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Alkalinity is the measure of how much acid it takes to lower the pH of a water sample enough to convert all bicarbonate (HCO₃^–) and carbonate (CO32–CO₃^2–) to carbonic acid (H2CO3H₂CO₃). Although total alkalinity is equal to the stoichiometric sum of all bases in solution, not just carbonates, ~97% of alkalinity in typical seawater is due to carbonates.

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To measure alkalinity, a pore water sample is titrated with an acid to an endpoint at which carbonate is converted to bicarbonate and bicarbonate is converted to carbonic acid. In seawater, this endpoint occurs at about pH = 4.2.

H⁺ + CO₃^2– à xHCO = HCO₃^–

H⁺ + HCO₃^– à xH = H₂CO₃

The alkalinity determination in this method (Gran titration) relies on a mathematical evaluation of the second equivalence point of carbonate titration in seawater using the most stable part of the titration curve (i.e., the part beyond the equivalence point on the low pH side). In essence, the Gran method linearizes the titration curve by means of a simple function:

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v             =             volume of acid added to the solution in the titration vessel,

V0                       V₀           =             original volume of the sample,

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

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

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

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

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

Main instrument panel

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Before the titrator can be used to measure samples, the electrode must be calibrated against pH buffers in the same range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range.

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1. Select Calibrate Electrodes from the main Alkalinity interface.
2. Enter your range of buffers (4, 7, 10).
3. Select your Drift Span. 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 measurement is compared to the drift tolerance specified in the rate program.
4. Place 3 mL of the first buffer solution in the vessel. Add stir bar and immerse electrode in vessel.
5. Select Cal 1 and then Start.
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, select OK-Save to save the calibration.
   

   Dispensing rate

   The rate at which the titrator dispenses the 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.

   Select Edit Rates from the main alkalinity interface.

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   Standard ratio

   Calculating the standard ratio (estimated vs. actual alkalinity, “fudge factor”) for the anticipated range of alkalinity values accounts for measurement error in acid strength. Standard ratio 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). The measurement is repeated until at least 3 consistent measurements are obtained within 5% of actual value for each standard:

   -         IAPSO = 2.21–2.44 mM

   -         20 mM standard = 19–21 mM

   -         40 mM standard = 38–42 mM

   To start creating a standard ratio, select STANDARDS from the main alkalinity interface.

   
   

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1. Place 3 mL of standard in vessel. Add stir bar and immerse electrode in vessel.
2. Select Continue.  
3. Click START.
4. Insert the acid dispensing probe when prompted.
5. When finished, clean vessel and electrode.

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1. Select the three measurements (or however many were done in the prior step) and click Average.
2. The window to the right shows the next step to which you can save the new ratio to a new name or replace a prior name. Usually we save as a new ratio (e.g. 371_13august). The window showing the name creation is not shown.
3. Click Done.
   

   To select a standard ratio for subsequence subsequent measurements go to Setup in the main alkalinity interface.

   
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This window also shows where to set the path to the datalog file, the default setting for the Drift Span, and where to select the standard ratio correction.

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Select the sample (IWS) from the LIMS database. Use the tree or type in the Text_ID in the appropriate field. There’s currently a minor ‘glich’ in the tree method if you don’t use the filter = “IWS.” So without explaining the entire issue, just remember to put “IWS” in the filter field.

1. Place 3 mL of standard in vessel. Add stir bar and immerse electrode in vessel.
2. Select Continue.
   
3. Click START.
4. The first measurement that the system measures, is sample pH. Record the value in the blue book (just in case…).
5. Insert the acid dispensing probe when prompted.

6. When the titration is complete, a window appears showing the results.
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   When finished, clean vessel and the electrode.

   Click Done/Save to save the results to the LIMS database. The value saved to LIMS , and the value you need to note in the blue book (just for completeness, in case…) is the “Corrected Alkalinity” value. This is the value with the standard ratio applied to it.