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National Field Manual for the Collection of Water-Quality Data


TWRI Book 9
(REVISED 9/2001)

Calibration is required for the pH instrument—follow instructions in NFM 6.4.

Alkalinity and ANC determinations require the use of an acid titrant with a known concentration. Accurate standardization of the acid titrant is essential.

Arrow The normality of locally prepared titrant or 0.01639N solution obtained from the OWQRL must be tested once per month. The acid is checked against a fresh standard solution of sodium carbonate, as described below.
Arrow Record on field notes any correction factors necessary to compensate for cartridge manufacturing anomalies and/or variations in the stated acid normality for the lot of prefilled cartridges to be used. This information is available from OWQRL for USGS personnel.

CAUTION: When diluting concentrated acids,
always add the acid to the water.



Prepare a sulfuric acid titrant solution:

To prepare your own acid titrant for the buret system, follow the procedure below (adapted from Fishman and Friedman, 1989).

  1. Cautiously add 0.5 mL concentrated H2SO4 (specific gravity 1.84 g/mL) to approximately 950 mL of fresh deionized water (DIW).
  2. Mix thoroughly; dilute with DIW to the 1-L mark.

This recipe results in an acid concentration of roughly 0.018N. To determine the actual acid concentration, the solution must be standardized—follow the procedure on the next page. If a concentration of exactly 0.01639N is desired, the standardized solution may be diluted with DIW and restandardized.

Prepare the standard solution:

To prepare a fresh standard solution of 0.01639N sodium carbonate (Na2CO3):

  1. Dry 1.0 g primary standard Na2CO3 at 150 to 160°C for 2 hours.
  2. Cool in a desiccator; weigh out 0.8686 g Na2CO3; add to a 1-L volumetric flask.
  3. Dilute with DIW to the 1-L mark.

Standardize the sulfuric acid:

  1. Calibrate the pH system following instructions in NFM 6.4.
  2. Pipet 25 mL of sodium carbonate standard solution into a 100-mL beaker, and titrate with the sulfuric acid solution. Record the pH and volume of titrant added, following the procedure in section 6.6.4.A.
  3. Determine the equivalence point of the titration using the inflection point method (section 6.6.4.B). The equivalence point is the point at which the change in pH per volume of titrant added is maximized, and generally will be found near pH 4.5.
  4. Determine the normality of the acid as follows:



Ca is acid normality, in equivalents (eq) per liter, and

Va is volume of sulfuric acid added to reach the equivalence point, in milliliters.

  1. Store the solution in a tightly sealed 1-L glass bottle.

Section 6.6.3

Field Manual Contents

Table of Contents Chapter 6
Table of Contents
Section 6.6
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