I,VDUSTRIAL A N D ENGI,VEERING CHEMISTRY
846
Vol. 19, s o . 7
Titanous Chloride for the Determination of- Iron and of Chloric Acid' By Paul S. Brallier NIACARA SMELTING CORPORATION, NIACARA FALLS,Ii. Y.
HE application of titanous chloride to the determination of iron and of chloric acid is not new. A simplified procedure has been devised, however, which is suitable for laboratories where the titanous chloride is to be used only occasionally, and where the setting up and maintenance of the elaborate apparatus usually specified does not seem warranted. For both iron and chloric acid the procedure is very rapid and sufficiently accurate for all ordinary purposes.
T
Historical
4
The use of titanous chloride as a quantitative volumetric reducing agent was apparently first proposed by Knecht and Hibbert about 1903. I n t'he original publication a procedure is described for the estimation of iron, and mention is made of the fact that the reagent may also he applied to the determination of chloric acid, although no definite method is given. Since that time the reagent has been successfully applied to many determinations, and its analytical qualifications have been studied in some detail. It has been found applicable to the estimation of dyes and organic nitro comp0unds,2-~to the determination of ~ o p p e r , ~i r- ~~ n , ~ ,lO-12 ~,'~ chromium in the form of chromate, and vanadium as vanadate,' the higher oxides of lead and manganese,* hydrogen peroside,6 and percarbonates and ~ e r b 0 r a t e s . l ~ I n addition, it has been found useful qualitatively in testing for the noble metals; for tungstates, molybdates, vanadates, selenates, tellurates, chromates; and such acids as formic, acetic, oxalic, succinic, tartaric, citric, lactic, benzoic, salicylic, and tannic.14 General Method
I n spite of these numerous applications, titanous chloride has not come into general use, probably because in the various methods described elaborate precautions were advised to prevent oxidation of the titanous chloride reagent solution during storage and titration. There is no doubt that titanous chloride solutions are easily oxidized by air; but the writer has found it possible to get satisfactory results with very simple apparatus and no other precautions than those of working with all solutions between 20" and 30' C. and keeping the time of the actual titration a t a minimum. The "standardization" of the titanous chloride solution is simple and may be quickly done, so that no attempt has been made to keep the titanous chloride solution a t constant strength, as it is entirely practicable to standardize it for each set of determinations. Presented before the Regional Meeting of the Rochester, Syracuse, Cornell, and Eastern and Western New York Sections of the American Chemical Society, Rochester, N.Y., January 28 and 29, 1927. * Rathsburg, Ber., 64B, 3183 (1921). 3 Knecht, J . Soc. Chem. I n d . , 92, 825 (1903). 4 Knecht and Hibbert, Bcr., 86, 1549 (1903). 5 English, THIS JOURNAL, 12, 994 (1920). 6 Mach and Lederle, J . Chem. SOC.(London), 112, 580 (1917). 7 Monnier, A n n . chim. anal. chim. apBl., 21, 109 (1916). 8 Moser, Chem.-Zlg., 86, 1126 (1912). 0 Rhead, J . Chem. SOC.(London), 89, 1491 (1906). Zink and Liere, J . Gasbel., 57, 956 (1915). '1 Thornton and Chapman, J . A m . Chem. Soc., 48, 91 (1921). l 2 Thornton and Wood, THIS JOURNAL, 19, 150 (1927). I s Moser and Seeling, Z . anal. Chem., 69, 73 (1913). 1 4 Monnier, Ann. chim. anal. chim. appl., 80, 1 (1915). 1
The procedure for both iron and chloric acid consists in pipetting a measured excess of titanous chloride into the solution to be tested, which has been made acid with hydrochloric acid, and titrating the excess titanous ckloride with a ferric ammonium sulfate solution of known strength, using ammonium thiocyanate as indicator. A blank test is run, using the same volume of titanous chloride; and the difference between the volume of ferric alum used in the blank and that used on the sample is the measure of the oxidizing power of the sample. Solutions TITANOUS CHLORIDE-The titanous chloride solution may be made up to any convenient strength. A solution containing 20 to 25 grams per liter titanous chloride and 30 to 40 grams per liter hydrochloric acid has been found most suitable in this laboratory. The titanous chloride usually marketed is in the form of a 15 per cent solution, containing 210 to 225 grams per liter TiC13. A satisfactory analytical solution may be prepared from this by taking one volume of the 15 per cent solution, adding an equal volume of concentrated hydrochloric acid, and diluting with eight volumes of water. This dilute solution is prepared in small quantities as required. FERRIC ALuv-The ferric alum solution is the permanent and reference solution in this procedure. It may be made up to any suitable strength; but should be equivalent to from a half to an equal volume of the dilute titanous chloride solution. I n this laboratory the solution is prepared by taking 50 grams of the crystalline ferric alum and 50 cc. concentrated sulfuric acid for each liter to be prepared. It may be made up in as large quantities as desired. The solution should be free of ferrous iron. This may be assured by adding potassium permanganate solution to the stock ferric alum solution until the first perceptible color change is noted. Ordinarily, only a few drops of permanganate are required per liter. There are various ways in which the ferric alum solution may be standardized, a suitable means being by reduction in a Jones reductor, and titration with carefully standardized 0.1 N permanganate. THIOCYANATE-The thiocyanate indicator is prepared by dissolving 100 grams of ammonium or potassium thiocyanate crystals and diluting to one liter; 5 to 10 cc. are required for each titration. Determination of Iron PROCEDURE-The solution of the sample to be tested is placed in a 250-cc. Erlenmeyer flask. The sample should preferably contain not over 150 mg. of iron for solutions prepared according to the above directions. Ten to fifteen cubic centimeters of concentrated hydrochloric acid and sufficient water are added to make the total volume about 75 cc. Potassium permanganate solution is carefully added until a slight pink coloration develops, and the excess permanganate is then reduced with 5 to 10 cc. of an approximately 0.1 N solution of sodium arsenite. The contents of the flask are thoroughly stirred and allowed to stand for 5 to 10 minutes. The thiocyanate indicator is added, and titanous chloride solution is pipetted in until the red ferric thiocyanate color fades out. For solutions highly colored
July, 1927
I S D r S T R I A L AXD ENGlAVEERIXGCHEMISTRY
with iron 25 cc. of the titanous chloride solution may be required; for less highly colored solutions 10 or 5 cc. may be sufficient. The excess titanous chloride is immediately titrated with ferric alum until a faint reddish color persists in the solution. A blank is then run, in which the same volume of titanous chloride as was used in the determination is titrated with ferric alum. The difference between these titrations, multiplied by the iron equivalent of the ferric alum, giyes the iron content of the sample. It is obvious that, where the ferric content rather than the total iron is required, the procedure would be the same as aboi-e except that no permanganate would be used for preliminary oxidation of ferrous to ferric iron. The writer has used hydrogen peroxide as an alternative preliminary oxidizing agent, adding a large excess to the acid solution, and then decomposing the excess by vigorous boiling for 35 to 45 minutes. This is a less convenient method and, from the results obtained, seems to be open to the additional objection that there is danger of loss of solution by spattering during boiling. DETERhmbTIONS-The procedure described above was used in the following determinations of iron in samples of known iron content. An accurately weighed amount of pure iron wire was dissolved in an excess of (2. P. hydrochloric acid, and the solution so prepared accurately weighed on an analytical balance. Portions of this solution were then accurately weighed out from a Lunge weighing pipet. I n Series A the weight of iron taken for each determination was calculated directly on the basis of the iron wire weighed out. I n Series B the iron in the solution of ferrous chloride used wns determined gral-imetrically as Fe20S, the portions taken for the gravimetric determination being weighed out from the Lunge pipet just as the samples analyzed volumetrically with titanous chloride. The weights used throughout this work were adjusted in accordance with National Bureau of Standards tolerances for Class S weights, but were without a Bureau of Standards certificate. Of the volumetric apparatus employed, only the buret used for measuring the ferric alum and the buret used for permanganate in standardizing the ferric alum were of the precision grade, both being graduated in accordance with Bureau of Standards specifications, but without certificate. Since blank titrations were run on the titanous chloride solution pipetted out, the calibration of the pipet used is not essential. The permanganate and ferric alum solutions were standardized a t 20" C. and the volume of the ferric alum used in the various titrations throughout this work was corrected, when necessary, to this temperature. I n general, the work has been done with apparatus and under conditions which are likely to be found in a small industrial laboratory rather than with equipment favoring the highest degree of accuracy. The results obtained are summarized in Table I. Different ferric alum solutions were used in the two series of determinations; that in Series A was equivalent to 4.924 mg. iron per cubic centimeter, and that in Series B to 5.819 mg. per cubic centimeter. The blanks vary because the determinations were run at different times. The same blank may be used for a considerable number of consecutive titrations. The high errors in Series A, 2 and 5, and Series B, 10, are believed t o be due to a n insufficient amount of arsenite or insufficient time for the arsenite to reduce the chlorine that may have been produced by the action of the excess permanganate on the hydrochloric acid present. The first five determinations in Series B were made with the amount of arsenite and the time of standing recommended in the directions above, and the results are much more consistent than
847
in the remaining determinations where less arsenite ivas used and shorter time allowed. Table I-Iron
No.
IRON
TAKEX Mg.
REAGENT SOLCTIONS USED Ferric Tic13 alum
cc.
cc.
Determinations BLANK
FERRIC ALUM Cc.
IRON
ERROR
FOUND Mg.
Mg.
36.14 42.49 111.77 126.30 168.99
$-0.05 +0.49 -0.23 -0.10 f2.09
0.14 1.15 0.21 0.08 L26
68.49 72.80 99.39 106.37 131.63 124.64 157.93 154.38 205.29 115.33
f0.21 $-0.09 +0.03 -0.12 -0.15 -0.14 -0.31 -0.44 4 - 0 48 f0.57
0.31 0,12 0.03 0.11 0.11 0.11 0.20 0.22 0.23 0.50
Per cent
SERIES A
1 2 3
1 a
36.09 42.00 112.0 126.4 166.9
10 25 25 25
7.97 6.68 17.07 14.12 5,4a
68.28 72.71 99.36 106.49 131,78 124.78 l%24 154.82 204.81 114.76
10 10 10 10 20 20 20 20 30 10
6.83 6.09 1.52 0.32 14.58 7.62 1.90 2.51 8.28 9.22
10
15.31 15.31 39.77 39.77 39.77
SERIES E .~ ._ ~ ~
1 2 3 4
5 6 7 8 9 10
18.60 18.60 18.60 18.60 37.20 29.04 29.04 29.04 43.56 29.04
EFFECTOF SODIUMARSEIiITE-The question has been raised as to whether the arsenite introduced to reduce the slight excess of permanganate might not also reduce some of the ferric salts in the solution. The consistency of the results obtained above seemed to indicate that, under the conditions of the analysis, this reduction did not occur. To demonstrate whether or not this conclusion is warranted, several experiments were run under the same conditions as a regular analysis, in which a known amount of ferric iron was treated with 5 to 10 cc. of 0.155 iV sodium arsenite solution in the presence of 12 to 15 cc. of concentrated hydrochloric acid for varying lengths of time, and the ferric iron then determined as usual. The total volume of the solution during the time of contact with the sodium arsenite was from 70 to 80 cc. The results are summarized in Table 11. Table 11-Effect of Sodium Arsenite on Ferric Salts FERRIC SODIUM TIME FERRIC
EXPT.
IRON
TAKEN
Mg. la b 2a b C
d
3 a b
4a
b
49.24 49.24 49.24 49.24 49.24 49.24 49.24 49.24 49.24 49.24
ARSENITE
OF
USED CONTACT Cc. Mznutes 5 2 5 2 5 30 5 30 30 5 30 5 30 10 10 30 30 10 30 10
IRON
FOUND DIFFEREXCE Mg. Mg. 49.19 49.34 49.19 48.71 49.17 49.21 49.21 49.46 49.21 49.27
-0.05 +0.10 -0.05 -0.53 -0.07 -0.03 -0.03 +0.22 -0.03 + O . 03
I n experiment 4 no hydrochloric acid was added. The temperature of the solution in all cases was 26" C. during the period when the arsenite was in contact with the ferric iron. When it is considered that a difference of 0.05 mg. is caused by a variation of 0.01 cc. in the buret reading, only two of the results, 26 and 3b, are really significant, and these are opposite in sign. It would, therefore, seem safe to conclude that under the conditions suggested for this procedure there is no perceptible reduction of ferric salts by sodium arsenite. Determination of Chloric Acid Titanous chloride is very well adapted to the determination of chloric acid and chlorates. Not only is it less cumbersome and more rapid than the usual methods for chlorates, but i t is quite dependable, and is especially useful where small amounts of chlorate are to be determined in the presence of chlorides, as in hypochlorite, or alkali solutions. PROCEDURE-The procedure is the same, in general, as that for ferric iron. The sample containing the chlorate is measured into a 250-cc. Erlenmeyer flask, water added to
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Yol. 19, X O . 7
INDUSTRIAL AND ESGISEERIhTG CHEMISTRY
make the total volume about 75 cc., and 10 cc. of concentrated hydrochloric acid are poured in. The titanous chloride is then pipetted into the flask. As long as chloric acid is present the Folution has a faint greenish color, which changes to a faint blue when the titanous chloride is in excess. Aftei the addition of the titanous chloride solution the contents of the flask are quickly and thoroughly stirred, and if the faint blue color persists, the thiocyanate indicator is added and the excess titanous chloride immediately titrated with ferric alum. The usual blank is run on a volume of titanous chloride equal to that used in the determination, and the difference between the blank and the titration multiplied by the chlorate factor of the ferric alum gives the weight of the chlorate in the sample. It is essential that the titanous chloride be in excess before the thiocyanate indicator is added; otherwise chloric acid will be consumed in oxidizing the thiocyanate. DETERMINATlOPr'S-The potassium chlorate used was prepared by twice recrystallizing reagent-quality potassium chlorate from distilled water and drying the resultant crystals for 24 hours a t 120" C. An accurately weighed sample of this chlorate was dissolved, the solution accurately weighed, and portions of it were weighed out on an analytical balance f:om a pipet. The results in Table I11 indicate the degree of accuracy which may be expected. All the determinations were made a t one time, the samples being weighed out together, and then titrated, one after the other, consequently the blanks are the same throughout. The ferric alum was the same as that used for the iron deter-
minations of Series A, Table I, and each cubic centimeter was equivalent to 1.802 nig. of potassium chlorate. Table 111-Determination of Chlorate REAGENT SOLUTIONS BLANK KC103 USED FERRIC KC102 TAKEN TiCh Ferric alum ALUM FOUND ERROR Mg. CC. cc. cc. ME. Mg. Per cent 7.17 8.23 10 12.21 7.17 fO.00 0.00 6.17 10.61 10 10.88 +0.27 2.54 3.96 14.67 10 14.88 +0.21 1.43 17.04 20.83 25 30.52 17.48 f0.44 2.58 19.94 19.37 25 20.10 + O . 16 0.80 18.74 21.11 25 21,24 f O . 13 0.62 15.82 26.48 25 26.48 +0.00 0.00 12.38 32.75 25 32.70 -0.05 0.15 35.62 2.5 10.76 35,59 -0.03 0.09 43.06 25 6.62 43,05 -0.01 0.02 4.94 25 45.92 46,08 +O.l6 0.35 50.39 25 2.52 50.44 +0.05 0.10 MODIFICBTlOPr'S FOR CllUSTIC APr'D HYPOCHLORITE SOZUTIoxs-This method has been found convenient for the determination of chlorates in caustic soda solutions from electrolytic cells and evaporators, and in hypochlorite solutions. For the caustic solutions, the sample is measured out, acidified with hydrochloric acid, being careful to keep the temperature of the sample solution below 40" C. during the addition of acid, the titanous chloride pipetted in, indicator added, and the excess titanous chloride titrated with ferric alum. For hypochlorite solutions, a slight excess of sodium arsenite is added over that required to reduce the hypochlorite present, the solution acidified with hydrochloric acid, the titanous chloride pipetted in, indicator added. and the excess titanous chloride titrated with ferric alum.
Direct Iodimetric Determination of Glucose' By Alexis Voorhies and A. M. Alvarado LOYOLAUNIVERSITY.S E W ORLEANS, L A .
REVIEW of the literature shows that most of the important methods for the determination of glucose are copper reduction methods, which, with sundry variations and modifications, are essentially the same as the original procedure devised by Fehling in 1848. rlniong the modifications of Fehling's method are Kendall's2 in which the alkaline tartrate solution is replaced by an alkaline salicylate solution, and that of Benedict,3 in which the alkaline tartrate is replaced by an alkaline citrate solution. One of the main difficulties in a volumetric copper reduction method for glucose is the accurate determination of the end point. Daggett, Campbell, and Whitman4 have proposed the electrometric determination of this end point. Recently Lane and Ey16 have developed a method using methylene blue as an internal indicator. I n seeking, if not a more accurate, at least a simpler, method of determining glucose, the writers have been led to the use of iodine directly as the oxidizing agent. The results here presented are merely preliminary and further work is being done on the problem. Theoretical
A
When glucose is treated with an alkaline solution of metallic salts, the aldehyde group in the sugar molecule should be oxidized to an acid carboxyl group. This oxidation Received March 14, 1927 Z J A m Chsm S o c , 34, 317 (1912) 3 J A m Med A s s o c , 61, 1193 (1911) 4 J A m Chem Soc, 46, 1043 (1923) s l n r e r n Sugor J 26, 143 (1923)
1
is only t'he beginning of bhe reaction, however, for the oxidation is attended by the breaking down of the carbon chain, bhe products of decomposition varying in proportion according to the conditions of experiment.6 It was hoped that by using an oxidizing agent of the type of iodine and working a t room temperatures the oxidation would proceed only to the gluconic acid stage. The general reaction is represented by the following equation: -C / O \H
+
I*
--f
-c
\OH
+
2HI
By removing the H I formed the reaction should proceed to completion to the right. By examining this equation we can see that 90.06 grams of pure anhydrous glucose are equivalent to 126 grams of iodine. Therefore, a solution containing 9.006 grams of anhydrous glucose per liter would be exactly 0.1 normal. Apparatus and Materials All burets and other measuring apparatus were carefully calibrated in the usual way. SODIUM THIOSULFATE-C. P. reagent quality. A 0.1 71'2 solution was made up and carefully standardized against C. P. reagent quality potassium iodate. IODINE-C. P. resublimed quality. A 0.1 N solution was prepared and the concentration checked daily against the thiosulfate. Gr.ueosS=C. F. anhydruus quality. A 0.1 N solution was prepared by dissolving 0.9006 gram of the anhydrous glucose in boiled and cooled distilled water and making up t o 100 cc. e Browne, Handbook of Sugar Analysis. p. 335.
