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1. | On the Alkalinity/Standard tab, ensure dispensing rate, drift span, and standard ratio are set correctly (Figure 7). |
2. | Ensure the water bath temperature is set to 25°C and pipette sample in titration vessel. |
3. | After rinsing the electrode with DI water and gently blotting dry with a Kimwipe, insert the electrode tip into sample (not touching the bottom of the cup). |
4. | Click SAMPLE. |
5. | Select measurement type from the list, enter operator name (this must be the same as LIMS user ID), text_ID, and sample volume (Figure 8). Click OK. |
6. | After taking the initial mV and pH readings, the software prompts the user to insert the acid dispensing tip and click GO (Figure 9). If you make a mistake, click ABORT and no acid will be added to the sample. After clicking GO, the sample will be titrated to completion. |
7. | Click Done/Save to upload data (see LIMS Integration). |
Figure 815. Measuring Alkalinity. The plot on the left side is the signal coming directly from the electrode in real time.
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The mV readings come from the final value reached upon satisfying the stability criteria.
Figure 16. Analysis complete.
Cleaning up after Measurement
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Outlier Gran factor points (Figure 1017) can be deleted from the linear portion of the curve to improve the accuracy of sample results as follows.
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1. | When the titration is complete, the Alkalinity Calculation screen opens. Unlock the Gran factor axis. |
2. | Zoom in on the Gran factor points. |
3. | Select the cursor and move to the point to be deleted. |
4. | Click Delete Data Point and then click Done/Save Ok - when finished. |
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Figure
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17. Gran Factor Points (blue = Gran factor F, red = optimum mV range for linearity: 220–240 mV).
Data Reports
Data reports are not available at the instrument, but data can be viewed on the View Datalog tab (Figure 11). Also, the software appends the alkalinity results to the DAT file C:\ALKALINITY\LOG\ALKALINITYSAMPLES.DAT. The best way to view the alkalinity values to be used in graphs and reports 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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The instrument is calibrated by the onboard laboratory specialist at the beginning of the expedition. Calibration is verified routinely during operation. (See Instrument Calibration. )
Analytical Batch
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 (see Precision and Accuracy).
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– | Samples are not reported as less than the detection limit for alkalinity. The only way a sample could not be analyzed is if its initial pH (before acid addition) is <4.2, which is rare. |
– | Results are reported to three decimal places. |
– | The titration uncertainty is ±0.003 mLml. When carried through the alkalinity calculation, this uncertainty results in the alkalinity difference being 50 µM, though it is also dependent on the starting pH. |
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Two primary data types are generated by this system: pH and alkalinity. The alkalinity measurement depends on a series of pH measurements, so sample pH thus the pH value of the sample is determined before the alkalinity titration begins.
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