Total Alkalinity Measurements

Total alkalinity is a measure of the amount of negatively charged ions in a given amount of water. In the oceans, most of the negatively charged ions present are bicarbonate (HCO₃^-) and carbonate (CO₃^--). Since the other ionic species which contribute to the amount of alkalinity in sea water are present in both small and relatively well known amounts, when we measure the alkalinity of a sea water sample, we are really measuring is the amount of these two major compounds.

Total alkalinity is important to us here at the Scripps CO₂ Program because it, along with the amount of DIC in a batch of sea water, can be used to calculate the amount of dissolved carbon dioxide gas (pCO₂) in that batch of sea water. Precise measurements of total alkalinity are thus vital to our efforts to understand how the oceans are responding to the increase of CO₂ in the atmoshphere. We have traditionally used the closed-cell titration method to measure total alkalinity.

Closed Cell Alkalinity Titration

We measure the amount of total alkalinty in sea water by carrying out a potentiometric titration of a weighed aliquot of sea water in a vessel which is sealed from the atmosphere. This is accomplished by adding precise amounts of 0.1 normal hydrochloric acid (HCl) to the vessel in small increments, and measuring the change in the electromotive potential (emf) of the water caused by this addition. The potential of the sea water is measured with a standard combination-type pH electrode, and the whole cell is encased in a jacket through which constant temperature water is pumped. To ensure that the sample maintains a constant temperature (temperature has a direct effect on the emf of the water), an electronic temperature probe is also mounted in the cell.The entire procedure is run by a computer, which also monitors and records the response of the probes. Once the titration is complete, the data (acid volume, emf, and temperature) are used to calculate the total alkalinity by the modified gran method. While the math behind all this is rather complicated, what we are actually doing is determining how many protons (supplied by the HCl) were consumed in converting all the carbonate and bicarbonate in the sample to CO₂.Looking at a plot of this titration data, it is easy to visualize just where this occurs. As the positively charged protons (H+) are added to the mix, they react with the carbonate (CO₃^--) to convert it to bicarbonate (HCO₃^-), causing the first of two inflection points on the titration curve. As more acid is added, all of the bicarbonate is converted to CO₂, which causes the second inflection point on our curve. The location of this second inflection point is what determines the total alkalinity of the sample.