Bi-Ortho-Anisidine as Internal Indicator in Bichromate Method for Iron MARYELVIRA WEEKS,University of Kansas, Lawrence, Kans.
IB
ECENT research in volumetric analysis has revealed several useful inside indicators suitable for titrating iron with a standard solution of potassium bichromate. Diphenylamine and diphenylbenzidine, which were introduced by Knop (1) in 1924, have proved to be very satisfactory under most conditions. Occasional failures result, however, because, as Kolthoff and Sarver (8) have pointed out, these indicators are sometimes oxidized, not to the blue holoquinoid, but to a green meriquinoid which cannot be distinguished from the chromic chloride formed by the reduction of the standard solution of potassium bichromate. Moreover, even when a mixture of concentrated sulfuric acid and concentrated phosphoric acid is used to bind the ferric ions, many chemists have difficulty in detecting the color change from blue-green to blue. In 1930 SzebellBdy (8) overcame the latter objection by using ammonium fluoride to bind the ferric ions and p-phenetidine as the inside indicator. Under these conditions the iron solution retains a light yellow-green color until the first excess drop of bichromate solution produces a rich violet. The author (4) found in 1931 that p-anisidine may be used exactly like p-phenetidine, and that the oxidation product is equally brilliant in color. Unless these indicators have been freshly prepared, however, the violet oxidation product will appear too soon, and the results reported for iron will be too low. For these reasons it seemed advisable to continue the search for oxidation indicators. Solutions of a number of aromatic amino derivatives were acidified with hydrochloric acid and treated with a slight excess of a dilute solution of potassium bichromate in order to observe any color changes that might appear. p-Phenylenediamine hydrochloride gave a green color which gradually changed to a rich purple, whereas ophenylenediamine hydrochloride, o-anisidine, and bianisidine all gave a blood-red color. Only the latter substance, however, proved t o be suitable for quantitative work.
0.0055696 and 0.005694 gram per cc. were used in a few of the titrations. BIANISIDINE AS INTERNAL INDICATOR Four portions of the Mohr's salt were titrated by the following method: The solution obtained by dissolving the weighed sample in 30 cc.'of 7 N hydrochloric acid was heated to the boiling point and carefully reduced with stannous chloride. After addition of one excess drop of stannous chloride, the iron solution was quickly diluted to 400 cc. with distilled water which had been cooled on ice. After adding 12 cc. of a mixture containing equal volumes of concentrated phosphoric acid and concentrated sulfuric acid, exactly 10 drops of the 1 per cent solution of bianisidine, and 30 cc. of a saturated solution of mercuric chloride, the iron solution was immediately titrated with the standard solution of potassium bichromate until the color passed through brownish green to red-brown. If the standard solution is added slowly toward the end, the color change is very distinct and the titrations can be duplicated very accurately. After standing a few minutes the titrated solutions acquire a blood-red color. The results presented in Table I show that iron can be titrated accurately by this method. Although the indicator is not sharply reversible, iron solutions which have been overtitrated may be treated with a small measured volume of a standard ferrous solution and then correctly titrated with the standard solution of potassium bichromate. AS INDICATOR IN TITRATION OF TABIXI. BIANISIDINE MOHR'SSALT
(After reduction with stannous chloride) 1 2 3 Mohr's salt, grams 1.5026 1.5046 1.5085 37.63 37.66 Potassium bichromate, 00. 37.49 Iron value 0.005696 Iron found % 14.21 14.24 14.22 Mean, 7d 14.22 Value from previous analysis 14.21
yiiiT
X4-
= 0.005695 gram per cc.
Standardization of this solution against the ferrous ammonium sulfate yielded the same result-that is, 10 grams of potassium bichromate dissolved in water and diluted to 1 liter gave an iron value of 0.005695 gram per cc. Two other bichromate ,solutions similarly standardized and having iron values of
14.23
AS INDICATOR IN TITRATION OF TABLE 11. BIANISIDINE SIBLEYIRONORE27
PREPARATION OF MATERIALS A 1 per cent solution of bianisidine in glacial acetic acid was prepared, and kept in a dark bottle. A quantity of Mohr's salt, ferrous ammonium sulfate, to be used as a standard was analyzed by weighing the iron as ferric oxide and by titration with a permanganate solution which had been standardized against sodium oxalate. The gravimetric and volumetric results agree closely. The standard solution of potassium bichromate was made by dissolving 10.0005 grams of the dry, recrystallized salt and diluting to the mark with distilled water at 28" C. in a calibrated flask whose capacity at that temperature was found to be 1999.7 cc. The iron value of this solution was therefore
4 1.6024 37.53
(After reduotion with stannous ohloride) 1 2 3 4 5 6 Ore gram 0.4012 0.4019 0.4003 0.4009 0.3999 0.4013 Potassium bichromate, cc. 48.64 48.80 48.61 48.64 48.46 48.72 Iron value 0.005695 Ironfound, % 69.04 69.15 69.16 69.09 69.01 69.14 Mean, % 69.10 Bureau of Standards value, % 69.2
As a further test of the usefulness of this indicator, six portions of the Sibley iron ore 27 provided by the Bureau of Standards were analyzed by the following method: Each weighed portion of the ore was treated with 30 cc. of 7 N hydrochloric acid and 5 cc. of stannous chloride solution and heated just below the boiling point for several hours until no brown residue remained. The solution was then filtered through ashless filter paper into a 600-cc. beaker. After washing with hot water, the filter was ignited in a porcelain crucible, the small amount of residue was carefully fused with potassium pyrosulfate, and the dissolved melt was added to the iron solution in the 600-cc. beaker. This was slightly oxidized with a few drops of a permanganate solution, care127
ANALYTICAL EDITION
128
fdly reduced with stannous chloride, and treated exactly like the samples of ferrous ammonium sulfate. The results found for iron, as shown in Table 11, average 69.10 per cent, whereas the value reported by the Bureau of Standards is 69.2 per cent, The results with bianisidine are therefore correct to within about 0.1 per cent. TABLE111. PERMANENCE OF BIANISIDINE SOLUTION (Titration of Mohr's salt after reduction with stannous chloride) 1 2 3 4 Mohr's salt, grams 1.5041 1.5022 1.5021 1.6034 Potassium bichromate, 0 0 . 37.48 37.49 37.48 37.64 Iron value 0.005694 Iron found, % 14.20 14.21 14.21 14.22 Mean, % 14.21 Value from previous analysis 14.21
PERMANENCE OF INDICATOR In order to test the effect of light on th'e indicator, four portions of the ferrous ammonium sulfate were titrated by the
Vol. 4, No. 1
above method, using a bianisidine solution that had been exposed to light in a north window for 11 days. The results given in Table I11 show that this treatment did not impair the usefulness of the bianisidine solution. ACKNOWLEDGMENT The author wishes to make grateful acknowledgment of the helpful suggestions received from F. B. Dains. LITERATURE CITED (1) Knop, J., J . Am. Chem. SOC.,46, 263-9 (1924). (2) Kolthoff, I. M:, and Sarver. L. A., 2. Elekfrochem., 36, 139-41 (1930). (3) SaebellBdy, L., 2. anal. Chem., 81, 97-103 (1930).
(4) Weeks, M. E., to be published in Trans. Kansas Acad. Sci.
RECEIVED JUIY
14,1931.
Aqua Regia and Base Metals Rate of Corrosion of Iron and Nickel by Aqua Regia as Function of Its Composition and Time of Mixture CHARLESF. BONILLA, Chemical Engineering Laboratories, Columbia University, N e w York, N . Y. a c t i o n on nickel t h a n had INCE the eighth century, T H E BEST C O N D I T I O N S for the rapid that of standard composition, when a q u a regia was solution in aqua regia of base metals, as ex4 volumes of h y d r o c h l o r i c first mentioned, accordemplified by iron and nickel, have been detera c i d to 1 of n i t r i c . It w a s in$ to Mellor (Y), much exmined, using ordinary laboratory concentrated also found that the activity of p e r i m e n t a l w o r k h a s been hydrochloric and nitric acids and controlling the the aqua regia as measured by carried out to d e t e r m i n e the t h e r a t e of s o l u t i o n of t h e nature of the reactions which temperature only roughly. For each ratio and m e t a l v a r i e d w i t h the time t a k e p l a c e when it i s pretime of mixing before introduction of the metallic during which the c o m p o n e n t pared and used, and the ratios sample, the time of immersion and loss in weight acids had been mixed. This of t h e c o n s t i t u e n t s which are recorded, the rate qf corrosion being calwould be expected by one who give the best results in dissolvculated and plotted. Ils greatest value is obhas worked with aqua regia and ing metals. Recently Priwozo b s e r v e d t h e color changes nik ( 5 ) , and Hoke and Moore tained at a definite acid ratio and time after which take place after mixing it. (2) determined the best compomixing, the former difering greatly f r o m the Since results of practical use sitions for dissolving gold and 4 volumes of hydrochloric to 1 of nitric widely were desired f o r t h e c h e m i s t p l a t i n u m , r e s p e c t i v e l y , as used f o r dissolving the noble metals. For nickel, who must quickly dissolve sama p p r o x i m a t e l y 4 volumes of 3 volumes of hydrochloric to 40 of nitric, and ples of metals, s t a n d a r d pure hydrochloric acid to 1 of nitric, concentrated laboratory acids for the c o n c e n t r a t e d acids. f o r iron as low-carbon steel, 7 of hydrochloric to were u s e d , a p p r o x i m a t e l y Schmitz (6) studied somewhat 20 of nitric constituted the best mixtures with 70 per cent nitric and 38 per the corrosion of n i c k e l a n d approximately 30 and 12 minutes' mixing, recent hydrochloric acid. These chromium steels by aqua regia, spectively. I n both cases the m a x i m u m rate of a c i d s were d i r e c t l y mixed b u t g a v e practically no data corrosion obtained is about 16 times that obtained from the bottle. The compoon t h i s s u b j e c t . Moore (4) sition of t h e r e s u l t i n g a q u a with a 4 to 1 mixture of hydrochloric and nitric made several suggestions as regia was expressed in terms of to the cause of the activity of acids used at the time of m a x i m u m corrosiverzess. percentage of hydrochloric acid aqua regia. Briner ( 1 ) has exto total acid. the water not a m i n e d the monovariant sgstem of phases with the two components, nitric and hydro- being considered in the calculations. It increased, of course, as the percentage of hydrochloric acid did, In practice, chloric acids (pure), and has shown that the reaction "03 3HC1 KOC1 Clz 2Hz0 is reversible. In all of the excess acid is invariably used, and therefore in these exliterature covered no work with aqua regia of low hydro- periments this was done, and the rate of solution of the metal chloric acid content nor a systematic study of the dissolving assumed approximately constant during each run. In every of base metals by this acid mixture has been mentioned. case the sum of the volumes of hydrochloric and nitric acids used was 50 cc. The acid to be present in larger volume EXPERIMENTAL PROCEDURE was first measured into a 250-cc. beaker, and then, a t a From some rough preliminary experiments it was found time that was recorded, the other acid was added with that aqua regia low in hydrochloric acid had decidedly more gentle stirring. The metal samples measured approxi:
S
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