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Kansas Geological Survey, Bulletin 202, pt. 1, originally published in 1971

Preparation of Detergent Samples for Determination of Arsenic

by Larry M. Magnuson, Truman C. Waugh, James E. Bredfeldt

Originally published in 1971 as part of Kansas Geological Survey Bulletin 201, pt. 1, p. 15. This is, in general, the original text as published. The information has not been updated. An Acrobat PDF version of the complete bulletin (7 MB) is also available.

Abstract

Reported here is a method of preparing samples of laundry products prior to determination of arsenic by a standard method (American Public Health Association, 1965) which requires the use of an arsine generator. [Note: The Fisher No. 1-405 arsine generator has been found satisfactory for the purpose, Cat. 70.] Wet ashing of samples prevents their subsequent foaming in the generator and assures that all the arsenic is inorganic. The use of low temperatures prevents the loss of arsenic which occurs with high-temperature ashing.

The procedure of preparation is as follows:

  1. Accurately weigh 0.1 to 3 gm of sample into a 125-ml flask and cover with a minimum of concentrated H2SO4,
  2. Digest resulting mixture at 100°C until thoroughly charred.
  3. Add 25 ml of concentrated HNO3 and reflux solution until color is discharged.
  4. Reduce solution to near its original volume by heating on a hot plate. Care must be taken to avoid reducing the volume to the point that SO2 evolves; however, no brown fumes should still be evolving. If blackening occurs, an indication of incomplete ashing, Steps 3 and 4 must be repeated.
  5. Cool sample, add 5 ml of 30-percent hydrogen peroxide, cover flask, and heat until effervescence ceases.
  6. Add 25 ml of concentrated HCl and reduce solution to near its original volume. Repeat this step until no nitric oxide is evolved, usually three times.
  7. Add 25 ml of water and again reduce solution to near its original volume, subsequently cooling and diluting solution to 50 ml. The sample is now ready for the standard method.

The weight of sample in Step 1 must contain between 5 and 20 micrograms of arsenic for proper use of the standard method. This weight may be determined by emission spectrography (Waugh, personal communication, 1970) or trial and error. Use of a minimum of H2SO4 in Step 1 is necessary to prevent later overheating in the arsine generator. Reduction of volume to the point that fumes of H2SO4 form causes significant loss of arsenic. Step 5 aids in the removal of NO3- in Step 6. NO3- causes results that are too low. Step 7 eliminates an excess of HCl which also causes overheating in the arsine generator.

In all the detergents analyzed in this laboratory, the concentrations of metals which interfere with the determination were too low to affect results.

We found a coefficient of variation of 3.5 percent for the preparation and standard method combined; this increased to 12 percent when samples were taken from different parts of the detergent box. The difference between standards of As20a subjected to the preparation described here and duplicates determined directly by the standard method was less than 2 percent. W. J. Traversy (personal communication, 1970) of the Canadian Department of Energy, Mines and Resources reported a mean recovery of 98 percent from spiked samples and a coefficient of variation of 11 percent using our procedure.

References

American Public Health Association, 1965, Standard Method for Examination of Water and Wastewater, 12th ed., New York, p. 56-58.

Kansas Geological Survey, Preparation of Detergent Samples for Determination of Arsenic
Placed on web May 7, 2009; originally published in May 1971.
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