Determination of Chromium by O x i d a t i o n with Perchloric A c i d S I G M U N D SCHULDINER
AND
B. C L A R D Y , Chemical Laboratory, Norfolk Naval Shipyard, Portsmouth, Va.
FREDERICK
T""'
perchloric acid method (8, S, 4 , 6 ) for the determination of chromium essentially consists of oxidation of chromium to the sexivalent state by fuming perchloric acid. The oxidized chromium is titrated with a standard ferrous ammonium sulfate solution to an o-phenanthroline end point, an electrometric end point, or a permanganate end point by using an excess of reductant and back-titrating with potassium permanganate solution. Xlthough the perchloric acid method of oxidation is widely used, the following sources of error have been observed: 1. Loss of chromium by volatilization as chromyl chloride ( 1 , 6). 2. Reduction of chromium by hydrogen peroxide formed in the reaction flask (which can be eliminated by rapid cooling of flask and contents). 3. Incomplete oxidation of the chromium due to incomplete heating (least serious of the three).
A.
Chromium Evolved
%
%
Chromium Not Evolved
Total Chromiuiii Found
70
5%
Chromium titrated with ferrous ammonium ized with potassium permanganate solution lOlB 18.50 1.07: 18.50 1.81 18.50 0.12 18.50 0.44 121s 18.68 0.07 18.68 I 0.18
5
sulfate solution standard17.45 16.67 18.36 18.05 18.63 18.58
18.52 18.48 18.48 18.49 18.70 18.76
MU. Jug. M g. Mg. 30.0 0.1 29.7 29.8 30.0 0.2 30.0 30.? 30.0 0.0 30.0 30. 30.0 0.1 29.6 29.7 Solutions fumed vigorously, intentionally t o drive off chromium.
I1 indicate that, by recovery of chromyl chloride, chromium can be accurately determined. In the modified procedure careful control of the oxidizing conditions is not necessary. Results shown in Table I, B, were obtained by using hydrogen peroxide to reduce the chromium in a solution of sodium chromate in perchloric acid; the chromium was then reoxidized by the modified procedure. Table I1 shows that, within the limits of experimental error, the chromium was completely oxidized. Hence by use of the apparatus and procedure described the chromium factor of the standard ferrous ammonium sulfate solution can be computed stoichiometrically. The chromium results therefore do not depend on an empirical factor but can be derived theoretically.
After experimenting with vertical air and water condensers, the authors found that the sources of error in the perchloric acid method could be overcome by an arrangement of apparatus as shoir n in Figure 1.
R - AIR CONDENSPR
Soo-rm. BOILING FLASK
Chromium Present
ized with N.B.S. sampIe lOlB (sodium chromate dissolved in perrhloric acid and reduced with H ~ O I )
EXPERIMENTAL
-f
N.B.S. Sample No.
B. Chromium titrated with ferrous ammonium sulfate solution standard-
Evgerienced analysts have various methods of overcoming these errors and obtaining consistently accurate results. This paper presents modifications of the conventional procedures which have been satisfactorily used in this laboratory for over t \ v o years.
JOIW
Table I. Recovery of, Evolved Chromium
PROCEDURE FOR HIGH-CHROMIUM STEELS
Place 0.2 gram of sample in a 500-ml. boiling flask with groundglass connections that can be fitted to an air condenser (Figure 1). Add 10 ml. of hydrochloric acid plus 5 ml. of 1to 1 nitric acid, and heat until sample is dissolved. Then add 25 ml. of perchloric acid, and heat until the perchloric acid just starts to fume. Connect the reaction flask to the air condenser and place the beaker with about 75 ml. of water in position as in Figure 1. Heat until the reaction flask is cleared of fumes and the perchloric acid refluxes down the sides of the,flask. Turn the heater current o f f and let stand for 2 or 3 minutes. Disconnect the reaction flask and connecting tube from the air condenser and cool the flask rapidly, using first a stream of air and then cold run-
?I
N
/I U
~~
Figure I. Apparatus
N.B.S. Sample No.
~~~
~
~~~
Results of Chromium Determinations
Chromium Chromium Weight Present Found Grams % % 121a 0.2 18.68 18.665 0.2 18.68 18.72" 18.68 0.2 18.76" 18.68 18.760 0.2 0.2 18.33 18.37' lOla 18.33 18.380 0.2 0.2 18.50 18.46" lOlB 18.60 18.47a 0.2 18.60 18.66'' 0.2 18.620 0.2 18.60 0,272 0.274, 2.0 111 0.272 0.274b 2.0 0.666 0.658b 72a 2.0 0,655 0.668b 2.0 2.18b 2.16 1.0 116 2.16 2.18b 1.0 a Determined by standardising ferrous ammonium sulfate solution with standard potassium permanganate solution. b Determined by standardiiing ferrous ammonium sulfate solution against N.B.S. samples lOlB and 101.
A 500-ml. boiling flask, connected with ground- lass connections by a U-tube to an air condenser, contained t i e chromium solution and was heated by means of an electric heater. The end of the air condenser was submerged about 0.6 cm. (0.25 inch) in a beaker of water which served to recover any chromyl chloride evolved. The water hydrolyzed the chromyl chloride to chromic acid and hydrochloric acid: Cr02Clz f 2Hz0 = H2Cr0,
~
Table 11.
+ 2HC1
This water solution was titrated and the chromium determined was added to the chromium figure from the reaction flask. The experimental results in Table I show the amount of chromium that can be volatilized during oxidation with perchloric acid. The first and second determinations of part A, where the chromium was oxidized rapidly by intentionally overheating the reaction flask, show that loss of chromium can be appreciable unless oxidizing conditions are carefully controlled. Tables I and
728
November, 1946
ANALYTICAL EDITION
ning tap water. Wash the inside of the connecting tube back into the reaction flask with distilled water and disconnect the tube from the flask. Wash the inside of the air condenser into the beaker of water containing evolved chromium. Pour this solution into the reaction flask. Heat flask and contents until chlorine is completely driven off. Volume should now be about 250 ml. Cool and titrate, following the conventional methods, with standard ferrous ammonium sulfate solution and ferrous o-phenanthroline indicator. The air condenser should be a t least 2.5 cm. (1 inch) in diameter and 60 cm. (2 feet) long in order to guardagainst back-suction of the water into the reaction flask. After the sample is in solution and the perchloric acid solution is taken to fumes on the hot plate, care should be taken not to oxidize the chromium before the reaction flask is connected to the air condenser as in Figure 1. The solution may be heated to a bright green color but should not turn orange before transfer t o the air condenser. All-glass connections must be used in the oxidizing unit; if fuming perchloric acid comes in contact with organic matter such as rubber, a serious explosion or fire may result. The reaction flask should be protected from air currents‘ otherwise the sudden cooling of the flask may cause a suction and some of the water from the water seal may be sprayed into it. A bank of several oxidizing units can be easily arranged if the work load requires it. If vanadium is present in the material analyzed, add an excess of ferrous ammonium sulfate solution and then back-titrate with
729
a potassium permanganate solution, omitting the o-phenanthroline indicator. ACKNOWLEDGMENT
The authors wish to express their appreciation to R. S. Gibbs, principal chemist, and Lt. L. D. Wilson, U.S.N.R., laboratory officer, Norfolk Naval Shipyard, for valuable suggestions offered during their review of this paper. LITERATURE CITED
(1) Hoffman, J. I . , and Lundell, G. E. F., J. Research Natl. Bur. Standards, 22,469 (1939); R P 1198. (2) Lichtin, J. J., IWD. ENG.CHEM., ASAL.ED.,2, 126 (1939). (3) Smith, G. F., “-Mixed Perchloric, Sulfuric and Phosphoric Acids
and Their Applications in Analysis”, p. 4, Columbus, Ohio, G. Frederick Smith Chemical Co., 1934. (4) Smith, G., “Ortho-Phenanthroline”, Columbus, Ohio, G. Frederick Smith Chemical Co., 1934. ( 5 ) U. S. Steel Corp. Chemists, “Sampling and Analysis of Carbon and Alloy Steels”, pp. 138-54, New- York, Reinhold Publishing Corp., 1938. (6) Willard, H: H., and Gibson, H. C., IND.ENG.CHEM., ASAL. ED., 3, 88 (1931).
THEviews presented in this article are those of the writers and are not t o be construed as the official views of the Navy Department.
Apparatus for Quantitative Low-Temperature Vacuum Distillation of Milliliter Volumes W. M O R T O N G R A N T , H o w e Laboratory of Ophthalmology, Harvard Medical School, Boston, Mass.
I’r IS
The material to be distilled is introduced into the bulbous side flask by means of a curved-tip pipet. The tube is then closed by a silicone-greased, one-hole rubber stopper carrying a stopcock for connection to a vacuum pump. The side flask is immersed in a dry ice mixture with the tubular portion horizontal. When the contents are frozen, the apparatus is evacuated, sealed by closing t,he cock, and disconnected from the vacuum line. This whole procedure requires less than a minute. The side flask is then taken from the cold bath and the test-tube portion of the apparatus is immersed in its place, leaving the flask exposed. Distillat,ion is allowed to proceed to completion without artificial heating. Four tubes of the dimensions specified fit conveniently into the mouth of a 0.5-liter Dewar flask. The apparatus was evacuated by means of a Welch two-stage duo-seal pump which readily reduced the pressure in an empty flask to approximately 1 micron, as judged by the faint bluegray color produced by a Tesla coil discharge.
the case of whole blood the degassing is facilitated by the presence of a small drop of xylene.. With a single evacuation, blood, plasma, and urine samples have been distilled a t the rate of 0.5 to 1.0 ml. per hour without artificial heating. Comparative distillations made with double evacuation showed an increase in rate of approximately 2570 for plasma and saline. Thus, where every saving in time is desirable, the extra manipulation appears warranted. The quantitative nature of distillation in this apparatus was determined by analysis of distillates from sulfosalicylic acid filtrates of blood to which known amounts of formic acid and methyl alcohol had been added. The distillation of a formaldehyde solution was also studied. Recoveries of formic acid in amounts of 30 t o 1000 micrograms per ml. of blood and of methyl alcohol in amounts of 150 to 3000 micrograms per ml. of blood averaged 98 and lOl%, respectively, for 9 determinationq of each. Solutions containing 1.2 and 3.6 micrograms of formaldehyde per ml. of a mixture of 3Y0 sulfosalicylic acid a n d 1% sulfuric acid yielded 1.2 and 3.6 micrograms per ml. of distillate. In separate experiments it was found that distillation must be carried to completion in order to obtain the same relative concentrations of volatile constituents in the distillate as in t,he original sample.
With aqueous samples frozen in dry ice, the vacuum appears to be limited by the vapor pressure of the water. During distillation, gases which are trapped in the sample are released and have an appreciable influence on the rate of distillation. Some of this interfering gas can be eliminated by a second evacuation of the apparatus after melting and refreezing the sample. I n
T h e a u t h o r wishes to acknowledge the assistarice of Helen E. Pentz in devising and testing thi. tipparatus.
frequently desirable to use lon--temperature vacuum distillation for quantitative separation of such substances as alcohols, lower fatty acids, and deuterium oxide from nonvolatile materials in small samples. However, this process is inconveniently slow when apparatus incorporates long or complex distillation paths to prevent contamination of the distillate from foaming of viscous solutions, especially those containing protein. The distillation apparatus shown in the accompanying figure was designed t o provide a short distillation path for rapid distillation, while minimizing the chance of contamination from foaming. After trial of several variations in the critical dimensions, this apparatus, which was constructed from parts of a 50-ml. Florence flask and a 25 X 150 mm. test tube, was considered to be of optimum size for the distillation of 1- to 3-ml. samples.
ACKNOWLEDGMENT
