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Before the titrator can be used to measure samples, the electrode must be calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range. The electrode is calibrated at the beginning of each expedition.
Figure 2. Electrode Calibration.
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- Make sure the water bath temperature is set to 25°C. Select Calibrate Electrodes from the main Alkalinity interface.
- 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 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.
Dispensing rate
The rate at which the titrator dispenses the acid into the sample can be adjusted according to the expected alkalinity value. Higher alkalinities may require faster dispensing rates. The dispensing rate can be selected from a list of predetermined programs, or a new dispensing rate program can be created.
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Sample Storage and Preservation
Use 3 mL ml of interstitial water sample. Store residual sample in a 5 mL ml snap cap tube for additional shipboard analysis.
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The readings will continue until the drift tolerance is satisfied, which provides a final mV reading.
The green trend on the plot right side of the figure is the trend of mV readings vs. acid additions in mLml.
The mV readings come from the final value reached in the red arc after upon satisfying the drift tolerancestability criteria.
Cleaning up after Measurement
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