National Field Manual for the Collection of Water-Quality Data

6.6.5 CALCULATIONS

TWRI Book 9
(REVISED 9/2001)

Concentrations of alkalinity or ANC and carbonate species can be calculated by hand (sections 6.6.5.A and 6.6.5.B), or by using a computer program (section 6.6.5.C). For hand calculations, first determine the equivalence points using the IPT (section 6.6.4.B) or Gran function (section 6.6.4.C) method. The equations below are valid for buret or digital titration equipment, and any acid concentration and sample volume.

6.6.5.A ALKALINITY OR ANC

Calculation of alkalinity or ANC is a simple accounting of the amount of acid used to neutralize the sample:

where

The correction factor CF is equal to 1.0.1 as determined by the NWQL, resulting from a bias caused by the configuration of Hach digital titrator cartridges.

6.6.5.B CARBONATE-SPECIATION EQUATIONS
FOR SAMPLES WITH pH 9.2

	To calculate bicarbonate and carbonate species using the equations below, the following assumptions must apply:
	–	Bicarbonate and carbonate are the only species providing significant contributions to alkalinity or ANC. (If the sample contains other titratable species in significant amounts, the calculated bicarbonate and carbonate concentrations will be in error.)
	–	he sample pH is 9.2.
	If sample pH is > 9.2, calculations are more complex and use of the Alkalinity Calculator computer program (section 6.6.5.C) is recommended.
	If the sample contains other titratable species such as ammonia, borate, or silicic acid that contribute significantly to alkalinity or ANC, use of a full geochemical model such as PHREEQC (Parkhurst and Appelo, 1999) is recommended.2

TECHNICAL NOTE: The equations provided below for bicarbonate and carbonate only apply for samples with a pH of 9.2 or less because the effects of hydroxide were neglected. The equations remain useful, however, because they still apply to a wide range of natural water samples. The error in using these equations should be less than 10 percent and/or less than 1 mg/L. Speciation equations for samples with a pH greater than 9.2 that account for the presence of hydroxide and the equilibrium chemistry of carbonic acid can be found at http://oregon.usgs.gov/alk/methods.html (refer to the documentation for the Alkalinity Calculator Program, section 6.6.5.C).

²A description of such geochemical models is beyond the scope of this manual.

1. Calculate bicarbonate and carbonate species as follows:
   
   

where

2. Double-check your calculations.

6.6.5.C COMPUTER-PROGRAM ANALYSIS OF
TITRATION DATA

Two USGS computer programs that simplify alkalinity or ANC plotting and calculation tasks include the Personal Computer Field Form (PCFF) program and the Web-based Alkalinity Calculator. These cover the full range of pH measured in natural waters and are useful for performing the complex calculations needed for samples with pH > 9.2.

PCFF Program

The PCFF program is a Windows-based tool that allows users to enter field-derived sample collection data into standard USGS field forms electronically.

	PCFF minimizes the manual input of information, helps eliminate transcription errors, and saves the user time.
	PCFF can perform some common field calculations, such as the analysis of titration data to determine alkalinity or ANC.
	PCFF uses the inflection point titration method (section 6.6.4.B) to find equivalence points in the titration data.

Alkalinity Calculator Program

The Alkalinity Calculator is a Web-based program that allows users to analyze titration data and determine alkalinity or ANC with several different methods. The current version of the Alkalinity Calculator provides a choice of the following recommended methods:

	The inflection point (IPT) method
	The Gran function plot method
	A theoretical carbonate titration curve method.

The IPT and Gran methods implemented by the Alkalinity Calculator are as described in this manual. The carbonate titration curve method attempts to fit the titration data with a theoretical titration curve derived solely from the chemistry of carbonic acid. This method shows the user whether the chemistry of carbonic acid alone can account for the shape of the entire titration curve. If the fit is poor, it is likely that more than just hydroxide, carbonate, and bicarbonate were titrated in the sample. An excellent fit with this method is evidence that the titration curve is dominated by carbonate and bicarbonate.

The Alkalinity Calculator can be found on the Internet from a link that can be accessed at the following URL: http://water.usgs.gov/owq/methods.html or can be accessed directly at the following URL: http://oregon.usgs.gov/alk

Methods used by the Alkalinity Calculator are documented at http://oregon.usgs.gov/alk/methods.html