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Main instrument panel

Figure 12. Main instrument panel.

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To ensure accuracy of sample measurements, the titrator settings must be optimized. Calibration of the system includes calibrating the electrode, selecting a dispensing rate program and calculating a standard ratio.

Calibrating the electrode

Before the titrator an electrode can be used to measure samples, the electrode 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. The electrode is calibrated at the beginning of each expedition.

Figure 23. Electrode Calibration.

1. Make sure the water bath temperature is set to 25°C. Select Calibrate Electrodes from the main Alkalinity interface.
2. Enter your range of buffers (4, 7, 10).
3. Select your Drift Span. The default drift span is 30.
4. 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.
5. 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.
6. When finished, clean vessel and the electrode.
7. Repeat steps 4–6 with each calibration buffer, selecting Cal2 and Cal3 when appropriate.
8. 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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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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1. Set your Stability Criteria for each step of the program: Measurement continues until Stability Criteria (mV/s) is satisfied.
2. 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.
3. Set the Time Out for each step of the program: Seconds until rate program times out if Stability Criteria is not satisfied.
4. Save To File.

Figure 34. Selecting a titration dispensing rate. A new dispensing rate program can also be created.

Standard ratio correction

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.

The measurement is repeated until at least 3 consistent values are obtained within 5% of actual value for each standard:

-         IAPSO = 2.21–2.44 mM

-         20 mM standard = 19–21 mM

-         40 mM standard = 38–42 mM

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.

To start creating a standard ratio correction, select STANDARDS from the main alkalinity interface.

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Figure 5. Entering standard information.

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Figure 6. Measuring a standard for standard ratio correction.

1. Place 3 mL of standard in vessel. Add stir bar and immerse electrode in vessel. Confirm that the frit is in the solution.
2. Select Continue.  
3. Click START.
4. Insert the acid dispensing probe when prompted.
5. When finished, clean vessel and electrode. Repeat steps 1–4 until you have at least three consistent measurements per standard.
   
   Now go to the STND Manager, selected from the main alkalinity interface.
   
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Figure 7. Averaging standard measurements.

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Figure 8. Saving the averaged standard correction.

1. Select the three measurements you want to average and click Average.
2. The small window shows the next step in which you can save the new standard ratio correction or replace an old one. Usually we save as a new ratio (e.g. 371_13august). The window showing the name creation is not shown.
3. Click Done.
   
   

   To select a standard ratio correction for subsequent measurements go to Setup in the main alkalinity interface.

   Image AddedFigure 9. Selecting the standard ratio correction file to use. Double-click to select.
   
   

This window also shows the electrode calibration values, the path to the datalog file, the default setting for the Drift Span, and where to select the standard ratio correction.

To select a saved standard ratio correction, double-click it.

Drift Span

A drift span of 30 (default) indicates that a minimum of 30 measurements will be taken after each addition of titrant (acid). The difference between the first and last measurements is compared to the drift tolerance specified in the rate program (see Creating a New Dispensing Rate Program). Generally, drift tolerance starts at 0.10 mV/s. Drift tolerance acts as follows:

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