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Apparatus, supplies and reagents
Laboratory apparatus
Figure 1. Metrohm Autotitrator (Note: dispenses 0.1 M HCl).
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Figure 4. Selecting a dispensing rate. A new dispensing rate program can also be created.
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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 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.
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To select a saved standard ratio correction double-click it.
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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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Sample preparation and analysis
Sample
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storage and
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preservation
Use 3 ml of interstitial water sample (usually pipetted from the IWS vial). Store residual sample in a 5 ml snap cap tube for additional shipboard analysis.
Sample
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preparation and
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analysis
The laboratory technician receives a whole-round sample at the catwalk to squeeze for interstitial water, which is passed through a 0.45 µm filter before analyzing pH and alkalinity. Note that RHIZON samples will produce incorrect pH and alkalinity values and should not be analyzed.
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- Pipette a 3 mL sample into the titration vessel and add stir bar.
- Take an initial pH reading.
- Titrate to determine total equivalent alkalinity value.
- Upload data to LIMS.
- Store titrated sample in a sealed polytube with a new label indicating volume and concentration of acid added.
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- cryo vial.
Entering sample information
The system should now be calibrated and dispensing rate as well as the standard ratio correction selected. Generally, start with the slowest dispensing rate, assuming the alkalinity will be around the value of IAPSO. Same with the standard ratio correction, start with the IAPSO standard ratio correction and adjust according to what is measured in the samples.
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.
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Select the sample (IWS) from LIMS tree. Alternatively, type or scan in a Text_ID. If you use 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.
Place 3 mL 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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After completing all the steps in the "We are watching you!" guide box, click GO. The pH measurement will commence.
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Insert acid dispenser probe when prompted. Click GO.
Measuring
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alkalinity
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.
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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.
Figure 17. Analysis complete.
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- Remove electrode from cup.
- Rinse electrode with DI water in squeeze bottle.
- Blot the electrode dry with a Kimwipe. Do not rub the electrode, which could cause static charge buildup.
- Place the electrode in a storage container containing 3 M KCl.
- Rinse and dry the acid dispensing tip and stir bar.
- Pipette or pour the titrated alkalinity IW sample into a container to ship to repository or scientist.
- Write on label how much HCl was added during titration.
- Rinse the titration vessel cup with DI water and dry.
Data
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handling
Editing Gran
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factor points
Outlier Gran factor points (Figure 17) can be deleted from the linear portion of the curve to improve the accuracy of sample results as follows.
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- When the titration is complete, the Alkalinity Calculation screen opens. Unlock the Gran factor axis
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- Zoom in on the Gran factor points.
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- Select the cursor and move to the point to be deleted
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- Click Delete Data and then click Ok - Save when finished.
Figure 1719. Editing Gran Factor Points (blue = Gran factor F, red = optimum mV range for linearity: 220–240 mV)factor points.
Data
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reports
Data reports are not available at the instrument, but data can be viewed on the by selecting View Datalog tab (Figure 11 20) from the Main instrument panel. Also, the software appends the alkalinity results to the DAT file C:\ALKALINITYProgramData\LOGIODP\ALKALINITYSAMPLES.DATAlkalinity. 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.
Figure 1120. Datalog.
Figure 12. Viewing alkalinity data in LIMS.
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Quality assurance/quality control
Overview
A quality assurance/quality control (QA/QC) program ensures that a measurement system is performing within control limits and therefore provides high-quality data. The QA/QC program for this system includes instrument calibration, calibration verification, and accuracy and precision monitoring.
Instrument
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calibration
The instrument is calibrated by the onboard laboratory specialist at the beginning of the expedition. Calibration is verified routinely during operation.
Analytical
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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).
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.
Calibration/
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calibration verification
– | pH buffers are calibrated at the beginning of the cruise |
– | Borax/IAPSO standard correction factors are calibrated at the beginning of the cruise |
– | Recalibration is done if the precision or accuracy is greater than ±5% |
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Precision is the degree to which further measurements will show the same or similar results.
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– | Duplicates are not run on samples because that would require using 6–10 mL of the interstitial water, which is usually too large a sample amount to justify. Instead, duplicate calibration verification samples (e.g., duplicate IAPSO standard measurements) are compared to calculate precision. Select a standard close to the alkalinity value of the IW samples being analyzed if possible. |
– | Precision is measured with every batch of 10 samples. |
– | Precision control limit allows a difference of ±5%. |
– | If the precision control limit is exceeded, the system must be recalibrated and all samples run since the previous in-control precision measurement must be repeated. |
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Accuracy is the degree of closeness of a measured value to the actual (true) value.
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– | Standards are run with every batch of 10 samples. Select a standard close to the alkalinity value of the IW samples being analyzed if possible. |
– | Accuracy control limit allows a difference of ±5% from true standard value. |
– | If the accuracy control limit is exceeded, the system must be recalibrated and all samples run since the previous in-control accuracy measurement must be repeated. |
Limits of
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detection and
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quantitation
– | 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 ml. 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. |
Software
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dataflow
Two primary data types are generated by this system: pH and alkalinity. The alkalinity measurement depends on a series of pH measurements, thus the pH value of the sample is determined before the alkalinity titration begins.
pH
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dataflow
Calibration
User-configurable variables include the following (Figure 1221). Refer to Drift Span drift span for more information about these values:
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$mVthreshold: slope threshold; maximum calculated value for the reading to be considered stable; also “drift tolerancestability criteria.”
Figure 1221. Alkalinity Electrode Calibration Data Flowelectrode calibration dataflow.
Alkalinity
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dataflow
User-defined variables (Figure 13), with values from the example given in the Setting the Dispensing Raterate 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 (30 µL15 µl)
$Rate2: rate for second mV threshold (4 µLµl)
$Rate3: rate for third mV threshold (1 µL3 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 drift tolerancestability criteria)
Figure 1322. Alkalinity Measurement Data Flowmeasurement data flow.
LIMS
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integration
In LIMS terminology, we perform two types of alkalinity analyses: ALKALINITY and ALK_QAQC:
ALKALINITY analyses include pH and alkalinity tests performed on interstitial water samples.
ALK_QAQC includes the same physical tests performed on a calibration standard or standard sample.
ALK_QAQC analysis has three different variations:
CALIBRATE: tests done to calibrate the autotitrator
STANDARD: standard analyses
REPLICATE: duplicate sample analyzed to check precision and/or accuracy
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Analysis | Component | Unit | Description |
ALKALINITY | alkalinity | mM | Ability of a solution to neutralize acid to the equivalence point of carbonate |
pH | NA | Acidity or basicity of a solution (–log[H+]) | |
acid_quantity | µL | Amount of 0.1 M HCl added to the sample during titration | |
correction_factor | — | Correction factor for non-ideal behavior of samples to adjust calibration | |
ALK_QAQC | alkalinity | mM | Ability of a solution to neutralize acid to the equivalence point of carbonate |
pH | NA | Acidity or basicity of a solution (–log[H+]) | |
acid_quantity | µL | Amount of 0.1 M HCl added to the sample during titration | |
correction_factor | — | Correction factor for non-ideal behavior of samples to adjust calibration | |
slope | — | Slope of the calibration equation | |
intercept | — | Intercept value of the calibration equation | |
corr | Rho | Correlation coefficient of the calibration |
Uploading
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data to LIMS
When the alkalinity titration finishes, the GRAN-METHOD screen shows the slope information, correction factors, and final alkalinity value (Figure 1418).
Edit outlier Gran factor points, if necessary (see Editing Gran Factor Points). If satisfied with the results, click Ok/Save to load the values into LIMS.
Figure 14. Load Data to LIMS.
Health,
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safety and environment
Safety
Wear personal protective equipment including close-toed shoes, lab coat, gloves and safety glasses when working with acids of any strength.
Use a fume hood when making solutions from concentrated acids.
Pollution
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prevention
Make reagent and standard solutions in quantities no larger than needed to complete sample analysis.
Waste
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management
Neutralize acid solutions before disposal.
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