ANALYTICAL EDITION
May, 1944
THE REAGENTBLANK. Certain lots of sulfuric arid have given difficulty with a reagent blank which is apparently not due to arsenic and isof significance only on determinations below 3 micrograms. With such lots the blank is irregular, and recoveries below 3 micrograms are correspondingly erratio. However, recoveries above this level are perfectly regular, and give no indication of the presence of the blank. Furthermore:, if to such distilled blanks one adds pentavalent arsenic in exwss of 3 micrograms, color develops only in a n amount proportional to the added arsenic, and the interfering blank is not observed. The authors have been unable t o identify this interfering substance, which is found only in certain lots of sulfuric acid. The most satisfactory sulfuric acid has been the regular reagent Krade rather than some of the “arsenic-free” grades. A “pseudo blank” may also appear if the molybdate solution is added down the sides of the test tube rather than directly to the distillate; the molybdate may decompose on the sides of the hot tube and result in a blue color. INTERFERING SUBSTANCES. During the distillation process vomplete separation from phosphorus is accomplished except for a mechanical carry-over of approximately 1 part in 100,000. Thus, if one adds 0.1 gram of phosphate to the digest, about 1 microgram will appear in the distillate. The importance of this phosphorus interference will obviously depend on the relative amounts of phosphorus and arsenic in the digest. I n most biologic work i t becomes important only when large amounts of urine, or qpecimens of nervous tissue or bone are to be analyzed. I n such vases, phosphorus interference may be eliminated by pouring first distillate into a second distilling flask, adding 2 to 3 cc. of conc-entrated nitric acid and 5 cc. of concentrated sulfuric acid, taking down to strong fumes of sulfuric acid, then redistilling. .\ntimony does not interfere with the determination.
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R x m OF DISTILLATION AND LIMITS OF ACID TOLERANCE. The rate of distillation of the arsenic under the heating conditions described depends upon the amounts of sulfuric acid and water present. The 4-minute distillation time will allow complete recovery of the arsenic unless the loss in sulfuric arid volume during digestion has been more than 40%. The amount of hydrobromic acid distilled in the 4-minute period varies between 2.5 and 4.0 milliequivalents. Using the color solutions as described there is no significant difference in the color intensity in the range between 1.0 and 5.0 milliequivalents. There is thus an adequate margin of safety with respect to acidity. SUMMARY
A rapid method for the determination of small amounts of arsenic in biological material is described in which the arJenic distillate is obtained in pentavalent form. This distillate ran be used without further treatment for the final colorimetric determination with ammonium molybdate. The method is rapid, and has given results comparable in accuracy to other published methods for microdetermination of arsenic. LITERATURE CITED
(1) Chaney, A. L., and Magnuson, H. J., IND. E N G .CHEM.,ANAL. ED., 12, 691 (1940). (2) Hubbard, D. M.,Ibid., 13, 915 (1941). (3) Maechling, E. H., and Flinn, F. B., J . Lab. C’lin. Med., 15, 779 (1930). ., ED., (4) Morris, H. J., and Calvery, H. O., IND.ENG.C H ~ MANAL. 9, 447 (1937). (5) Scott, W. W . , “Standard Methods of Chemical Analysis”, 5th ed., pp. 99, 115, New York, D. Van Nostrand Co.. 1939. (6) Sultzaherger, J. .I.,IND.ENG.CHEM., ANAL.ED., 15, 408 (1943).
Determination of Nitric O x i d e Using Solid Reagents D. J. LE ROY
AND
E. W. R. STEACIE,
National Research Laboratories, Ottawa, Canada
S
M I T H and Leighton (A) have desccribed a micromethod for the determination of nitrir oxide in mixtures of hydrogen or nitrogen which consists in a modifiration of the macromethod of Baudisch and Klinper (1). Their procedure is based on the oxidation of nitric oxide by the addition of oxygen and the rapid absorption of the resulting oxides of nitrogen by a moist potassium hydroxide bead. The method is indirect, because the amount of oxygen required is variable and consequently the amount of hydrogen or nitrogen in the original mixture must be calculated by analyzing for residual hydrogen after removing all the remaining oxygen by combustion with an excess of hydrogen (a known volume of hydrogen being added if necessary). A further disadvantage is the possibility of contaminating the mercury surface through the presence of the oxides of nitrogen. The present method is an adaptation of that of Diverb (S), who found that alkaline sodiqm sulfite solution readily absorbed nitric oxide with the formation of sodium hyponitrososulfate. Further work by Moser and Herzner ( 4 ) showed that the reagent was superior to ferrous sulfate in its capacity for absorbing nitric oxide but the rate of absorption was somewhat less.
A pellet of potassium hydroxide is ground in a mortar and sodium sulfite crystals (Na&30*.7HzO) are added till a thick paste is formed. No water is required, as the mixture becomes moist. The paste is then formed into a bead on a platinum loop and if sufficient sodium sulfite has been added very little drying is necessary. If the bead is thoroughly dried no absorption takes placc, but aside from this the moisture content does not appear to be critical. When placed in the gas containing nitric oxide absorption is complete in 5 to 10 minutes. Table I is indicative of the accuracy obtainable by this method, using the Blacet-Leighton apparntus ( 2 ) .
Table
Determination
I.
Determination of Nitric Oxide
Nitric Oxide Volume of Sample Theoretical Determined Difference Cu. mm. % % 7c Nitric oxide-hvdrogen mixtures 0.0 13.4 21.6 38.4 50.2 66.6 80.4
0.2 13.5 21.4 38.6 50.0 66.2 80.0
-0.2 -0.1 0.2 -0.2 0.2 0.4 0.4 Av. 0 . 3
0.2 8.4 19.0 35.0 49.7 65.0
-0.2 0.4 0.4 0.4 0.1 0.8 Av. 0 . 4
Nitric oxide-ethvlene mixtures 0.0 8.8 19.4 35.4 49.8 65.8
I n contrast to the method of Smith and Leighton, the present method can be used in the presence of combustible gases other than hydrogen, and very satisfactory results have been obtained in the presence of hcetylene as well as ethylene and hydrogen. LITERATURE CITED
(1) Baudisch, O., and Klinger, G., Be?., 45, 3231 (1912). (2) Blacet, F. E., and Leighton, P. A., IND.ENG.CHEM.,ANAL.E D . , 3, 266 (1931). (3) Divers, E., J . Chem. SOC.,75, 82 (1899). (4) Moser, L., and Herzner, R., 2. anal. Chem., 64, 81 (1924). (5) Smith, R. N., and Leighton, P. A., IND.ENG.CHEM.,ANAL.ED., 14, 758 (1942). CONTRIFJVTION 1181, National Research Laboratoriea, Ottawa, Canada.
