User Guide: Autotitrator pH/Alkalinity
Manual information
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User Guide contents
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Introduction
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Generally, the slope is ~59 mV at 25°C. Slope is determined during calibration.
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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- 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 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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Selecting a dispensing rate
Select Edit Rates from the Main instrument panel.
To select a dispensing rate, double-click on the desired rate on the Rate List. Click Done.
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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 instrument panel.
Before any measurement, press the red STOP/FILL button on the titrator itself (Figure 1). This will fill the syringe pump and ensure you will not run out of acid during the titration.
To start creating a standard ratio correction, select STANDARDS from the Main instrument panel and enter the information for the standard in question.
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Select STND Manager from the Main instrument panel.
Figure 7. Averaging standard measurements.
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- Select the three measurements you want to average and click Average.
- 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).
To select a standard ratio correction for subsequent measurements go to Setup in the Main alkalinity interface.
Figure 9. Selecting the standard ratio correction file to use.
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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 stability criteria specified in the dispensing rate program. Stability criteria acts as follows:
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Before any measurement, press the red STOP/FILL button on the titrator itself (Figure 1). This will fill the syringe pump and ensure you will not run out of acid during the titration.
Select SAMPLE from the Main instrument panel.
Figure 11. Entering sample information.
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The alkalinity titration is automatic once you have inserted the acid dispenser probe and clicked GO. The plot on the left side displays the signal coming directly from the electrode in real time. The y-axis is the mV reading, and the x-axis is time in seconds. The readings will continue until the stability criteria 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 ml. The mV readings come from the final value reached upon satisfying the stability criteria.
Figure 16. Measuring Alkalinity.
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The alkalinity titration will continue through the three stages of the dispensing rate program. The software will let you know once the analysis is complete.
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The Gran-Method window appears with the results of the titration. Write the pH and the Alkalinity Cor value in the blue book. Alkalinity Cor is the alkalinity with the standard ratio correction applied to it. Confirm that your MUT uploader is active. Click Ok/Save. This will upload the result to LIMS.
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Data reports are not available at the instrument, but data can be viewed by selecting View Datalog (Figure 20) from the Main instrument panel. Also, the software appends the alkalinity results to the DAT file C:\ProgramData\IODP\Alkalinity. The best way to view the alkalinity values is via LIMS Reports. It is also highly recommended to record the alkalinity and pH values in the blue laboratory notebook to protect against inadvertent data loss.
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Instrument calibration
The instrument is calibrated by the onboard laboratory specialist at the beginning of the expedition. Calibration is verified routinely during operation.
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An analytical batch is a group of samples run together in one sequence, sharing a calibration curve, blanks, reference materials, and verification samples. The alkalinity batch size is 10 samples. Each batch of 10 unknown samples contains a sample to verify precision and a sample to verify accuracy.
Blanks
Blanks are not run for this method because DI water has no buffering capacity and would therefore fail the slope program. Thus, blanks are not applicable to this chemistry.
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User-configurable variables include the following (Figure 21). Refer to drift span for more information about these values:
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User-defined variables (Figure 13), with values from the example given in the Dispensing rate section:
$MV1: rate 1 mV threshold (150 mV)
$MV2: rate 2 mV threshold (220 mV)
$MV3: rate 3 mV threshold (240 mV)
$Rate1: rate for first mV threshold (15 µl)
$Rate2: rate for second mV threshold (4 µl)
$Rate3: rate for third mV threshold (3 µl)
$SlopeSpan: number of samples used to calculate the slope (default = 30)
$StabilityThreshold: maximum slope value to ensure a stable reading (also referred to as stability criteria)
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When the alkalinity titration finishes, the GRAN-METHOD window shows the slope information, correction factors, and the final alkalinity value (Figure 18).
Edit outlier Gran factor points, if necessary. When satisfied with the results, click Ok/Save to load the values into LIMS.
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