A COMPARATIVE STUDY OF THE USE OF DIPHENYLAMINE AND POTASSIUM FERRICYANIDE AS INDICATORS IN TITRATIONS WITH POTASSIUM DICHROMATE J. P. MEmIG, OREGONSTATE AGRICULTURAL COLLEGE, CORVALLIS, OREGON As a standard solution for use in iron determinations potassium dichromate has many advantages over potassium permanganate. I t can be obtained in a very pure condition, the solution is easily made, is very constant, and can be used in the presence of chlorides. The one big disadvantage has been that until recently no suitable inside indicator has been available and the tedious procedure known as Penny's method1 using potassium ferricyanide on a spot plate has had to be used. J. KnopZproposed the use of diphenylamine which gives an intense blue color at the instant of complete oxidation of all ferrous salts: The interference of the troublesome yellow color of ferric chloride he avoided by the use of a mixture of sulfuric and phosphoric acids. The phosphoric acid here acts as it does in the Zimmermann-Reinhardt solution, suppressing hydrolysis of femc chloride and causing the formation of the poorly ionized ferric phosphate, which is not highly colored. The present work was undertaken to give Knop's method a thorough try-out and to see what sort of results it would give in the hands of students. The students were sophomoxe chemical engineers and the work was carried out in connection with their regular course in quantitative analysis. Comparative analyses were made oa each sample by both the Knop and Penny methods. The results obtained by the instructor, J. P. Mehlig, are also given. Solutions Required in Titration
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1. Approximately 0.1 N potassium dichromate solution. 2. A mixture of sulfuric and phosphoric acids made as follows: 150 cc. sulfuric acid (d. 1.84) is mixed with 150 cc. phosphoric acid (d. 1.7) and the mixture diluted with water to 1000 cc. 3. Diphenylamine solution, consisting of 1 gm. of diphenylamine dissolved in 100 cc. concentrated sulfuric acid.
Standardization of Potassium Dichromate The potassium dichromate solutions were standardized against Mohr's salt using both the spot plate and the diphenylamine method. In each case about 1gm. of Mohr's salt was dissolved in 50 cc. of recently boiled and cooled water. For the spot plate method 15 cc. 6 N sulfuric acid were added and enough water to make a total volume of 100 cc. For the diphenylamine method 25 cc. 6 N sulfuric acid, 15 cc. of the snlfuric-
' Chen. Gas.. 8,330 (1850). 8
J. Am. Chem. Soc., 46,263
(1924).
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DIPHENYLAMINB AND POTASSIUM FERRICYANIDE AS INDIUTORS 825
phosphoric acid titration mixture, and three drops of indicator were added and the total volume made up to about 200 cc. A correction of 0.05 cc. for the oxidation of diphenylamine was subtracted from the volume of standard dichromate solution added as suggested by Knop. In most cases a slightly lower normality factor was obtained with diphenylamine. The average of fourteen different solutions was 0.1013 N by the spot plate and 0.1012 N by diphenylamine. Analysis of Ferrous Salts The determination of iron in ferrous salts was carried out in the same manner as the standardization of the dichromate solution. In each method the corresponding normality factor was used in the calculation. Table I shows the very close agreement that was obtained. All results are averages of closely agreeing duplicate determinations. TABLEI ANALYSISOP IRON M FERROUS SALTS Mohr'r salt. 14.23% Fe Spot plate Diphenyiamine %Re % Fe
Student A Student B Student C Student D Student E Student ave. Mehlig
14.18 14.19 14.24 14.20 14.21 14.20 14.25
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14.21 14.11 14.16 14.23 14.20 14.18 14.21
Ferrous sulfate, 20.08% Fe Spot plate Diphenylarnine % Fe % Fe
Student F Student G Student H Student I Student J Student K Student L Student m e . Mehlig
20.01 20.02 20.14 20.10 20.24 20.05 20.17 20.10 20.10
20.00 19.91 20.12 20.10 20.25 20.13 20.16 20.10 20.08
Analysis of Chromium in Steels and Ores Knop3 found that, since the presence of a large amount of femc salts does not have a bad effect on the end-point, it was possible to use this method in the determination of chromium in chrome steel by dissolving a steel sample in 1 : 1 sulfuric acid, oxidizing the solution by ammonium persulfate, filtering off the precipitate of MnO(OH)%,and determining the LOC.
