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Generally, the slope is ~59 mV at 25°C. Slope is determined during calibration (see Calibrating the electrode).
The function F, when plotted as a function of the volume of acid added (v), is linear when sufficiently removed from the equivalence point. We measure mV instead of pH to determine the endpoint because this method offers better precision. The optimum range of millivolts for linearity is 220–240 mV. The value of v at F = 0 is the equivalence point from which the alkalinity is evaluated.
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- 3 M KCl solution (223 224 g KCl in 1 L reagent water)
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- 100 mM alkalinity (pipet 10 mL 0.5 M Na2CO3 into 90 mL 0.7 M KCl)
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Before an electrode can be used, it must be calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range.
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- Make sure the water bath temperature is set to 25°C. Ensure no air bubbles are present in the acid dispensing line. Press DOS on the body of the titrator to push acid through the line to remove potential air bubbles. Select Calibrate Electrodes from the main Alkalinity interface Main instrument panel.
- Enter your range of buffers (4, 7, 10).
- Select your Drift Span. The default drift span is 30.
- Place 3 mL of the first buffer solution in the vessel. Add stir bar. Remove the electrode from the storage solution, rinse with DI water, and blot dry with a Kimwipe. Do not rub the electrode, as this can cause a static charge. Insert the electrode tip into the titration vessel (not touching the bottom of the cup or the stir bar). Confirm that the frit is in the solution.
- Select Cal 1 and then Start. Measure until the drift gets close to 0.0. Usually approximately 500 seconds will be adequate. Select Stop when satisfied with measurement.
- When finished, clean vessel and the electrode.
- Repeat steps 4–6 with each calibration buffer, selecting Cal2 and Cal3 when appropriate.
- When all three buffers have been run, the slope value of the regression curve should be close to –59 pH/mV. Select OK-Save to save the calibration.
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- Set your Stability Criteria for each step of the program: Measurement continues until Stability Criteria (mV/s) is satisfied.
- Select your Increment for each mV level (initial to 150, 150 to 220 and 220 to 240): How much acid is added in each increment.
- Set the Time Out for each step of the program: Seconds until rate program times out if Stability Criteria is not satisfied.
- Save To File.
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Calculating the standard ratio correction (estimated vs. actual alkalinity) for the anticipated range of alkalinity values accounts for measurement error in acid strength. Standard ratio correction can be calculated using borax solution, sodium bicarbonate solution or IAPSO standard seawater, as necessary, to most closely match alkalinity values (within 5 mM, to preserve the first-order transfer function) of the unknown samples. Generally, IAPSO standard seawater is used to establish this ratio, and additional calibration standards are used if samples deviate >5 mM from the alkalinity of IAPSO (~2.325 mM). It is good practice to have IAPSO, 20 mM and 40 mM standard ratio corrections calculated before arriving at the first site. This prepares you for alkalinities up to 40 mM.
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Make sure to select a correct dispensing rate program for the standard in question. You can access the rates by selecting Edit Rates from the main alkalinity interface Main instrument panel.
To start creating a standard ratio correction, select STANDARDS from the main alkalinity interface.
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The system is now calibrated, dispensing rates selected (generally start with the slowest rate, assuming that alkalinity will be around the value of IAPSO) and standard ratio correction selected (again, generally start with the IAPSO standard ratio and adjust according to what is measured in the samples).
Select SAMPLE from the main alkalinity interface Main instrument panel.
Select the sample (IWS) from the LIMS database. Use the tree or type in the Text_ID in the appropriate field. If you type in a Filter Code IWS, the software will only bring up the IWS sample, which can be handy if your IW has a lot of children.
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Place 3 mL of the sample in the vessel. Add stir bar. Remove the electrode from the storage solution, rinse with DI water , and blot dry with a Kimwipe. Do not rub the electrode, as this can cause a static charge. Insert the electrode tip into the titration vessel (not touching the bottom of the cup or stir bar). Confirm that the frit is in the solution.
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Click Ok/Save to save the results to the LIMS database. The value saved to LIMS and the value you need to note in the blue book is the “Alkalinity Cor” value. This is your result with the standard ratio correction applied to it.