0.2
0
0.4
0.6
As
Figure 5.
I. 0
0.8
(pg)
Calibration curves of arsenic
A . By flameless AAS B. By argonhydrogen-entrained air AAS
Several precautions should be noted when applying the arsine evolution and the electrically-heated absorption tube methods. Since the absorbance of arsenic varies with the temperature, as previously mentioned, a constant temperature should be maintained in the absorption tube. Second, during wet digestion, the arsenic must be maintained in As5+state to avoid losses and this can be accomplished by constant addition of nitric acid to maintain oxidizing conditions (8). However, the nitric acid must be removed in order to maintain arsenic in a trivalent state before the chemical conversion of arsenic to arsine. This can be achieved by addition of a concentrated ammonium oxalate solution and heating the digest to expel the oxides of nitrogen. Third, since arsine is immediately evolved after the addition of zinc granules (5-9, the adapter with the balloon should be inserted immediately to prevent the escape of arsine and the reduction of absorbance. Finally, the balloon should be flushed with argon before each analysis to ensure that no residues of arsine are left in the balloon. Fernandez and Manning (6) reported that a balloon would lose elasticity after 5-10 determinations because of acid fumes. With a curved adapter as used in the present study, a balloon could be used for 15-20 determinations before it needed to be replaced. In conclusion, the flameless AAS method for arsenic determination not only significantly lowered the background __ (8) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, N. Y., 1959, p 278.
absorption compared to that of the argonlhydrogen flame method, but also increased the sensitivity and gave an excellent reproducibility and recovery. RECEIVED for review December 13, 1971. Accepted March 17, 1972. Authorized for publication on Dec. 3, 1971, as paper no. 4101 in the journal series of the Agricultural Experiment Station.
Correction Intrinsic End-Point Errors in Titration with Ion Selective Electrodes. Chelometric Titrations In this paper by Peter W. Carr [ANAL. CHEM.,44, 452 (1972)], an error appears in Equation 12, p 453. This should read -2ba3 -
cy2
- 4bpa
- 4bzp2
=
0
The data presented in the paper are correct and were computed on the basis of the above equation.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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