INDUSTRIAL AND ENGINEERING CHEMISTRY
38
TABLEI. SOLUBILITY OF CARBON DIOXIDE IN SALT SOLUTION SA~T
(Temperature 25O C., total pressure 783.5 mm.) CONVOLUMEOF CARBON DIOXIDE BUNSEN
USED
OlNTRATION
WeQht % None Has04 HB0r NaCl NaCl NaCl His04 NaCl -*m. NrtCl nu NaaSOc NaaSOc Ha604 NaaPOd HsPO4 CaClz MgCL ZnCh AlCla A h (804)I 0 Milliliters of 25' C.
*.
5 10 10 20 20 5
SOLVTION
DIBSOLVED
M1.
M1.
24.54 24.54 24.54 24.54 49.54 49.54
20.20 18.30 17.60 13.52 16.54 16.29
COEFFICIENT
Ml./ml.a soh. 0.823 0.746 0.717 0.551 0.334 0.328
25 49.54 12.75 49.54 17.48 20 1 20 49.54 12.9 20 13.07 49.54 5 (vol.) 13.89 10 49.54 7 49.54 7.82 40 30 49.54 8.12 19.12 50 49.54 25 49.54 12.00 20 49.54 10.96 COS at 25" C.,760 mm., dissolved
0.754 0.683 0.657 0.505 0.306 0.302
0.257 0.352
0.236 0.323
0.260 0.263
0.237 0.242
0.280
0.256
0.158 0.164 0.386 0.243 0.221 per ml. of
0.144 0.150 0.354 0.222 0.203 solution at
DISCUSSIONOF RESULTS From Table I it is seen that the concentrated solution of calcium chloride dissolves the least carbon dioxide. However, this has three disadvantages as a confining liquid: the solution is quite viscous, it seems to become easily contaminated with impurities and precipitate, and when spilled it does not evaporate and leave crystals but remains as a sticky fluid. The solution may be satisfactory for a confining liquid in gas storage containers, but does not seem suitable for use in the gas analysis apparatus. The magnesium chloride solution is much less viscous than the calcium chloride solution. The solubility of carbon dioxide is almost equal in the calcium and magnesium chloride solutions, but is much lower than for the sodium chloride or sodium sulfate soIutions. Its use in the gas analysis apparatus has the disadvantage that whenever the concentrated
Vol. 7, No. 1
hydroxide absorbents come into contact with the magnesium chloride, even though acidified, a precipitate of magnesium hydroxide is formed, which plugs up the capillary tube. The use of magnesium chloride is recommended when this contact with alkalies is not possible. The solubilities in 20 per cent sodium sulfate and 25 per cent sodium chloride are nearly equal; however, the 25 per cent sodium chloride is more nearly saturated than the 20 per cent sodium sulfate, so the latter is recommended. The addition of 5 per cent by weight of sulfuric acid to 20 per cent sodium chloride and 5 per cent by volume of sulfuric acid to 20 per cent sodium sulfate has been shown to produce a negligible change in the solubility of carbon dioxide in these solutions. As a confining solution must never be alkaline, the addition of acid is necessary, and no error is introduced as suggested by Hoffmann (2). CONCLUSIONS A 20 per cent by weight sodium sulfate and 5 per cent by volume sulfuric acid solution is recommended as the most practical confining liquid for use in technical gas analysis equipment. The addition of acid to sodium sulfate or sodium chloride solutions causes no appreciable increase in the solubility of carbon dioxide, and is necessary to prevent the solution from becoming alkaline a t any time. LITERATURE CITED (1) Hoffmann, F. G., Feuermngstech., 14,98(1926). (2) Hoffmann, F.G . ,Z . angew. Chem., 39,23(1926).
(3) International Critical Tables, Vol. 3,pp. 260,279. (4) Nahocsy, A.,Bdnydsz. KoMsz. Lapok, 66,332(1933). (5) Passauer, H., Feuermngstech., 19, 142 (1931). (6) Tropsch, H., 2. angew. Chem., 39,401 (1926). (7) U. S. Steel Corp., "Methods for Sampling and Analysis of Gases," Pittsburgh, Carnegie Steel Co., 1927. (8) Wolf, O.,and Krause, Arch. Wtirmewirt., 8, 216 (1927). RECEIVED November 5, 1934.
Determination of Chloride A Modification of the Volhard Method JOHN R. CALDWELL AND HARVEY V. MOYER,Ohio State University, Columbus, Ohio HE Volhard method for the determination of chloride has been subjected to many modifications. Kolthoff (8) in a review of the subject points out the fundamental errors in several of the proposed methods and concludes that only one, the method of Schoorl (4,gives accurate results. This procedure is somewhat tedious, and since it requires special precautions it is limited in application. The method in general use at the present time requires the removal of the precipitated silver chloride by filtration before back-titrating with potassium thiocyanate. A modification is desirable which eliminates this filtration. Recently Stschigol (6) has proposed covering the chloride solution with a layer of toluene or benzene. These immiscible liquids cause the silver chloride to be drawn to the interface and thus remove it from the aqueous solution. This principle was suggested earlier by Rothmund and Burgstaller (4, but the method has certain disadvantages as shown by Kolthoff (5). Furthermore, the authors have observed that the end point is partially obscured because the precipitated silver chloride turns dark more rapidly in the presence of these organic liquids. The principal objection to the Volhard method is the fading end point when the silver chloride is not removed, due to the fact that silver chloride is more soluble than silver thiocya-
T
nate. Kolthoff (3) suggests the subtraction of 0.7 per cent of the percentage of chloride found in order to correct for adsorption of silver nitrate on the silver chloride and on the silver thiocyanate. In this paper a modification is proposed which reduces the errors mentioned above to negligible values and considerably shortens the time required for a determination. Repeated trials in the hands of several operators have given accurate results with errors no greater than the probable errors in reading the volumetric apparatus. In view of the successful use by Caldwell of the addition of an organic substance to improve the end point in the iodometric determination of copper (I), this principIe was applied to the Volhard titration. It was found that nitrobenzene had the desired properties. The experimental evidence seems to indicate that in the presence of nitrobenzene no appreciable amount of silver nitrate is carried down and that the nitrobenzene forms an insoluble layer over the precipitate, so that the rate of solution of the silver chloride is reduced to such an extent that it does not interfere in the thiocyanate titration. EXPERIMENTAL SILVERNITRATE. A 0.1 N solution of silver nitrate was prepared by dissolving the appropriate weight of reagent silver
anuary 15, 1933
ANALYTICAL EDITION
nitrate which had been recrystallized twice and dried at 70” C. in a vacuum desiccator over sulfuric acid. POTASSIUM THIOCYANATE. An approximately 0.05 N solution was prepared and standardized against the silver nitrate solution. SODIUMCHLORIDE,This was prepared by precipitating a sample of reagent quality sodium chloride three times from aqueous solution with concentrated hydrochloric acid. It was dried at dull redness for 30 minutes. NITROBENZENE. A reagent grade gave no precipitate with alcoholic silver nitrate and was used without further purification. FERRICALUM INDICATOR. Concentrated, freshly boiled nitric acid was added to a saturated solution of ferric alum until the solution became greenish yellow. Titrations were made in 250-ml. glass-stoppered bottles.
A solution (25 to 50 ml.) containing from 0.0483 to 0.2606 gram of sodium chloride, free from the usual interfering ions, was acidified with 8 to 10 drops of concentrated nitric acid and 1 ml. of nitrobenzene was added for each 0.05 gram of chloride. Standard silver nitrate was added until an excess of 1 to 4 ml. of 0.1 N solution was present. The bottle was then tightly
stoppered and shaken vigorously until the silver chloride settled out in large spongy flakes. Usually 30 to 40 seconds’ agitation was required. It was found that a perfectly clear supernatant solution mas not necessary. Fine droplets of nitrobenzene were often left in suspension. However, nearly all the nitrobenzene seemed so closely attached to the silver chloride that there was little evidence of it as a separate phase. One milliliter of ferric alum indicator was added and the titration completed with standard thiocyanate solution. The ferric alum acted as an effective flocculating agent and coagulated any suspended matter which was present. Standard potassium thiocyanate solution was added slowly with gentle swirling until a pink color was produced. Usually a false end point appeared one drop before the true end point; it faded in about 30 seconds and may have been due to the desorption of the la& traces of silver nitrate from the preci itate. The next drop of thiocyanate produced a decided copor change which persisted 10 to 15 minutes. Titrations should be made a t temperatures below 25” C., as is customary in other titrations with thiocyanate.
Samples of pure sodium chloride were dissolved and titrations were made according to the method just described. The results are shown in Table I. TABLEI. RESULTSOF TESTANALYSES SODIUM CHLORIDE FOUND
Gram
Qram
0.0483 Q . 0486 0.0491 0.0492 0.1005 0.1011 0.1011 0.1132 0.1210 0.2006 0.2211 0.2508 0.2518 0.2605 0.2606
0.0484 0.0487 0.0489 0.0491 0.1003 0.1013 0.1010 0.1132 0.1209 0.2004 0.2207 0.2506 0.2520 0.2605 0.2607
liquids. A very small volume is required, and since it is heavier than water it does not form a troublesome layer over the aqueous solution. Most of it attaches itself t o the silver chloride, so that its presence is hardly noticed in the subsequent thiocyanate titration. Divalent ions, such as calcium and barium, have no apparent effect on the functioning of the nitrobenzene. The method was used with success by 0. C. Dermer in this laboratory for the titration of solutions containing piperidine hydrochloride.
LITERATURE CITED
PROCEDURE
SODIUM CHLORIDE TAKEN
39
DIFFERBNCE Gram
+o .0001
+0.0001 -0.0002 -0.0001 -0.0002 +o ,0002
-0.0001 0.0000
-n nnni
-0.0002 -0.0004 -0.0002 +o ,0002 0.0000
+0.0001
The method was checked by another operator who used a standard silver nitrate solution prepared with specially purified silver supplied by C. W. Foulk. The silver was prepared by Foulk and Pappenhagen (2) in a study of silver as a n ultimate standard in acidimetry. It was dissolved in nitric acid a n d the solution diluted to a definite volume, so that the final strength was approximately 0.1 N . Several titrations were made with this solution with results equally as good as those shown in Table I.
DISCUSSION Nitrobenzene, as used in this determination, exhibits the interesting property of inhibiting the darkening of silver chloride in light and this also improves the end point. In several other respects it is superior to other proposed organic
(1) Caldwell, J. R., accepted for publication in J. Am. Chem. Soc.
(2) Foulk, C. W., and Pappenhagen, L. A,, IND. ENQ.CHEM.,Anal. Ed., 6, 430 (1934). (3) Kolthoff, I. M , “Volumetric Analysis,” Vol. 11, p. 227, New York, John Wiley Sons, 1929; 2. anal. Chern., 56,568 (1917). (4) Rothmund, V., and Burgstaller, A., 2. anorg. Chem., 63, 330 (1909). ( 5 ) Schoorl, N., Pharm. Weekblad, 42, 233 (1905). (6) Stschigol, M. B.. 2. anal. Chem., 91, 182 (1932). RECEIVEDOctober 11, 1934. Presented before the Division of Physical and Inorganic Chemistry at the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
Determination of Perchlorates M. L. NICHOLS, Cornel1 University, Ithaca, N. Y. ELLOR states (2) that “Williams (4) found that the perchlorates are reduced by titanium trichloride, while the chlorates are not affected, and based a process for the volumetric determination of perchlorates-in the presence of chlorates and chlorides-on this reaction.” However, he also states (3) that “chlorates and perchlorates are reduced to chlorides’’ by titanium trichloride. Knecht and Hibbert (1) give directions for the determination of both chlorates and perchlorates with titanous chloride. The chlorates are quantitatively reduced in the cold while the perchlorates are not appreciably reduced in dilute aqueous solution, even on prolonged boiling. The perchlorates are completely reduced, however, by a strong solution of titanous chloride in a fairly strong sulfuric acid solution. The two statements of Mellor and that of Knecht and Hibbert do not agree, and a closer investigation of the work of Williams shows that he boiled weighed quantities of potassium chlorate and ammonium perchlorate with an excess of titanous chloride in the presence of sulfuric acid and then titrated the excess of titanous chloride with ferric alum. This should reduce both the perchlorate and chlorate and he states in his results that “the table below gives results (for perchlorate) after the chlorate had been allowed for.” He, therefore, undoubtedly did not find that the chlorates were unaffected by titanous chloride, as stated by Mellor, but corrected his results for the known amount of chlorate present. LITERATURECITED (1) Knecht and Hibbert, “New Reduction Methods in Volumetric Analysis,” pp. 5 , 25, London, Longmans, Green & Co., 1925. (2) Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 11, p. 381, London, Longmans, Green & Co., 1922. (3) Mellor,Ibid., Vol. VII, p. 78, 1927. (4) Williams, J. G., Chem. News, 119,s (1919). RECEIVED December 4, 1934.
