titration with methyl violet as indicator is well suited. Table IV gives suitable sample sizes to cor’er the range of acids and nitrogen compounds in using the titration flask shoTTn in ~i~~~~ 1, ~h~ flask can be
(2) (3) (4)
used for other color-indicator methods, provided the free space in the neck (about 10 nil.) is not exceeded. LITERATURE CITED
(1) Am. Soc. Testing hIateriale, Standards on Petroleum Products and
(5)
Lubricants, D 974-55T; D 664 (1955). Clark, J. R., ]Tang, S. M., ANAL. CHEW26, 1230 (1954). Deal, V. Z., Weiss, T. T., White, T. T., Zbid., 25,426 (1953). Fanale, D. T., Fricioni, R. B., Hutton, D. IT., Snyder, R. E., Clark, R. O., Twentieth Mid-Year Meeting, Division of Refining, American Petroleum Institute, May 10, 1955, St. Louis, &lo. Fritz, J. S., “Acid-Base Titrations in Nonaqueous Solvents, G. Frederick Smith Chemical Co., Columbus, Ohio, 1952.
(6) Kukin, I., ANAL. CHEM. 29, 461 (1957). (7) Moore, R. T., McCutchan, P., Young, D. A., Zbid., 23, 1639 (1951). (8) Pozefsky, A,, Kukin, I., Zbid., 27, 1466 (1955). (9) Tucker, E. B., Section A, RD VI, ASTM Committee D-2, “Indicators for Neutralization Numbers,” D 974, Houston, Tex., February 1955.
RECEIVED for review December 20, 1956. Accepted February 13, 1958. Seventh Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 28, 1956.
Determination of Trivalent Chromium in the Presence of Chromate R.#W. CLINE’, R. E. SIMMONS, and W. R. ROSSMASSLER Union Carbide Nuclear Co., A Division of Union Carbide Corp., Paducah Planf, Paducah, Ky.
,A method i s given for the determination of small amounts o f trivalent chromium in water containing approximately 300 p.p.m. o f chromate. The chromium(lll) i s separated from chromate b y precipitating with ammonium hydroxide using aluminum hydroxide as a carrier. The chromium(lll) i s then oxidized to chromate with bromine and determined spectrophotometrically. Reproducibility i s good.
A
DEVELOPED, the method determines chromium(II1) in the range from 0 to 0.5 p.p.m., but this range could be extended. The principal problem involves the separation of chromium(II1) from much larger quantities of chromate (300 p.p.m.). The chromium(II1) may then be oxidized and determined colorimetrically by well known methods. The method of separation \vas adapted from an analytical procedure ( J ) , which described the determination of trace quantities of chromium in lithium salts and utilized an aluminum hydroxide carrier for separation of the chromium from the lithium. As adapted by the authors, the separation consists of a double precipitation of chromium hydroxide with an aluminum hydroxide carrier. The hydroxide precipitate is dissolved in sulfuric acid and diluted to 25 ml. with water. Two 10-ml. portions of this solution are then aliquoted. The chromium in one portion is oxidized to chromate with bromihe. The excess bromine is destroyed with phenol. This portion of the
s
Present address, U. S. Army, Fort Detrich, Md.
sample along with the unoxidized portion is then colored with diphenylcarbazide. The chromium content. of each is determined spectrophotometrically a t a wave length of 540 mp by comparison with a standard curve. The difference in chromium content of the two portions is due to the chromium(II1) in the sample. The chromium present as chromate in the unoxidized portion contributes not more than 0.03 p.p.m. to the analysis. This amount is usually insignificant; however, this step serves as a check on the separation efficiency of the analysis. EXPERIMENTAL
A curve conforming to Beer’s law was obtained by carrying standard solutions of chromium(II1) through the entire procedure. The standard solutions contained 0.1, 0.2, 0.3, 0.4, and 0.5 p.p.m. of chromium(II1). Transmittance measurements of the colored solutions were made in 5-cm. cells, using the Beckman spectrophotometer a t a wave length of 540 mp and a slit m-idth of 0.03 mm. Apparatus and Reagents. Color reagent. Dissolve 0.15 gram of diphenylcarbazide in 25 ml. of acetone and dilute t o 50 ml. with water. Prepare daily. Spectrophotometer, Beckman Model DU, %em. cells. Procedure. Pipet a 10-ml. sample into a 15-ml. centrifuge tube. Add 1 ml. of 1% aluminum nitrate solution, and precipitate the aluminum hydroxide with ammonium hydroxide. Centrifuge, and then wash the precipitate with 1 ml. of water. Dissolve the
Table I. Determination o f Chromium(lll)
Cr(II1) Pre‘sent, P.P. M , 0.1 0.2 0.3 0.4
0.5 0.6
Cr(II1) Found, P.P.M. 0.13,O. 16 0.19,0.18,0.22 0.28,0.28,0.28 0.39,0.38,0.40,0.40,0.40 0.53,0.60,0.55
0.57,0.63
precipitate in 4 drops of sulfuric acid, dilute to 10 ml., and reprecipitate as before. Again dissolve the precipitate in acid and dilute t o 25 ml. Take two 10-ml. aliquots of this solution, reserve one, and proceed with the oxidation of the other. Add 30 drops of bromine water. Then fade the bromine color to a faint yellow by adding 10M sodium hydroxide dropwise, finally adding 5 drops in excess ( 3 ) . Add more bromine, if necessary, to obtain the faint yellow color. Heat to 90’ to 95’ C. for 15 minutes, then add 1 to 5 sulfuric acid until the bromine color reappears, adding 2 drops in excess. Boil to remove the bromine. Cool, and add 4 drops of 1 to 5 sulfuric acid and 1drop of phenol. To both the oxidized aliquot and the reserved aliquot, add 1 ml. of the color reagent, dilute t o 25 ml., and determine their transmittance against a water blank a t 540 mp. The difference in two aliquots represents the chromium(111) in the sample. RESULTS A N D DISCUSSION
Although this method was developed for the determination of chromium(II1) in high chromate water, it may be apVOL. 30, NO. 6, JUNE 1958
11 17
plied to other types of samples. The results obtained by carrying standard solutions of chromium(II1) containing 250 p.p.m. of chromate through the procedure are given in Table I. Reproducibility is good. Recovery is essentially complete, as indicated by comparison of the standard curve with one prepared from standard potassium dichromate. The procedure has been used successfully for the determination of chromium(II1) in water treated with relatively large amounts of sodium chro-
mate. In this procedure, the chromium is oxidized by the alkalinebromate method ( 1 ) . Other possible oxidation steps which were not attempted include permanganate ( 2 ) or perchloric acid (3).
p. 93, hlcGrav-Hill, New York 1944.
(3) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., p. 269, Van Nostrand, New
York, 1949. (4) Union Carbide Kuclear Co., ‘‘Determination of Chromium In Lithium Salts,” unpublished.
LITERATURE CITED
(1) American Public Health iissociation, “Standard Methods For ?amination of Water and Sewage, 9th ed., p. 56, 1946. (2) Snell, F. D., Biffen, F. hI., “Commercial Methods of Analysis,” l e t ed.,
RECEITED for review May 10, 1957. Accepted February 21, 1958. Work carried out under Contract KO.W-7405-eng-26 at the Paducah Plant operated by Union Carbide Nuclear Co. , A Division of Union Carbide Corporation, for the ‘c. S.Atomic Energy Commission.
Determination of SiIanoI with Grignard Reagent F. 0.GUENTHER’ General Elecfric Research laborafory, The Knolls, Schenecfady,
b The use of methylmagnesium iodide was investigated for the gasometric determination of the silanol group. A solution of the sample was added to 2N Grignard reagent using butyl ether as solvent and methane as the A variety of samples, inert gas. including commercial silicone resins, was successfully analyzed a t room temperature. It was possible to complete 12 analyses in an 8-hour day. A least squares analysis of the data indicated that the absolute error, independent of the gas volume, is & 0.2 ml., 95% confidence limits.
A
for the determination of silanol was needed to aid in the identification of hydrolysis products from alkoxysilnnes ( I d ) . Karl Fischer reagent m-j,s considered because of the excellent results with simple silanols reported by Gilman and Miller (4) and Grubb (6). However, these hydrolysis products, like silicone resins, react too slowly (6). A gasometric method using a butyl ether solution of lithium aluminum hydride was also evaluated. Reaction was rapid and complete at room temperature with simple as well as complex silanols, provided the samples were added in solution. However, cleavage of siloxane bonds with resulting formation of silanes limits this reagent to samples without siloxane bonds or to samples which give silanes of lorn vapor pressure. As Grignard reagent was used to analyze silanols (9),a procedure, essentially that of Fuchs, Ishler, and Sandhoff (S), was evaluated. The apparaMETHOD
1 Present address, Major Appliance Laboratories, General Electric Co., Louisville, Icy.
1 1 18
ANALYTICAL CHEMISTRY
N. Y.
tus in Figure 1was used, but the sample, contained in a steel cup (1/4 X 1 inch), was introduced magnetically from the sidearm of the reaction flask into the Grignard reagent under a nitrogen atmosphere. Some samples reacted so slowly at room temperature that extended heating periods were necessary. Not only was considerable time involved (1 to 1.5 hours), but accuracy was low (&lo%), partially caused by the solubility of methane. These difficulties were overcome by adding a solution of the sample to 2N Grignard reagent a t room temperature, using butyl ether as the solvent and methane as the inert gas. Wright gives a complete background on the use of organometallic compounds for the determination of active hydrogen (13). MATERIALS, REFERENCE SAMPLES, AND APPARATUS
DI-%-BUTYL ETHER (Eastman Kodak Co.) was purified by refluxing over sodium with stirring for 7 hours under a slow nitrogen stream. The ether was filtered and distilled over sodium, and the process repeated with the distillate. The purified ether was stored in brown bottles in a nitrogen atmosphere over sodium wire. GRIGNARDREAGEKT.A solution of 2N methylmagnesium iodide ( I O ) in dibutyl ether was prepared and the concentration determined. Khen stored under nitrogen in glass-stoppered bottles which were well greased, the reagent was stable for a t least several months. GASES. Line nitrogen (dew point -60’ C.) and CP methane (Matheson) were used. SILANOLS. Table I gives the preparation and constants of these com-
a pounds. It is assumed that all silanols were 100% pure. A diagram of the apparatus appears in Figure 1. The volumetric portion was essentially that of Fuchs, Ishler, and Sandhoff (S), except that a water jacket was added to the buret. The reaction flask was designed to permit addition of an aliquot of a sample in solution. A 4-ml. pipet, B, was adapted for sample addition by sealing a stopcock to the upper end and attaching a standard taper joint by means of a ring seal 3 inches from the delivery tip. The pipet was found to deliver 81.937, of the 5-ml. volumetric flask used to prepare the solution of sample. A 5-ml. volumetric pipet, A , was used for adding butyl ether to the sample and Grignard reagent to the reaction flask. Prior to reading the buret, the bulb of mercury was placed in a position approximately level with the mercury in the buret. Stopcock 111, ‘to the dibutyl phthalate manometer, was then opened and final leveling mas accomplished with the screw adjustment on the leveling bulb support. This combination made it possible to take a reading in 5 to 10 seconds xith a reproducibility to 0.02 ml. The temperature of the water bath was adjusted to that of the buret and held constant by the addition of small pieces of ice. This compensated for the heat given off by the rheostat in the magnetic stirrer apparatus. All joints and stopcocks were lubricated with Celvacene medium vacuum grease (Consolidated Vacuum Corp.). Steel springs Rere used to hold joints together. PROCEDURE
Standard Procedure. Pipets A and B and the reaction flask were rinsed with reagent grade acetone and dried in a stream of nitrogen. The reaction flask, with its side arm
