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

Supplies

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Pipettes, Class A: 0.5, 2, 3, 5, 8, 10, and 20 ml

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Adjustable pipettor, 100–5000 µl

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Volumetric flasks, Class A: 100 ml, 1 l

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Combination electrode (Metrohm, combined pH glass electrode, model 6.0234.100)

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pH paper, to bracket pH 7 (pH 1–12 paper)

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5 ml snap-cap vials

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Magnetic stir bars

Laboratory reagents

• IAPSO standard seawater (alkalinity ~2.325 mM)
• Potassium chloride (KCl)
• Sodium bicarbonate (NaHCO₃)
• Sodium carbonate (Na₂CO₃)
• 18.2 MΩ millipore reagent water
• 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)

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

Laboratory reagents

Dry reagents

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Hydrochloric acid (HCl), ultrapure

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Potassium chloride (KCl), certified ACS

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Borax (Na₂B₄O₇·10H₂O)

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Sodium bicarbonate (NaHCO₃), certified ACS

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Sodium carbonate (Na₂CO₃), certified ACS

Reagent solutions

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0.1 N HCl solution (premade from Fisher, AMS# CH5009)

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• in 1 l reagent water)

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• 1 M Na₂CO₃ (10.6 g sodium carbonate in 1 l reagent water)

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Standard solutions (

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0.1 M borax solution (38.1 g borax in 1 l reagent water)

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0.5 M NaHCO₃ (42 g sodium bicarbonate in 1 l reagent water)

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0.1 M Na₂CO₃ (10.6 g sodium carbonate in 1 l reagent water)

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0.5 M Na₂CO₃ (53 g sodium carbonate in 1 l reagent water)

Standard solutions (100 ml)

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5 mM Na₂CO₃ alkalinity (pipet 2.5 ml 0.1 M Na₂CO₃ into 97.5 ml 0.7 M KCl)

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20 mM Na₂CO₃ alkalinity (pipet 10 ml 0.1 M Na₂CO₃ into 90 ml 0.7 M KCl)

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40 mM Na₂CO₃ alkalinity (pipet 20 ml 0.1 M Na₂CO₃ into 80 ml 0.7 M KCl)

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50 mM NaHCO₃ alkalinity (pipet 10 ml 0.5 M NaHCO₃ into 90 ml 0.7 M KCl)

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100 mM Na2CO₃ alkalinity (pipet 10 ml 0.5 M Na2CO₃ into 90 ml 0.7 M KCl)

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IAPSO standard seawater (alkalinity ~2.325 mM)

Reagent water

18.2 MΩ millipore deionized water

Buffer solutions

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).
Tris buffer: tris(hydroxymethyl)amino methane or 2-amino-2-(hydroxymethyl)-1,3-propanediol
Bis buffer: bis(hydroxymethyl)methylamino methane or 2-amino-2-methyl-1,3-propanediol (not commercially available because of its highly hygroscopic nature; prepare as follows):

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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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Main Main

Main instrument panel

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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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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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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).
   
   

   
   

   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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Once the pH measurement is complete, you will be reminded about insert the acid dispenser probe when prompted. Click GO. The alkalinity measurement will commence.

Figure 15. Acid dispenser reminder prior to starting the alkalinity measurement.

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measurement.

Measuring 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.

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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1. When the titration is complete, the Calculations window opens.
2. Zoom in on the Gran factor points.
3. Select the point to be deleted.
4. Click Delete Data. This will only delete the selected point. 

Image Added
Figure 19. Editing Gran factor points.

Uploading data to LIMS

Data is uploaded to LIMS using MUT uploader.

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Make sure MUT uploader is open, set to the correct expedition, and set to automatic upload.

Click OK - Save in the Calculations window.

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

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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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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.

Calibration/calibration verification

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The electrode is calibrated against pH buffers at the beginning of each expedition.

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IAPSO/Na₂CO₃ standard ratio corrections at generated at the beginning of each expedition.

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

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Precision Precision

Precision

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.

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Precision is measured with every batch of 10 samples.

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Precision control limit allows a difference of ±5%.

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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

Accuracy

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.

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Accuracy control limit allows a difference of ±5% from true standard value.

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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 detection and quantitation

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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.

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Results are reported to three decimal places.

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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 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.

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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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Figure 21. Alkalinity electrode calibration dataflow.

Alkalinity dataflow

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)

Figure 22. Alkalinity measurement data flow.

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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

LIMS stores several components of each analysis, which are detailed in the chart below.

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Uploading data to LIMS

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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Maintenance/Troubleshooting

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Make sure the probe storage container is filled with 3 M KCl solution and the reservoir is filled with 0.1 M HCl solution.

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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 and remove air bubbles.

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Change out the Drierite trap when ~50% of the color turns from blue to pink.

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In case of a bubbling noise, top up your temperature-controlled water bath. Use tap water. Check titration vessel for air space and eliminate if present.

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Black AgS₂ may precipitate in the diaphragm of the electrode from sulfide containing samples. You will most likely notice a decline in the electrode's performance, and the diaphragm that has turned black. Treat the diaphragm with freshly prepaired 7% Thiourrea solution in 0.1 mol/l HCl.

Consumables

Electrode: Metrohm 6.0234.100 or equivalent.

0.1 M HCl, Fisher CH5009

Microvalve buret tip, Metrohm 020683244, CM5129

Archive Version

LMUG-pH_Alkilinity Autotitrator_UG-230220-1718-112.pdf

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