm CaCO3
Mg+2 2.43X105 2.43X104 2.43X103 Zn+2 6.53X105 6.53X104 6.53X103 Ba+2 9.60X105 9.60X104 9.60X10 K
+1
1.56X105 1.56X104 1.56X103
3
Na+1 4.60X104 4.60X103 4.60X102 Ni+2 2.94X104 2.94X103 2.94X10 Cu+2 2.54X104 2.54X103 2.54X10 Fe+2 1.11X104 1.11X103 1.11X10 Sr+2 5.20X103 5.20X102 5.20X10 H Hg
+1
1.4 pH 2.4 pH 3.4 pH
+2
8.0X103 8.0X102 8.0X101
Pb+2 2.0X101 2.0 2.0X10
Complexation
2 2 2 1
-1
Sulfate, bicarbonate, and carbonate are the most common species that complex calcium ions. The level of calcium ions, the level of the complexing ion, the pH of the solution, and the total ionic strength of the solution determine the extent of the complexation. Complexation reduces the free calcium ion concentration and, since the electrode responds only to free calcium ions, a false reading results.
To avoid formation of CaSO4, the sulfate concentrations must be less than 5X10-4M (50 ppm). To avoid formation of CaCO3 or formation of the CaHCO
+
complex, the pH of the solution should be
3
less than 7, and the total carbonate/bicarbonate concentration should be less than 3X10-3M (280 ppm carbonate).
Temperature Influences
Samples and standards should be at the same temperature, since electrode potentials are influenced by changes in temperature. A 1oC difference in temperature results in a 4% error at the 1.0X10-3M level.
Provided that temperature equilibrium has occurred, the calcium electrodes can be used at temperatures from 0o-40oC. Room temperature measurements are recommended, since measurements at temperatures quite different from room temperature may require equilibrium times up to one hour. Table 4 indicates the variation of theoretical slope with temperature.
TABLE 4: Temperature vs. Value for the Electrode Slope
o
Temp (
C) "S" 0 27.10 10 28.10 20 29.08 25 29.58 30 30.07 40 31.07
12
Instruction Manual Calcium Electrode
Electrode Response
Plotting the mV potential against the calcium concentration on semi-logarithmic 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 one minute or less for calcium concentration above 1.0x10-4M to several minutes near the detection limit. (Refer to Figure 3.)
Limits of Detection
The upper limit of detection in pure calcium chloride solutions is 1M. In the presence of other ions, the upper limit of detection is above 1.0x10-1M, 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 theoretical response. Calibrate the electrode at four or five intermediate points, or dilute the sample,
-1
to measure samples between 1.0x10
M and 1M.
The lower limit of detection is influenced by the slight water solubility of the ion exchanger used in the sensing portion of the electrode. Refer to Figure 1 for a comparison of the theoretical response to actual response at low levels of calcium chloride.
pH Effects
The operating range of the calcium electrode is from pH 3 to pH 10. Use at other pH values can adversely affect the membrane. Hydrogen ion interferes with measurements of very low levels of calcium. Hydroxide ion will complex calcium ions.
Electrode Life
The calcium electrode will last six months in normal laboratory use. On-line measurement might shorten operational lifetime to several months. In time, the response time will increase and the calibration slope will decrease to the point calibration is difficult and electrode replacement is required.
13
Calcium Electrode Instruction Manual
Electrode Storage
The calcium electrodes may be stored for short periods of time in 1.0X10-2M calcium standard. For longer storage (longer than two weeks), rinse and dry the calcium membrane and cover the tip with any protective cap shipped with the electrodes. 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
Eutech Calcium Electrode consists of an electrode body containing an ion exchanger in a sensing module. This sensing module contains a liquid internal filling solution in contact with a gelled organophilic membrane containing a calcium selective ion exchanger.
An electrode potential develops across the membrane when the membrane is in contact with a calcium solution. Measurement of this potential against a constant reference potential with a digital pH/mV meter or with a specific ion meter depends on the level of free calcium ion in solution. The level of calcium 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 calcium ions in solution
The activity, X, represents the effective concentration of the ions in solution. Total calcium concentration, Ct, includes free calcium ions, Cf, plus bound or complexed calcium ions, Cb. Since the calcium electrodes only respond to free ion, the free ion concentration is:
Cf = Ct - Cb
The activity is related to the free ion concentration, Cf, by the activity coefficient, γ , by:
X = γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½ Σ CxZ
2
x
where: Cx = concentration of ion X Zx = charge on ion X Σ = sum of all of 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 (ISA) is added to samples and standards. The recommended ISA for calcium is potassium chloride, KCl.
14
Instruction Manual Calcium Electrode
Solutions other than this may be used as ISA's as long as ions that they contain do not interfere with the electrode's response to calcium ions.
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 into 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 equitransferent. No junction potential can result if the rate at which positive and negative charge carried into the sample is equal.
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through checking each of the system components in turn: the meter, the glass-ware, the electrodes, the standards & reagents, the sample, and the technique.
Meter
The meter may be checked by following the check-out procedure in the instrument instruction manual.
Glass-ware
Clean glass-ware will drain clean. That is, when rinsed with distilled or deionized water, the water does not bead on the inside walls of the glass-ware.
Electrodes
The electrodes 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 section Measuring Hints. Repeat the slope check.
3. If the electrodes still fail to respond as expected, substitute another calcium 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 standards and/or reagents 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.)
15
Calcium Electrode Instruction Manual
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 of the solution according to the method being used for the analysis.
- 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 of standards, poor quality distilled/deionized water, or a simple mathematical miscalculation.
Sample
Look for possible interferences, complexing agents, or substances which could affect the response or physically damage the sensing electrode (or the 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 electrodes' 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 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.
16
Instruction Manual Calcium Electrode
TROUBLESHOOTING HINTS
Symptom Possible Causes Next Step Out of Range defective meter check meter with shorting strap Reading (see meter instruction manual)
defective electrode check electrode operation
electrodes not unplug electrodes and reset plugged in properly
reference electrode be sure reference electrode is filled not filled
air bubbles on remove bubbles by membrane re-dipping electrode
electrodes not put electrodes in solution in solution
Noisy or Unstable defective meter check meter with Readings (readings shorting strap continuously or randomly changing air bubble on remove bubble by membrane re-dipping electrode
meter or stirrer ground meter or stirrer not grounded
outer filling solution fill electrode to level just too low below fill hole
defective electrode replace electrode
electrode exposed to soak electrode in calcium standard interferences
"Incorrect incorrect scaling plot millivolts on the linear Answer" of semi-log paper axis. On the log axis, be sure (but calibration concentration numbers within curve is good) each decade are increasing 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:
1.0x10-3M = 40 ppm Ca+2 = 100 ppm as CaCO3 sample carryover rinse electrodes thoroughly
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Calcium Electrode Instruction Manual
between samples
Drift (reading samples and standards allow solutions to come to room slowly changing at different temperature before measurement in one direction) temperatures
electrode exposed soak electrode in to interferences calcium standard
incorrect reference use recommended filling solution filling solution
incorrect pH adjust to pH 3-10 with NaOH or HCl
Low Slope or standards contaminated prepare fresh standards No Slope or incorrectly made
defective electrode check electrode operation
air bubble on membrane remove bubble by re-dipping probe
electrode exposed to soak electrode in interferences calcium standard
standard used as ISA use ISA
ISA not used use ISA
18
Instruction Manual Calcium Electrode
SPECIFICATIONS
Concentration Range: 1M to 5x10-6M
pH Range: 3 to 10
Temperature Range: 0o to 40 oC
Resistance: 100 Mohms
Reproducibility: ±4%
Samples: Aqueous solutions only; no organic solvents
Size: 110 mm length; 12 mm diameter; 1 m cable length
Storage: Store in dilute calcium standard
ORDERING INFORMATION
CODE NO. DESCRIPTION
EC-CAL-03 Calcium Electrode, epoxy body combination
EC-SCS-CA1-BT Calcium Standard, 0.1M CaCl2
EC-SCS-CA2-BT Calcium Standard, 1,000 ppm Ca
+2
EC-SCS-CA3-BT Calcium Standard, 100 ppm Ca+2 as CaCO3
EC-ISA-CA1-BT Calcium ISA (Ionic Strength Adjuster), 4 M KCl
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