Alkalinity Calculator

The U.S. Geological Survey has developed an "Alkalinity Calculator" (Initial version - 1/14/2000) which takes titration data and performs all calculations using a variety of methods. The "Alkalinity Calculator" is available for use as an online tool or is available for use as a free downloadable stand-alone version.

Data input into the alkalinity calculator is relatively straightforward. The user is asked to enter a site name, site ID, collection date, collection time, sample temperature (at time of titration), sample conductance, analyst name, analysis date, analysis time, sample volume, filtered (yes or no), acid normality, acid lot number, acid correction factor (if Hach® digital titrator cartridges are used), acid expiration date, stirring method (magnetic or manual), titration type (digital titrator or burette titration), and any comments. The user must also enter or paste from a spreadsheet, two columns of data which include the pH and titrant volume. The user is also asked to select the data analysis method which includes the inflection point method, gran function plot method, and theoretical carbonate titration curve method. Although the gran function plot method is always selected, it is often instructive to select all analysis methods for comparative purposes! Also, the user is asked to select a speciation method for calculation of carbonate and bicarbonate. While the default and preferred method is the advanced speciation method, the simple mass balance speciation method can also be selected. In essence, the alkalinity calculator can be instructed to perform all calculations that it is capable of at once! The Maine Geological Survey finds this to be a good approach in best studying the alkalinity of a sample and documenting the results.

For readers interested in trying the alkalinity calculator, a data set from a sample actually collected from Easton, Maine on July 17, 2001 is provided in Table 1. Titration data may be copied from the table and pasted into the calculator.

Using the Gran Function Plot Method for this sample, total alkalinity was determined to be 378.9 mg/L as calcium carbonate (CaCO₃) using the F1 Gran function. From the F1 alkalinity and sample pH, hydroxide (OH^-) was determined to be 0.0 mg/L, carbonate (CO₃^-2) was determined to be 0.2 mg/L, and bicarbonate (HCO₃^-) was found to be 461.6 mg/L. For an explanation of the various "F" functions in the Gran Function Plot Method, refer to USGS National Field Manual Chapter 6, Section 6.6 and the USGS Oregon Water Science Center Alkalinity Calculation Methods. Also, it should be noted that it would likely be better to have titrated to a pH of less than 3.5 (say pH of about 3.0) for even more accurate calculation.

This sample from Easton is somewhat atypical with regard to alkalinity in that the level is very high. Total alkalinity in Maine ground water is often 100 mg/L or less as CaCO₃, unless the underlying bedrock tends to be rich in calcium which is the case in this area of the state. Hardness was determined to be 494.44 mg/L which explains why the homeowner installed a water softener and reported salt usage in the ion exchange process of 200-250 pounds per month!!

Last updated on January 13, 2009