ANALYTICAL EDITION
March, 1944
tion within the period being -0.000021. I n another series, the initial normality was 0.00500 and a t the end of 56 days was 0.004977 with a maximum variation of -0.000020. The other two series gave comparable results. As a check on the accuracy of sodium hypochlorite solution as a titrimetric reagent, arsenic and antimony were determined in standard solutions, using both the potassium bromate and the sodium hypochlorite methods. DISCUSSION
I n the determination of microquantities of antimony, the potassium bromate method (1) was not entirely satisfactory for a number of reasons, the principal ones being that titrations had to be performed almost a t the boiling point in strong hydrochloric acid solution with the consequent production of relatively copious and irritating fumes of hydrogen chloride and that the blank, using methyl orange as the indicator, was large. The blank obtained using some of the indicators suggested by Smith and Bliss (11) was even larger. Standard sodium hypochlorite solution has several marked advantages. It is an economical reagent. One can perform direct titrations with it. It is unnecessary to perform the titrations a t elevated temperatures, eliminating any danger from the irritating fumes of hydrochloric acid. The blank is smaller than that obtained with potassium bromate titrations. Hypochlorite titrations can be performed under conditions of low acid concentration without apparent decrease in accuracy. From 0.1 to 1mg. of antimony or arsenic per 10 ml. of sample solution can easily be estimated. Titrations can be made in glass-stoppered bottles,
207
glass-stoppered Erlenmeyer flasks, or iodine flasks, if desired, to minimize losses attributed to volatility. Many of the indicators mentioned by Smith and Bliss, and Kolthoff and Stenger can be used instead of methyl orange without increase of the blank. The precision compares favorably with that of other methods, as can be seen from the reproducibility of results. Several precautions must, however, be observed in using sodium hypochlorite solution as a titrimetric reagent. It must be preserved in brown, glass-stoppered bottles. It may be kept a t room temperature without deterioration over considerable periods of time. Keeping the solution at lower temperatures is perhaps preferable. The optimum condition3 for the titrations are a volume of a t least 35 to 40 ml. with an acid concentration equivalent to 5 ml. of concentrated hydrochloric acid. LITERATURE CITED
Anderson, ISD. ENQ.CHEM.,A s . 4 ~ ED., . 11, 224 (1939). Chapin, J . Am. Chem. Soc., 56, 2211 (1934). DenigBs, J . pharm. chim., [5] 23, 101 (1891). Jackson and Parsons, IXD. ENG.CHEM.,ANAL.ED.,9, 14 (1937). Jellinek and Kresteff, 2. anorg. Chem., 137, 333 (1924). Jellinek and Kuhn, Ibid., 138, 81 (1924). Kolthoff and Stenger, IND.ENQ. CHEM.,ANAL. ED., 7, 79 (1935).
Koppeschaar, 2. a n d . Chem., 15, 233 (1876). Lunge-Berl, “Chemisoh-technische Untersuchungsmethoden”, Vol. 1, 7th ed., Berlin, J. Springer, 1921. Manchot and Oberhauser, Z . anorg. Chem., 130, 161 (1923). Smith and BlisJ, J . Am. Chem. SOC.,53, 2091 (1931).
Thiosulfate Washers in Alkoxy Microdeterminations E.
P. WHITE, Chemical Laboratory,
Animal Research Division, Department of Agriculture Wellington, N e w Zealand
Determinations of methoxy and methylimide groups in which thiosulfate alone is used as a washer give values considerably lower than theoretical. This is due to the solubility of the methyl iodide in the washer, and a subsequent reaction. Ethoxy and ethylimide determinations are not subject to this loss. The effect of thiosulfate can be eliminated b y using as washer thiosulfate dissolved in saturated sodium chloride, or b y adding cadmium sulfate as in the standard gravimetric procedure. The minor errors in determinations using as washers water, phosphorus suspension, or 0.570 sodium carbonate, are insignificant in comparison with that due to thiosulfate. A rapid distinction between ethoxy and methoxy can be made b y doing a determination with a good washer, such as 0.5% carbonate, then with 5% thiosulfate) the methoxy value will be reduced to 55 to 70% of the original, while ethoxy remains unchanged.
W
ORK on alkaloids in this laboratory required the development of micromethods of alkoxy and akimide
determination. The apparatus used was that of Pregl for alkoxy and that of Friedrich for alkoxy and alkimide determinations, and the procedure was essentially that of modern textbooks of microchemistry. Three to 5 mg. of material were weighed on tinfoil, dissolved in phenol and acetic anhydride, heated with hydriodic acid, and passed through a washer of 5% thiosulfate containing 0.5% sodium carbonate. Final estimation was by the Viebock-Brecher method. Preliminary experiments with the Friedrich apparatus showed that the values obtained with vanillin and several alkaloids did not agree with theory, calculation being from first principles. The titration obtained with all methoxy-containing substances was only 50 to 70% of the theoretical, while ethoxy values agreed closely with theory. The method was then examined in detail and many
of the more obvious possible sources of error were eliminated. The same effect was found in the Pregl apparatus. The only factor not eliminated appeared to be the washing solution. Consequently, red phosphorus suspension, the original washer of Pregl ( 9 ) , as well as water and 0.5% sodium carbonate was tried. These gave theoretical results in the Pregl apparatus, and values some 5% low in the Friedrich apparatus. The use of thiosulfate as a washer was then investigated, and the literature searched for counterindications to its use. Thiosulfate washers are almost universally used and recommended by the later workers in microchemistry. EXPERIMENTAL
Known amounts of methyl iodide were introduced into the Friedrich apparatus without any hydriodic acid, drawn through various washers, and titrated In the ordinary way. With no washer, or with water, phosphorus suspension, or 0,5y0carbonate the recovery was almost theoretical. With thiosulfate there was only 50 to 65% recovery, thus confirming the effect of thiosulfate.
A survey of results obtained with various washers in the Pregl apparatus is given for vanillin in Table I, and for phenacetin in Table 11, which show that the effect of thiosulfate on the methoxy value is detected when 1 mi. of a 1.57, solution is used. This effect becomes much larger when 5 to 10% thiosulfate is used, while with very high concentrations (40%) theoretical values are again obtained. With ethoxy there was no detectable effect in any concentration. DISCUSSION
The effect of thiosulfate is explained as a result of two factors: (1)the solubility of the alkyl halide in the washing solution, with
208
Table I. Washing Solutions
Volume of Washing Solution
substance
Methoxy Values on Vanillin (Theory 90.36)”
Titration, 0.0188 N Thiosulfate
blethoxy
Washing Solutions
Volume of Washing Solution
.Vf 1. Water
0.5
...
1.0
...
0.5% sodium carbonate 1.0% sodium bicarbonate 1.5% sodium thiosulfate
0.5
...
... ... 0.5
...
0.5
...
... ...
1.0 ,..
...
... 5% thiosulfate
0.5
... ...
1 .o
Table
II. Ethoxy Values
Washing Solutions
Volume of Washing Solution
M1. Water
5
Vol. 16, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
% thiosulfate
0.5
... ... ... 1.0 ...
10% thiosulfate
0.5 1.0
20% thiosulfate
1.0
...
3.790 3.210 4.526 5.511 3.546 4.292 3.930 3.552 3.193 4.110 3.250 3.881 4.117 3.350 3.492 5.454 3.217 3.112 3.388 3.770 5.710 4.038 4.026 3.668
7.44 6.66 9.39 11.80 7.49 8.52 8.07 7.56 6.66 8.52 6.69 7.57 8.34 6.69 7.29 10.11 6.61 5.60 6.53 6.00
9.50 7.68 7.49 5.80
19.1 20.1 20.2 20.7 20.5 19.3 20.0 20.7 20.2 20.1 20.0 19.0 19.7 19.7 20.3 18.0 20.0 17.8 18.7 15.5 16.2 18.5
18.0 15.4
on Phenacetin (Theory 25.14)” Substance
Mg .
4,744 5.334 2.787 4.213 4.051 5.680 3.666 4.815 4.117 5.211
Titration, 0.0188 N Thiosulfate
Ethoxy
M1.
%
8.75 9.62 4.94 7.57 7.47 9.91 6.59 8.65 7.50 9.25
26.0 25.4 25.1 25.3 25.8 24.6 25.3 25.3 25.7 24.9
Several blanks gave 0.05 t o 0.07 ml. of thiosulfate with t h e washers used. t h o u e h a few with hinh thiosu
a consequent dependence on the volume of washer used, and (2) the rate of reaction of the dissolved halide with thiosulfate. Methyl iodide and thiosulfate react in water according to a mellknown bimolecular reaction investigated by Slator (11). At 25’ C. his data show for 0.035 N thiosulfate and 0.018 N methyl iodide a half-time of 10 to 12 minutes, with Kz = 0.85. The ethyl iodide reaction is slower with K Za t 25‘ = 0.050. This reaction is expected to ocrur in methoxy determinations because of the appreciable solubility of methyl iodide in water. At 20’ 1.40 grams of methyl iodide dissolve in 100 grams of tvater ( 7 ) . With very concentrated thiosulfate the solubility is apparently depressed, and little if any reaction can take place. The solubility of methyl iodide in concentrated neutral salt solutions would be expected to be much less than in water. With 1 ml. of 5% thiosulfate in saturated salt, no loss was found. Addition of an equal volume of 5 % cadmium sulfate also completely suppressed any reaction, apparently through more than a solubility effect. I n the case of ethyl iodide the lower rate of reaction and the lower solubility (at 20’ 0.401 gram of ethyl iodide dissolves in 100 grams of water, 7) combine to give no detectable effect. Satisfactory results were obtained by Pregl (9), using phosphorus sus ension as a washer for alkoxy and alkimide determinations g y Viebock and Brecher (IS), and by many later workers using modified apparatus. The main purpose of the washer appears to be to remove hydrogen iodide vapors, rather than iodine itself, and for this purpose an aqueous washer appears ,effective even in alkimide determinations. Phosphorus suspension was criticized by Friedrich (6) as it is incapable of removing iodine rapidly. He therefore used for the gravimetric method 3% thiosulfate to which was added an equal volume of
5% thiosulfate 5% thiosulfate with 0 . 5 7 carbonate 10% tiiosulfate
1.0 ,..
0.5
...
1.0
...
20% thiosulfate
1.0
40% thiosulfate
1 0
80% thiosulfate 5% thiosulfate in saturated sodium ohloride
1 0 1 0
... ,..
2.5’7, thiosulfate and 2.5’7, cadmium sulfate
1 0
Substance
Titration, 0.0188 N Thiosulfate
Mg.
M1.
5.298 4.022 5.885 3.568 4.531 5.284 4.172 3.680 3.692 3.423 3.767 3.892 5.252 5,470 3 780 4 900 4,959 3.008 5.090 4.420 3.545 3.944 5 049
7.15 5.97 8.51 5.24 6.08 7.89 6.61 5.80 6.05 5.60 8.08 8.30 10 39 10.80 7.87 10.29 10.32 6.21 10.45 9.11 7.43 8.33 10.71
Methoxy
% 13.1 14.4 14.1 14.3 13.0 14.5 l5,4 l5,6 15.9 15.9 20.2 20.7 19.2 19.2 20.2 20 4 20.2 20.1 20.0 20.0 20.4 20.8 20.6
5% cadmium sulfate. The use of the latter material for removal of hydrogen sulfide was due to Edlbacher (3). Friedrich’s values were correct, as would be expected from this study, but he does not insist on the admixture with cadmium sulfate. Friedrich (6) states that use of cadmium sulfate is not necessary if the acid is free from sulfide. The standard washer for the gravimetric method (Roth, 10) is 1 ml. of equal volumes of 5y0 thiosulfate and cadmium sulfate. As indicated in the table, this washer is perfectly satisfactory, and because of a chance effect which is not quite expected, the thiosulfate effect is eliminated, and results are free from error. This washer has also been used in modified volumetric methods by Elek (4) Christensen, Friedman, and Sato ( I ) , and Cooke and Hibbert ( 2 ) with good results. Slotta and Haberland (12) used for the volumetric method 1 ml. of 1.5% thiosulfate to which was added 0.5% sodium carbonate, and gave a few examples which were close to theory. From Table I it is seen that with 1 ml. of thiosulfate of this concentration a lowering is detected, but with 0.5 ml. little if any lowering, The washer for the volumetric method generally adopted by Friedrich (6) and by Roth (10) is 1 ml. of 5 % thiosulfate with optional addition of 0.5% carbonate. Roth states that this washer was used by Slotta and Haberland. The carbonate was found to have no effect on the thiosulfate reaction, and this washer Ras incapable of giving anything near Bhe theoretical results. Friedrich (6) states that he used this washer for 5 years nTith completely satisfactory results. He gives no example of calculation and makes no mention of empirical corrections. In the volumetric methylimide determination he states that 1 ml. of 0.01 AT thiosulfate = 0.1502 mg. of CHa ( S ) ,which, divided by 6, is in agreement with theory. Roth (10) also makes no mention of low results or empirical corrections. He gives 1 ml. of 0.02 N thiosulfate =0.6204 mg. of OCHa, which divided by 6 is theoretical. His factors given for calculation of methoxy, ethoxy, methylimide, and ethylimide are all in agreement xith theory, assuming 100% recovery of alkyl iodide. LITERATURE CITED
(1) Christensen, B. E., Friedman, L., and Sato, Y., IND.ENQ. CHEM., ANAL.ED.,13, 276 (1941). (2) Cooke, L. M., and Hibbert, H., Ibid.,15, 24 (1943). (3) Edlbacher, S., Z. physiol. Chem., 101, 278 (1918). (4) Elek, A., IND. ENG.CHEM.,A N ~ LED., . 11, 174 (1939). (5) Friedrich, A., “Die Praxis der quantitative organische Mikroanalyse”, pp. 133-60, Leipaig and Vienna, F. Deuticke, 1933. (6) Friedrich, A., Z. physiol. Chem., 163, 141-8 (1927). (7) International Critical Tables, 1st ed., Vol. 3, New York, McGraw-Hill Book Co., 1928. (8) Nanji, H. R., A n a l y s t , 59, 96 (1934). (9) Pregl, F. (tr. by Fyleman), “Quantitative Organic Microanalysis”, pp. 150-63, London, J. A. Churchill, 1934. (10) Roth, H., “Die quantitative organische Mikroanalyse yon Fritz Pregl”, pp. 211-35,4th ed., Berlin, Julius Springer, 1935. (11) Slator, A., J . Chem. SOC., 85, 1291 (1904). (12) Slotta, K. H.. and Haberland, G., Ber., 65, 127 (1932). (13) Viebock, F., and Brecher, C., Ibid.,63,3207 (1930).
