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586
INDUXTRIAL A N D ENGINEERING CHEMISTRY
The data thus obtained can be applied to any other distillation problem. Whereas, the a c t u d efficiency will differ with the mixture which it is desired to fractionate, the variation of efficiency with operating conditions will be substantially independent of that factor.
Vol. 16, No. 6
ACKNOWLEDGMENT Thanks are due to W. K. Lewis under whose direction this investigation was conducted, and to W. H. McAdams and W. A. Peters, Jr., for valuable suggestions concerning the interpretation of the data.
T h e Determination of Manganese in Water by t h e Sodium Bi sm IIt h at e M et h od'j2 By W.D. Collins and Margaret D. Foster
u. s. GEOLOGICAL SURVEY,
DEPARTMENT OF THE INTERIOR, 'WASHINGTON, D.
T
HE sodium bismuthate method for the determination of manganese in water has been recommended for a number of years in authoritative publications3which refer back to a paper on the subject published by Weston4 in 1907. A study of the method with reference to the saving of time and reagents resulted in the adoption of the procedure given below. The discussion is based on about one hundred experiments on the effects of various modifications of details of the method. METHOD Use 100 cc. of water or a sample small enough to contain less than 1 mg. of manganese. Add 10 cc. of dilute nitric acid (1 part strong acid to 3 of water), 1 cc. of concentrated sulfuric acid, and heat in a beaker on a hot plate until most of the sulfuric acid has been driven off. Cool, take up with about 50 cc. of water and 20 cc. of dilute nitric acid through which air has been bubbled to remove oxides of nitrogen. Add 0.10 gram of sodium bismuthate, stir for 1 or 2 minutes, allow the excess of bismuthate to settle, and filter through an alundum crucible or a Gooch crucible with a mat of ignited asbestos which has been washed with permanganate solution. Dilute the filtrate to a definite volume and compare with standards. To prepare the standards measure out appropriate volumes of standard permanganate solution, add to each the quantity of nitric acid used for the samples, and make to the same volume. DISCUSSION Evaporation of the sample with nitric and sulfuric acids insures the removal of chloride and of organic matter. By stopping the evaporation just short of dryness the manganese sulfate is obtained in an easily soluble condition. It is well to evaporate nearly all the sulfuric acid, because the shade of the permanganic acid is not the same in sulfuric acid and in nitric acid solutions. The usual directions call for the use of 40 cc. of nitric acid (1to 3 ) . This quantity assures a concentration of acid within the range found by Cunningham and Coltmans to be the best. Unless special care has been taken to remove all the lower oxides of nitrogen from the acid, the color will fade more rapidly with this concentration than with less nitric acid in the solution. I n a series of tests with 1.0 mg. of manganese and 0.10 gram of sodium bismuthate, the full color was developed with 2 cc. of nitric acid (1to 3 ) in a volume of 50 cc. and nearly 90 per cent with only 0.5 cc. of acid. With small quantities of acid appreciable quantities of manganese will be precipitated after several hours. I n a series of tests with quanReceived January 22, 1924. Published by permission of the Director, Geological Survey. I Am. Pub. Health Assoc , Standard Methods of Water Analysis, 1912, p. 49; 1917, p. 49; 1923,p 51; Assoc. Official Agr. Chem., Methods, 1920, 0. 36. 4 J. 4 m . Chem. SOC., 29, 1074 (1907). 6 THIS JOURNAL, 16, 58 (1924). 1 2
c.
tities of aerated acid ranging from 5 to 40 cc., the full cblor was developed in all samples, but the loss on standing was least with 20 cc. of acid. Manganese precipitated in the solutions containing less acid and the color faded more in those with more acid. Older directions call for the use of 0.5 gram of sodium bismuthate. According to the results of Cunningham and Coltmans this is enough bismuthate for 19 mg. of manganese. A series of tests using from 0.50 to 0.025 gram of bismuthate with 5 cc. of nitric acid (1 to 3) for 1 mg. of manganese in a volume of 50 cc. showed no difference in color G t h from 0.50 to 0.075 gram of bismuthate. With 0.050 gram bismuthate the color was 85 per cent of the standard. Taking 0.10 gram of bismuthate as the minimum makes an appreciable saving of reagent in examining any large number of water samples. The experiments reported were made with bismuthate from a lot that contained 83 per cent of NaBi03, determined as recommended by Cunningham and Coltman.6 A lot received later contained only 68 per cent of NaBi03. Practically all directions call for double treatment with bismuthate. This is necessary if the sample, when first treated with bismuthate, contains material that will be slowly oxidized by the permanganic acid after filtration from the bismuthate. If a water sample is treated as directed in the method given above, there will be no such material left and the heating to destroy the permanganic acid color is an unnecessary step which wastes time and bismuthate. When large quantities of manganese are determined in samples containing unoxidized material, some manganese may be precipitated. Directions for steel analysis provide for the solution of this precipitated manganese with the aid of a reducing agent such as sulfur dioxide, sodium thiosulfate, or sodium bisulfite. That this precipitation has rarely occurred in water analysis is indicated by the persistence of the direction to use ammonium or sodium bisulfate for this purpose. If precipitation of manganese a t this point in the analysis had taken place many times, the misprint probably would have been corrected. Cunningham and Coltman6 confirm the statement of Blumc that sodium bismuthate oxidizes manganese perfectly to permanganic acid, so that a solution of potassium permanganate of known strength is as useful as a manganese sulfate solution for the preparation of standards. There is no advantage in reducing the manganese in a permanganate solution with oxalic acid in the presence of sulfuric acid and then using this manganese sulfate for the preparation of Btandards with nitric acid and bismuthate. The usual precautions in preparation and preservation of the permangsnate standard solutions must be observed and a fresh standard solution should be made rather than trust one that has been kept for a long time. 6
J . A m . Chem. Soc., 54, 1379 (1912).
