Volumetric Determination of Carbon and Hydrogen In Organic Microcombustion Analysis AXEL JOHANSSON Royal lnstitute o f Technology, Stockholm, Sweden
The weighing of small amounts of carbon dioxide and water in comparatively large glass vessels is one of the main sources of error in organic microcombustion analysis. This difficulty can be avoided by a volumetric determination, so that only the sample need be weighed on a microbalance. The wTater formed during combustion is absorbed in a mixture of methanol and pyridine and determined by a Karl Fischer titration. The carbon dioxide is absorbed in an aqueous solution of sodium hydroxide and barium chloride. The precipitated barium carbonate is collected, converted to the iodate, and determined iodometrically.
0
NE of the main sources of error in the determination of
carbon and hydrogen on the micro scale is the weighing, in comparatively large glass vessels, of small amounts of carbon dioxide and water. By volumetric determination of carbon dioxide and water, this difficulty is avoided and only the sample need be weighed on a microbalance. This is of special importance in the case of submicro quantities of organic material, where the weighing of the absorption vessels causes great errors. Lnterzaucher ( 7 ) has published a volumetric method for carbon and hydrogen determinations similar to his method for the determination of oxygen in organic compounds, but it requires a rather complicated apparatus and is rather difficult to perform. The present method is simple, and the apparatus can be coupled to an ordinary combustion train The method has been perfected only for determinations on a milligram scale, but it is now being tested on a 0.1-mg. scale. The water formed during the combustion is absorbed in a mixture of pyridine and methanol and titrated with Karl Fischer solution. The carbon dioxide is absorbed in a mixture of sodium hydroxide and barium chloride solutions. The precipitated barium carbonate is filtered, dissolved in hydrochloric acid, and then precipitated as barium iodate. The iodate is filtered, washed, dissolved in hydrochloric acid and potassium iodide solution, and then titrated with sodium thiosulfate solution. In this way one atom of carbon gives rise to 12 atoms of iodine, as shown by the following sequence of transformations:
G
-+
CO,
+
BaCOa -.t Ba(IO&
-*
612
In an actual analysis 1 mg. of carbon is equivalent to about 10 ml. of 0.LV sodium thiosulfate solution. DETERMINATION OF CARBON DIOXIDE
Two points in the experimental procedure which require special attention are filtration of the barium carbonate and precipitation of the barium iodate. The carbon dioxide is absorbed in a solution of barium chloride and sodium hydroxide, and it is neceseary t o avoid interference by atmospheric carbon dioxide when performing the filtration. Many types of apparatus have been constructed for this purpose, but they are too complicated to be used for routine work. If, however, after the combustion, the absorption solution is adjusted to such a pH that no appreciable amount of barium carbonate is dissolved and no appreciable absorption of atmospheric carbon dioxide takes place, the filtration can be performed without any further precautions. On the assumption that less than 0.1% of the barium carbonate formed should he
dissolved, the lowest pH permitted may be calculated in the following way: The barium ion concentration in the absorption solution is rather high and, therefore, very little influenced by the small quantity passing into solution. It may be considered constant, and because the solubility product of barium carbonate is a constant, the carbonate ion concentratiw is also constant. The concentration of carbonic acid is negligible a t a high pH compared with the concentration of bicarbonate ion. The effect of the neutralization and the barium carbonate consequently being dissolved is, therefore, an increase of the bicarbonate ion concentration. Using the law of mass action and substituting the actual concentrations, the lowest pH permitked is calculated to be 9.1. The most suitable reagent for adjusting the excess hydroxide to the required pH was found to be ammonium chloride, which is added in excese to the absorption solution before the latter is exposed to the atmosphere. Because the vapor pressure of carbon dioxide over the resultant solution is less than the partial pressure of carbon dioxide in the atmosphere, atmospheric carbon dioxide should be absorbed, but a t pH 9.1 this absorption may be expected to be very slow. As the solution has a high buffering capacity, a moderate quantity of absorbed carbon dioxide does not change the pH. Thus the amounts of carbonic acid, carbonate, and bicarbonate ion do not change. The amount of carbon dioxide absorbed must therefore be precipitated as barium carbonate. The calculation of the amount absorbed per minute is rather inaccurate, but as shown in the following experiments, no absorption could be observed in 2 hours.
Table I. Sodium Carbonate, Mg. 1Y.76 18.53 19.63 20,68
Recovery of Known Amounts of Sodium Carbonate Barium Carbonate, M g . Found Calod. 33.27 34.48 36.58 38.50
33.07 34.50 36.55 38.51
Error, Mg. +0.20 -0.02 +0.03 -0.01
When 25 ml. of the absorption solution, 0.2N with respect to sodium hydroxide and 0.05M with respect to barium chloride, were mixed with more than 5 ml. of 2 5 ammonium chloride solution and the mixture was kept in an open beaker, no precipitate was formed during 2 hours. On the other hand, 15 ml. of the ammonium chloride solution gave too acid a solution, which dissolved a little of an added amount of barium carbonate. The most suitable quantity was therefore chosen as 10 ml. For testing this part of the analysis, known amounts of sodium carbonate were weighed, transferred to a flask, and treated with sulfuric acid. The carbon dioxide evolved was passed through the combustion tube and then absorbed as described in the procedure. The barium carbonate formed was determined gravimetrically, and the results were as given in Table I. When 15 ml. of ammonium chloride solution were used, results which were 0.5 to 1.5% too low were obtained in five experiments. The determination of barium as iodate is described by Bogdanov ( 1 ) . According to his procedure, barium is precipitated with potassium iodate, and the resulting precipitate is washed
1184
ANALYTICAL CHEMISTRY
with cold water, dissolved, and then titrated iodometrically. The precipitate is, however, rather soluble in water; hence washing causes some loss of iodate. Guthrie (S), therefore, proposed that the barium should be precipitated with a known amount of potassium iodate, the solution made up to a known volume, and the excess of iodate titrated in an aliquot of the solution. Preliminary investigation showed that i t was necessary to work in rather concentrated solutions or with a large excess of iodate to get complete precipitation in a reasonable time. In both cases the accuracy in working by Guthrie's method was very low. Further, it was difficult to get results which were independent of the excess of iodate used. A small excess gave low results, and when a larger excess was used, values greater than the theoretical were obtained. It was decided, therefore, t o reinvestigate the determination of barium as iodate, but to confine the investigation to amounts of barium commensurate with those occurring in the carbon analyses. The solubility of barium iodate in water is comparatively high, 8.1 X 10-4 mole per liter a t 25" C. ( 4 ) . I t is much less soluble in some organic solvents, and by using, for example, methanol or ethyl alcohol instead of water, it is possible to carry out the determination directly and avoid using Guthrie's indirect method. The choice of precipitant is also important. In the present case it is necessary to use acid solutions, and it was found that iodic acid is the best precipitant. R h e n potassium iodate is used, coprecipitation of potassium hydrogen iodate is a serious complication, and even with iodic acid there is slight coprecipitation. It is necessary, therefore, to standardize the experimental procedure. For the carbon determination a consumption of 20 to 30 ml. of sodium thiosulfate solution is desirable for accurate results. Trial experiments with corresponding amounts of barium chloride showed that a total volume of 25 ml. and an approximate fourfold excess of iodic acid gave reproducible results. I n some of these experiments 5 ml. of 0.5N hydrochloric acid and various amount8 of barium chloride were mixed with water to a total volume of 15 ml. T o these solutions were added 10 ml. of 0.17M iodic acid, and the precipitates formed were treated as described in the procedure. The results, recorded in Table 11, show that the precipitates formed under these conditions contain 1.9% of iodic acid, so it is necessary to reduce the experimental values by a factor of 1.019 to obtain the correct result. As can be seen from Table 11, the correction factor does not vary appreciably even for large variations in the amount of barium. DETERMINATION OF WATER
Methanol may be used for the absorption of water. It should be made as water-free as possible, for which purpose the method of Lund and Bjerrum ( 5 ) is convenient. As it is impossible to make absolutely water-free methanol, it is necessary to use a known amount of the absorbent and to run a blank experiment in order to make the necessary correction. A better method, in which the water is absorbed in a mixture of pyridine and methanol, has been tried in some analyses. Both solvents should be as water-free as possible. The last traces of water are removed by adding Karl Fischer reagent until the color changes to brown. This addition is made, with oxygen bubbling through the apparatus, in the absorption tube after it is connected to the combustion tube. The excess of sulfur dioxide which may cause trouble in the carbon determination is partly bubbled away and partly oxidized in a few minutes. After the combustion, the absorbed water is again titrated with Karl Fischer solution; only 1 or 2 ml. of reagent are consumed. A micrometer syringe may conveniently be used for this titration. During the combustion some of the methanol evaporates and passes into the next absorption tube. Because carbon dioxide was found to be less readily absorbed in sodium hydroxide solution which is contaminated with methanol, it is necessary to interpose a scrubber containing water between the absorption tubes.
Table 11. Titration of Known Amounits of B a r i u m Ba
+
+
Added,
Mg.
10.96 10.96 13.71 13.71 27.41 27.41 27.41 27.41 27.41 27.41 27.41 54.82 54.82 54.82
NamSaOs, M1. 0.1N 9.69 9.71 12.21 12.21 24.40 24.37 24.42 24,41 24,40 24.38 24.35 48.64 48.66 48.61
Ba
Found, hlg. 11.09 11.12 13.98 13.98 27.93 27.90 27.96 27.94 27.93 27.91 27.88 55.68 55.71 55.65
+ +
Ba++ Found, Mg. Ba Added, Mg. +
1.012 1.015 1,020 1.020 1.019 1.018 1 020 1,019 1,019 1.018 1.017 1.016 1.016 1,015
APPARATUS AND PROCEDURE
Apparatus. Any good microapparatus may be used for the combustion. I n the present investigation no special attention has been given to the combustion train; however, a very simple apparatus was found to give excellent results with the substances analyzed so far. The oxygen used was purified by passage through a tube similar to the combustion tube, which was heated t3 900" C., and then through absorbents to remove water and cnrbon dioxide. For the majority of the combustions, a GroteKrekeler semimicrotube ( 2 , 6), heated to 900' C. in an electric oven, was employed. For substances containing carbon, hydrogen, sulfur, and nitrogen, the tube was used without filling. For substances containing a halogen, a 2-cm. layer of silver wool was placed after the combustion zone. The sample was vaporized by means of a Bunsen burner. For the absorption of water and carbon dioxide from the sample, tubes of the type shown in Figure 1 were used. The dimensions given there are not critical. Solutions. Sodium hydroxide, 0.2,V, and barium chloride, 0.05M. A solution of 12.2 grams of barium chloride dihydrate is added to an aqueous solution containing 8 grams of sodium hydroxide, and the mixture is diluted to 1 liter. This solution is filtered and then stored in a bottle fitted with a 25-ml. buret and with tubes for excluding atmospheric carbon dioxide. Ammonium chloride solution, 2AV. Hydrochloric acid solution, 0 . 5 5 . Iodic acid solution, 0.17M. Potassium iodide solution, 10%. Sodium thiosulfate solution, 0.1N. Procedure. The absorption tubes and the scrubber containing about 2 ml. of water are attached to the heated combustion tube, stopcock grease is used to lubricate the connections, then oxygen is passed through at a rate of flox of 10 ml. per minute. One milliliter of a mixture of equal parts of methanol and pyridine is added to the first absorption tube. The tube is fitted with the capillary stopper, and Karl Fischer reagent is added bv means of a syringe with a platinum needle, until the color just changes to brown. Then 3 to 4 mg. of the sample are m-eighed in the boat. As no heavy absorption tubes are weighed, a beam balance which can take a 20-gram load is not necessary. A torsion balance is sufficient, and it is much more rapid to use. I n this case the sample is a-eighed in a small bottle and transferred to the boat. The boat is placed in the tube through which oxygen is then passed for about 1 minute. The first two tubes are closed with the ground-glass stoppers, and then 25 ml. of the 0.2N sodium hydroxide and 0.05M barium chloride solution are introduced in the last absorption tube. During this process, the buret tip is introduced through the taper joint in the top of the tube, XT-hich is then closed with the glass capillary stopper. The ox!-gen stream is readjusted to 10 ml. per minute, and the combustion is started and performed in about 20 to 30 minutes. When the combustion is finished, 10 ml. of 0 . 2 5 ammonium chloride solution are added to the carbon dioxide absorption tube. The contents are mixed by bubbling oxygen through the solution for 2 minutes, after which this tube and the water scrubber are disconnected. The stopper in the water absorption tube is replaced by a capillary stopper, and the water absorbed in the tube is titrated with Karl Fischer solution using a micrometer syringe. During the titration oxygen ie bubbled through the tube to stir the contents.
V O L U M E 2 6 , NO, 7, J U L Y 1 9 5 4
1185 CALCULATIONS
The calculations are best shown by an example. A sample (3.004 mg.) of diphenylamine was weighed. The water formed was titrated with Karl Fischer solution. On the micrometer screw used t o move the plunger in the syringe, an 11.38-mm. change was read. Every millimeter corresponds to 0.1535 mg. of water (determined in a separate titration), which gives 1.747 mg. of water or
C
In the carbon determination 25.75 ml. of 0. 10IOLV sodium thiosulfate were consumed. As C is equivalent to 612, the carbon content is: 25.75 X 0.1010 X 12.01 X 100 = 85,03% carbon 12.00 X 3.004 X 1.019 The factor 1.019 arises from the coprecipitation of iodic acid in the barium iodate as mentioned above.
Figure 1. Absorption Tubes A. B. C. All
ACCURACY
Water absorption tube Scrubber Carbon dioxide absorption tube dimensions i n millimeters
Table 111. Test Analyses of Known Compounds Hydrogen. 5% Calcd. Found 4.93 4 86 .., 5.01
Substance Benzoic acid Diphenylamine
...
69.01
. .. ...
6.57 6.59 6.51
85.17
.
84.82 85.11 85.03
5.12
5.12 5.13
40.68
.,
40.75 40.51
6.63
6.55
Succinic acid
...
Xylose
Carbon, 5’-. Calcd. Found (38.85 6Q.11
6.71
, , ,,,
,
...
40.00
6.80
.. .
39.77 39.93
Dichlorodimethoxyhydroquinone diacetate
3.74
3.75
44.59
44.59
Succinic acid
5.12
5.04
40.68
40.58
Some results o b t a i n e d f r o m s u c c e s s i v e analyses according to this procedure are shown in Table 111. I n a series of tests with succinic acid the following carbon figures were obtained: 40.74, 40.61, 40.75, 40.54, 40.58, 40.71, 40.52, and 40.58%. This gives a mean value of 40.63% compared with the calculated value of 40.68010, and a standard deviation of 0.08. For the corresponding hydrogen analyses the values were: 5.07, 5.11, 5.21, 5.04, 5.15, 5.06, 5.14, and 4.99%. The mean value of these figures is 5.10%, and the standard deviation is 0.08; the calculated hydrogen content is 5.12%. The samples except the dichlorodimethoxyhydroquinone diacetate were of reagent grade, recrystallized and dried. They have been used as test substances in ordinary carbon and hydrogen determinations. The purity of the dichlorodimethoxyhydroquinone diacetate was also tested by ordinary carbon and hydrogen determinations. ACKNOWLEDGMENT
The precipitate in the carbon dioxide absorption tube is filtered through a glass funnel having a sintered disk of porosity M. The absorption tube and the precipitate are washed with 50% aqueous ethyl alcohol. The precipitate remaining in the tube and on the filter is dissolved in 5 ml. of 0.5LV hydrochloric acid, which is then sucked down into a 100-ml. filtering flask. The tube and the funnel are washed three times using a total of no more than 10 ml. of water. The contents of the flask are mixed, and then 10 ml. of 0.17M iodic acid are added. After 3 minutes the solution is sucked off by means of an immersion a t e r with a sintered disk of porosity M. The barium iodate is Jvashed five times with 5-ml. portions of 96% ethyl alcohol. When the last wash liquid is sucked off, the precipitate is disintegrated in 2 ml. of 96% ethyl alcohol; 20 ml. of 10% potassium iodide solution and 5 ml. of 2LV hydrochloric acid are added, and the solution is titrated with 0.1N sodium thiosulfate solution. When the solution becomes colorless, any barium iodate remaining in the filter is dissolved by sucking some of the solution up and down through the filter. Starch solution is then added, and the titration is continued until the solution is colorless again. The process is repeated until a permanent end point is obtained.
/
G
The financial support given by Statens Saturvetenskapliga Forskningsrdd is gratefully acknowledged. The author also wishes to thank Karin Lindgren for her skillful experimental assistance. LITERATURE CITED
(1) Bogdanov, K. -4., Zauodskaya Lab., 7, 793 (1938). (2) Grote, W., and Krekeler, H., Angew. Chem., 46, 106 (1933). (3) Guthrie, F. C., J . SOC.Chem. I n d . , 59, 98 (1940). (4) Kolthoff, I. >I., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 58, New York, Macmillan Co., 1952. ( 5 ) Lund, H., and Bjerrum, J., Ber. deut. chem. Ges., 64, 210 (1931). (6) Schoberl, A,, Angew. Chem., 50, 334 (1937). (7) Unterzaucher, J., C h e m Ing. Tech., 22, 39 (1950). RECEIVED for review J u n e 2 9 , 1953. Accepted February 1.5, 1954.
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