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Figure 1. Metrohm Autotitrator (Note: dispenses 0.1 M HCl).

Laboratory apparatus

Hardware and software

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Equipment

• Metrohm 794 Basic Titrino autotitrator
• Metrohm 728 Stirrer
• Haake P5 water bath
• Combination electrode (Metrohm, combined pH glass electrode, model 6.0234.100)
• LabVIEW Alkalinity program v6.
• Laboratory oven
• Desiccator
• Water bath
• Analytical balance system

Laboratory reagents

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Reagents

• 0.1 M HCl solution (premade from Fisher, AMS# CH5009)
  • Used as titrant. Fills titration reservoir.
• 3 M KCl solution (224 g KCl in 1 l reagent water)
  • Electrode solution
• NBS buffers: commercially obtained low ionic strength solutions at pH 4.00, 700, and 10.00 (stable indefinitely; store in the chem lab refrigerator when not in use)
  

Reagent solutions

• 0.1 M HCl solution (premade from Fisher, AMS# CH5009)
• 3 M KCl solution (224 g KCl in 1 l reagent water)
• 0.7 M KCl solution (26.1 g KCl in 500 ml reagent water)
  • Used to calibrate the electrode

Standards

• IAPSO standard seawater (alkalinity ~2.325 mM)
• Higher alkalinty standards:
  • Stock standard solutions (1 l)
    • 0.5 M NaHCO₃ (42 g sodium bicarbonate in 1 l reagent water)
    • 0.5 M Na₂CO₃ (53.0 g sodium carbonate in 1 l reagent water)
    • 0.1 M Na₂CO₃ (10.6 g sodium carbonate in 1 l reagent water)
  • Standard solutions (100 ml)
    • 100 mM alkalinity (pipet 10 ml 0.5 M Na₂CO₃ into 90 ml 0.7 M KCl)
    • 50 mM alkalinity (pipet 10 ml 0.5 M NaHCO₃ into 90 ml 0.7 M KCl)
    • 40 mM alkalinity (pipet 20 ml 0.1 M Na₂CO₃ into 80 ml 0.7 M KCl)
    • 20 mM alkalinity (pipet 10 ml 0.1 M Na₂CO₃ into 90 ml 0.7 M KCl)

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Instrument Setup and Calibration

Figure 2. Main instrument panel.

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Calibrating the electrode

Before an electrode can be used, 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. Make sure the gray side-hole plug for the electrode is unplugged before starting a measurement.

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1. 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.
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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Dispensing rate

The rate at which the titrator dispenses 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.

Selecting a dispensing rate

Select Edit Rates from the Main instrument panel.

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Figure 4. Selecting a dispensing rate.

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Standard ratio

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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.

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-         40 mM standard = 38–42 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.

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1. Place 3 ml of standard in vessel. Add stir bar and turn stirrer on
2. Rinse and dry the electrode and then immerse the electrode in the vessel. Confirm that the frit is in the solution. Check that your Drift Span is 30.
3. Select Continue.  
4. Click START.
5. Insert the acid dispensing probe when prompted.
6. When finished, clean vessel and electrode. Repeat steps 1–4 until you have at least three consistent measurements per standard.
   
   

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

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Drift span

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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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If the difference between the first and the last measurement is larger than the stability criteria, measurement will continue at that increment until the stability criteria is satisfied.

Image Removed
Figure 10. Drift Span.

Sample preparation and analysis

Sample storage and 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.

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Figure 10. Drift Span.

Instrument Operation

Sample preparation and 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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When finished, transfer the residue into a labeled 5 ml cryo vial. Clean the vessel, the stir bar, the electrode and the acid dispensing probe.

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Instrument Clean Up

1. Remove electrode from cup.
2. Rinse electrode with DI water in squeeze bottle.
3. Blot the electrode dry with a Kimwipe. Do not rub the electrode, which could cause static charge buildup.
4. Place the electrode in a storage container containing 3 M KCl.
5. Rinse and dry the acid dispensing tip and stir bar.
6. Pipette or pour the titrated alkalinity IW sample into a container to ship to repository or scientist.
7. Write on label how much HCl was added during titration.
8. Rinse the titration vessel cup with DI water and dry.

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Data Handling and Upload

Editing Gran factor points

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A text file will be created and placed in the MUT upload directory with the sample data. MUT uploader then uploads the data to LIMS.

Data reports

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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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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Recalibration is performed when precision or accuracy is not within ±5%.

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Precision

Precision

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Precision is the degree to which further measurements will show the same or similar results.

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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

Accuracy is the degree of closeness of a measured value to the actual (true) value.

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Alkalinity dataflow

User-defined variables (Figure 13), with values from the example given in the Dispensing rate section:

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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.

Health, safety and environment

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