FERROUS ETHYLENEDIAMINE SULFATE AS AN OXIDIMETRIC STANDARD KATHRYN P. CARAWAY and RALPH E. OESPER University of Cincinnati, Cincinnati, Ohio
SEvmuL double sulfates containing ferrous sulfate have been proposed for use as oxidimetric standards. Ferrous ammonium sulfate, recommended by Mohr (3) in 1870, has been widely used. Graeger ( 1 ) recommended sodium ferrous sulfate and also zinc ferrous sulfate if properly prepared. None of these compounds is entirely satisfactory because of d i c u l t y in preparation of the pure salt or because of instability under ordinary laboratory conditions. This paper describes the preparation of ferrous ethylenediarnine sulfate, (CH2NHa)2S0~FeS04.4Hz0, and its use as an oxidimetric standard. This compound, prepared from ethylenediamine and ferrous sulfate, has the advantages that it is easily prepared in adequate' purity and that it is stable under ordinary laboratory conditions. It may he 'employed a s an oxidimetric standard in direct titrations. MATERIALS AND SOLUTIONS
Redissolve the precipitate in slightly acidulated water and add alcohol to obtain a 3:2 solution-alcohol ratio. Filter on a Biichner funnel, wash with 65 per cent alcohol and 95 per cent alcohol. Dry in oven overnight a t 50°C. A yield of 53.5 grams or 84 per cent was obtained. The resultant preparation was found to contain: Fe, 14.66 per cent; SO1, 50.22 per cent; N, 7.31 per cent. Calculated: Fe, 14.62 per cent; Sod, 50.28 per cent; N, 7.33 per cent. Results given are an average of four closely agreeing determinations of each component. Method of Standardization Employing Ferrous Ethylene-Diamine Sulfate: For the standardization of 0.05 N oxidizing solutions, dissolve approximately 0.4 gram sample of ferrous ethylenediamine sulfate in 150 ml. of water and 10 ml. (1:1) sulfuric acid. For potassium dichromate, add an additional 5 ml. of (1: 1) phosphoric acid. Titrate with the oxidant using the proper indicator: Ceric sulfate, erioglaucine A or ferroin. Potassium dichromate, diphenylamine or diphenylamine sulfonic acid. Potassium permanganate,..noindicator.
Diphenylamine indicator, 1 per cent solution in concentrated sulfuric acid. Erioglaucine A indicator, 1per cent aqueous solution. Ferroin indicator, 0.025 molar solution. EXPERIMENTAL Ethvlenediamine. 98 Der cent solution. "c."P."ferrous 'snlfke (FeSOx7HzO) dissolved in The merit of the ferrous ethylenediamine sulfate 0.5 N sulfuric acid and precipitated with ethyl alcohol. (FES) as an oxidimetric standard was determined by Filtered and washed with alcohol and ether and then air using it to standardize solutions of ceric sulfate, potasdried until no ether odor remains. sium dichromate, and potassium'permanganate. These Ceric sulfate solution, prepared by dissolving the salt, solutions were also standardized by the usual methods Ce(S04)2,in 1N sulfuric acid and standardized against described under Materials and Solutions. The results Bureau of Standards' sodium oxalate using ferroin indi- are shown in Table 1. As a further check, a solution of ceric sulfate was cator. Standard potassium dichromate solutions, prepared standardized by three methods: by direct titration by direct weighing. Potassium uermannanate solutions, filtered through TABLE 1 Normalities of Oxidizing S o l u t i 0 ~ asbestos and standarzized against sodium oxalate two days before use. Standard Against Sodium thiosnlfate solution standardized against method FES potassium dichromate, using starch indicator. Potassium dichramrtte 0.025 0.0249 PROCEDURE
Preparation of Ferrous Ethyknediamine Sulfate: The method of preparation is essentially that described by Grossman and Schuck (8). Add 60 ml. of 6 N sulfuric acid to 10 erams of 98 Der cent ethvlenediamine solution. Dissolve 4%.3grams ferrous sulfate in the acidified solution and add water to bring volume to 300 ml. To this solution add 300 ml. of ethyl alcohol. Filter on a Biichner funnel and wash with 50 per cent alcohol. 235
0.200 Potassium permnganate
ceric
n 0.0218 n m ~
0.0600 0.0979
0.0600 0.0600 0.0979 0.0979
JOURNAL OF CHEMICAL EDUCATION TABLE 2 Standardization of Ceric Sulfate Sdution Standard
Nmlilu
Pot&an dichromte (through NsnB01)
0.0500 0 .04119 O.O5oO 0.0499 0.0500
Ferrous ethylene-diamine sulfate
against Bureau of Standards' sodium oxalate, against potassium dichromate through sodium thiosulfate solution, and against ferrous ethylenediamine sulfate. The results are shown in Table 2. DISCUSSION
Ferrous ethylenediamine sulfate, when prepared by the method described, is a 6ne-grained, crystallime compound. It is easy to prepare, purify, and dry. The data in Tables 1 and 2 indicate that it reacts stoichio-
metrically with standard solutions of three powerful oxidizing agents commonly employed in oxidimetric titrations. The compound shows no apparent tendency to undergo oxidation, deliquescence, or efflorescence. A sample left loosely covered in the laboratory air for 149 days showed no change in composition and no positive test for ferric ironvith potassium thiocyanate. The material is easy to preserve in the pure state and as a reductant haa a high equivalent weight (382). It may be concluded that ferrous ethylenediamine sulfate is an acceptable standard since the foregoing hdings show that this compound meets the requirements of a good primary standard as specified by many authorities. LITERATURE CITED (1) GRAEGER, N., Chum.Z d r . , 18, 377 (1872). H., AND B. SCHUCK, Z. anorg. Chem., 50, 26 (2) GROSSMAN, IlanR~
(3) Mom. C. F., "Lehrhuch der Chemisch-hlytischen Titriermethode," Fried. Vieweg & Sohn. Braunschweig. Germany, 1870, p. 169.
