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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– ¿ =HCO₃^–
H⁺ + HCO₃^– ¿ = 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:
F = (v + V₀) × 10^E/A,
where:
F = Gran factor,
v = volume of acid added to the solution in the titration vessel,
V₀ = original volume of the sample,
E = EMF (millivolts) at v, and
A = slope of electrode determined on the basis of the electrode calibration.
Generally, the slope is ~59 mV at 25°C. Slope is determined during calibration (see Calibrating the Electrode).
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.
The slope of the F vs. v plot changes with variations in the sulfate content of the samples. This is because at lower pH values the following reaction
H⁺ + SO₄^2– = HSO₄^–
plays an important role in establishing the pH of the solution through a buffering effect. This change in slope, however, has no effect on the Gran extrapolation intercept with the y-axis and is not accurate enough to estimate sulfate concentrations.

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Laboratory Reagents

Reagent Solutions

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