Percentage of reaction Figure 1. Zones of color change of warning indicators shown on cerium(1V)-iron(1l) titration curve
a. N-Methyldiphenylaminesulfonicacid h. 3,4,7,8-Tetramethyl-l,lO-phenanthroline ferrous sulfate c.
Diphenylaminesulfonic acid
ferroin showed a warning zone closer
ta the end point than the preceding indicator. It started when 98.170 of the iron was oxidized. This indicator, however, was unsuitable because its color change is practically the same as that of the ferroin, the reddish orange colors of the reduced forms of both indicators being almost identical. The diphenylaminesulfonic acid, the only one of the three indicators commercially available, gave the warning zone closest to the end point. It starts when 99.1% of the iron is oxidized and reaches its end at 99.94%-i.e., just short of the end point and the abrupt color change of the ferroin. I t s color change does not interfere with that of
the main indicator. It goes from colorless to dark violet, whereas the ferroin changes from reddish orange to a pale blue which is almost colorless. The color change of the first indicator is distinct in the presence of the orange color of the ferroin, the intermediate mixed color being a dark wine-red. The whole range of the combined color changes goes thus from reddish orange over dark wine-red to violet, and the latter change is abrupt and easily perceptible. The warning actually starts even earlier than the first persistent coloration by the warning indicator, because the temporary color changes prior to the persistent one are also distinct. The warning thus develops
gradually and extends over a t least 1 to 2% of the total volume of titrant solution. Results were identical with the sodium and barium salts of the indicator. The diphenylaminesulfonic acid sodium salt is suggested as an auxiliary or warning indicator in the proportion of 2 to 3 drops of 0.01iM solution (270 mg. of sodium salt per 100 ml. of water) for each 100 ml. of titrated solution, together with 1 drop of 0.01M solution of ferroin. It can be easily calculated that this additional amount of indicator does not significantly affect the accuracy of the titration. \Then the titration is done as described above-Le., to reach a total volume of almost 300 ml. a t the end point-6 to 8 drops or about 0.3 ml. of warning indicator will have been used. As this quantity requires 0.06 ml. of 0.1N cerate solution for its color change, the error due to the warning indicator amounts to 1 part in 667 parts or to 0.15%. The error caused by the ferroin is about one sixth of this value according to the proportion in which the two indicators are used. The combined indicator error amounts thus to approximately 0.18%. For work of high accuracy this error can be taken into account. LITERATURE CITED
Brandt, W. K., Smith, G. F., ANAL. CHEM.21, 1313-19 (1949). Diehl, H., Smith, G. F., “Quantitative Analysis,” p. 249, Wiley, Kew York, 1952. (3) Ibid. , p. 275. (4) Hume, D. Ii., Kolthoff, I. M., J . Am. Chem. SOC.65, 1895-7 (1943). Knop, J., Kubelkova-Knopova, O., 2.anal. Chem. 122, 183-201 (1941). Sarver, L. A., Kolthoff, I. M., J. Am. Chem. Soc. 53, 2903 (1931). Smith, G. F., “Cerate Oxidimetry,” p. 22, G. Frederick Smith Chemical Co., Columbus, Ohio, 1942. Ibid., p. 25. RECEIVEDfor review August 6, 1956. Accepted January 31, 1957.
Determination of Sulfur in Nickel by the Evolution Method C. L. LUKE Bell Telephone Laboratories, Inc., Murray Hill,
b The evolution method for the determination of sulfur in nickel has been improved by using platinic chloride to accelerate the solution of the metal in hydrochloric acid.
A
method for the determination of sulfur in nickel is needed in certain phases of vacuum tube developRAPID
N. J.
ment. Attempts to develop a n oxygen combustion-iodometric titration method using a n induction furnace with automatic sulfur determinator were not successful because no way was found to eliminate the high and variable blanks due to the ceramic crucibles. I n considering other possible methods, it seemed probable that an evolution
method might be used, if means were found for speeding up the solution of nickel in hydrochloric acid and it could be demonstrated that all the sulfur in nickel is present as sulfide (1). Dissolution of nickel in hydrochloric acid can be greatly accelerated by the addition of a small amount of platinic chloride. During the dissolution VOL. 29, NO. 8, AUGUST 1957
1227
platinum is deposited as platinum black on the nickel, thus forming electrolytic couples in which the platinum is cathodic. Because of the low overvoltage of hydrogen on platinum black, dissolution of the nickel is very rapid. Thus. 1 gram of a reasonably well subdivided sample of nickel, that would normally require about a n hour for dissolution, can be dissolved in lees than 15 minutes. (Curiously enough, accelerated dissolution can also be accomplished by the addition of a small amount of hypophosphorus acid to the hydrochloric acid, but a black insoluble residue of unknown composition is formed.) T o show that all the sulfur in nickel is present as sulfide, a number of different types of nickel were analyzed by the proposed evolution method and by a n independent method which measures the total sulfur content regardless of state of oxidation. The method for the determination of total sulfur was similar to the author’s photometric method (2), but hydrogen sulfide was evolved and titrated iodometrically as recommended in the proposed evolution method. The validity of the total sulfur method was established by showing that results were quantitative when aliquots of a standard solution of potassium sulfate were carried through the analysis for sulfur. The results obtained by the proposed evolution method and by the independent method are shown in Table I. Agreement between the two methods is excellent. This indicates that the sulfur in the nickels analyzed is all present as acidsoluble sulfide and that the proposed method gives correct results. -4s a n additional test to prove that the conditions for evolution and titration used in the proposed method are satisfactory, a 0.5-gram sample of Bureau of Standards standard sample of ingot iron S o . 55-d was analyzed for sulfur. A value of 0.013% sulfur m s obtained, which checks the recommended value of 0.013% sulfur. APPARATUS
The distillation apparatus recommended is shown in Figure 1. REAGENTS
AMMONIACAL ZINC SULFATESOLVDissolve 50 grams of zinc sulfate heptahydrate in 250 ml. of water in a 500-ml. borosilicate glass flask. Add 250 i d . of ammonium hydroxide and mix. Stopper and alloi7 to stand orernight. Filter into a 500-nil. borosilicate glass flask. Keep stoppered a t all times when not in use in order to prevent loss of ammonia. P L a T I X I C CHLORIDE SOLUTIOS, 1%. Dissolve 1 gram of platinic chloride in TIOS.
1228
ANALYTICAL CHEMISTRY
Table I.
Determination of Sulfur in Various Samples of Nickel
% Sulfur Found Sample Nivac Pure nickel Grade A nickel Cathalloy A-30“ Inco 225“ Inco 220 (melt 95). Sample 999. Sample 999*
Independent method
PropoEed method 0 0 0 0 0 0 0 0
0005 0015 0025 0025 0030 0025 002i 0027
0.0004 0,0016 0.0026 0.0023 0.0030 0,0023 0.0026
Somewhat similar in composition to Grade A nickel. flow of nitrogen used to sweep out hydrogen sulfide during evolution was 50 rather than 30 ml./minute. a
* Rate of
PROCEDURE
Transfer 1.000 gram of the subdivided sample to a 50-ml. standard taper conical flask. Add 15 ml. of hydrochloric acid-platinic chloride solution, cap n i t h the distilling head, and dip the side arm into 30 ml. of water plus 3 ml. of ammoniacal zinc sulfate solution contained in a second 5O-ni1. flask. Place the distilling flask on a 150’ C. hot plate with the receiving flask resting in a cool r\-ater bath (Figure 1). Pass a gentle stream of nitrogen or hydrogen (ca. 30 ml. per minute) through the solution. If the sample has not been adequately subdivided and the time required for solution of the sample is excessive-e.g., over 30 niin2 ml. of the ammoniacal utes-add zinc sulfate solution to the receiving flask. Finally, when solution of the sample is complete, allon the distillation to continue for 1 minute and then remove the receiving flask. Vithout delay add 1 ml. of 1% potassium iodide solution, l ml. of 1% starch solution, and 5 ml. of hydroWOODEY BLOCK HOT PLATE l), swirling after chloric acid (1 each addition. Titrate immediately Figure 1. Distillation apparatus nith standard potassium iodate solution. added from a 10-nil. buret, until the blue starch-iodine color shows signs of persisting. Cap the flask, shake vigorously to recover any hydrogen sulfide in the neck of the flask, 100 ml. of hydrochloric acid. (The and then continue the titration dropplatinic chloride sample used in this wise until the first permanent blue work contained 37.68% of platinum.) HYDROCHLORIC ACID - PLATINIC color appears in the solution. If the blue starch-iodine color does not appear CHLORIDESOLUTION.Mix 2.5 nil. of momentarily on adding the first drop 1% platinic chloride solution nith 500 of potassium iodate solution, the soluml. of hydrochloric acid (1 1). tion is not sufficiently acid and a few POTASSIUM IODIDE SOLUTIOS, 1%. 1) milliliters of hydrochloric acid (1 Dissolve 1 gram of potassium iodide must be added before proceeding. Durin 100 nil. of nater. Prepare fresh if ing the titration a pink color may be any iodine appears in the solution. produced iii the solution, but this color STARCH SOLUTIOS,1%. Pour a slurry should be ignored and the titration of 5 grams of “soluble” starch in 35 nil. continued until a permanent blue or of formamide a t room temperature into purple color is obtained. 65 ml. of 100” to 110’ formaniide. Stir until solution is complete. Cool LITERATURE CITED to room temperature. Mix this solution with 400 ml. of formamide a t room (1) Am. SOC.Testing Materials, “ASTRI Methods of Chemical Analysis of temperature. Metals,” p. 235, 1956. STAKDARD POTASWJM IODATE SOLU(2) Luke, c. L., -kN.%L. CHEW. 21, 1369 TION, 50 y of sulfur per ml. Dissolve ( 1949). 0.1112 gram of pure dry potassium iodate in water and dilute to 1 liter in a RECEIVED for review January 31, 1957. volumetric flask. Accepted ilpril 5, 1957,
+
+
+
