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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–) to carbonic acid (H2CO3). Although total alkalinity is equal to the stoichiometric sum of all bases in solution, not just carbonates, ~97% of alkalinity in seawater is due to carbonates.
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Note that this method should only be used on interstitial water (IW) squeezed from the core material by the titanium squeezers. The RHIZON samplers alter the pH and alkalinity of the IW samples and should not be analyzed for alkalinity or pH.
Method theory
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+ + CO32– = 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) × 10E/A
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where:
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F = Gran factor,
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v = volume of acid added to the solution in the titration vessel,
V0 = original volume of the sample,
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E = EMF (millivolts) at v, and
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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
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electrode)
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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+ + SO42– = 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.
Alkalinity System
Figure 1. Metrohm Autotitrator (Note: dispenses 0.1 M HCl).
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Before the titrator can be used to measure samples, the electrode must be calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range.
The electrode is calibrated at the beginning of each expedition.
Figure 2. Electrode Calibration.
- Make sure the water bath temperature is set to 25°C. Select Calibrate Electrodes from the main Alkalinity interface.
- Enter your range of buffers (4, 7, 10).
- Select your Drift Span. A The default 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 programis 30.
- Place 3 mL of the first buffer solution in the vessel. Add stir bar and immerse electrode in vessel. 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.
- 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.
- When finished, clean vessel and the electrode.When finished, clean vessel and the electrode.
- Repeat steps 4–6 with each calibration buffer, selecting Cal2 and Cal3 when appropriate.
- When all three buffers have been run, select the slope value of the regression curve should be close to –59 pH/mV. Select OK-Save to save the calibration.
Calibrating the Electrode
The pH electrode must be standardized for linearity across the pH range of the samples. Generally, calibration at pH 4, 7, and 10 covers the necessary range. The electrode is calibrated at the beginning of each expedition as follows (calibration dataflow is shown in pH Dataflow).
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1.
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Select the Calibration tab from the main menu of the Alkalinity program (Figure 2).
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2.
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In the Buffers fields, enter pH of at least 2 buffers to bracket the expected sample pH values (generally 4, 7, and/or 10).
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3.
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Place enough buffer solution in the titration vessel to cover the electrode tip and add a stir bar.
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4.
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Immerse the electrode tip in the buffer solution (not touching the bottom of the vessel).
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5.
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Set water bath temperature to 25°C, and activate stirrer.
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6.
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Set the Drift Span field (below plot) to a value >0 (30 readings is recommended), and click Measure 1.
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7.
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Measure with stirring until drift (mV) vs. time (sec) on the plot is near 0.0 (this may take an hour or longer).
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8.
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When the drift is nearly 0.0, click Done.
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9.
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Repeat Steps 2–8 with each calibration buffer, selecting Measure 2 and Measure 3, as applicable.
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10.
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Click Calibrate to view the regression curve. The slope value should be close to –59 pH/mV.
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11.
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Click Save Calibration to accept values.
Setting the Dispensing Rate
The rate at which the titrator dispenses the acid into the sample can be adjusted according to expected alkalinity value. Based on experience, for samples with alkalinity <20 mM, a 3 mV rate is usually appropriate. A 3 mV rate dispenses as follows:
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Volume of 0.1 N HCl (µL)
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Until what voltage (mV)
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30
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150
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4
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220
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1
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220–240: through the linear Gran portion of the titration curve
Higher alkalinities may require faster dispensing rates. The dispensing rate can be selected from a list of predefined rate programs or a new dispensing rate program can be created.
Selecting a Predefined Dispensing Rate Program
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1.
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Open the Rates & Standard Corrections tab in the Alkalinity main menu (Figure 3).
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2.
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Under Rates, select a predefined rate from the list. These rates are multistep rate programs such as the 3 mV rate example in the previous section.
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3.
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Click Load This Rate to use the predefined rate program.
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Creating a New Dispensing Rate Program
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1.
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Open the Alkalinity/Standard tab in the Alkalinity main menu (Figure 4).
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2.
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Edit the Rates fields for each step in the rate program as follows:
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–Level (mV) up to which this rate is applicable (150 in the example shown below).
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–Acid (mL) to add in each increment (0.030 mL = 30 µL as below).
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–Drift tolerance (mv/s) for acid increment (0.02 as below). Measurement continues until drift tolerance is satisfied.
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–Seconds until rate program times out (timeout(sec)) if drift tolerance is not satisfied (180 as below).
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3.
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Click Save This Rate.
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4.
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Enter a rate program name in the dialog box and click OK.
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.
To create a new rate program:
- Set your Stability Criteria for each step of the program: Measurement continues until Stability Criteria (mV/s) is satisfied.
- Select your Increment for each mV level (initial to 150, 150 to 220 and 220 to 240): How much acid is added in each increment.
- Set the Time Out for each step of the program: Seconds until rate program times out if Stability Criteria is not satisfied.
- Save To File.
Figure 3. Selecting a titration rate. A new dispensing rate program can also be created
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Figure 4. Creating a New Dispensing Rate Program.
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 measurements is compared to the drift tolerance specified in the rate program (see Creating a New Dispensing Rate Program). Generally, drift tolerance starts at 0.10 mV/s. Drift tolerance acts as follows:
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