ANALYTICAL EDITION
32
The use of nearly 60 grams of combined dish, sand, and rod and the many weighings necessary to determine moisture in 1 gram of sirup make a cumbersome task and it was with great pleasure that the author welcomed the method of Bidwell and Sterling.* As soon as apparatus could be made the method was tried out, but with only partial mccess because the same continuous decomposition was encountered and no definite end point was found. Several tests showed that by using identical conditions and a definite time interval concordant results were possible on sirups of moderate or low invert sugar content, but with invert sugar of 25 per cent or over the results were unreliable and the residue in the flask after distillation was seriously blackened. At about this stage an oil bath was substituted for a sand bath with improved results, but the method was considered as limited to blackstraps and sirups of low invert sugar content. With the thought in mind to precipitate any calcium present as chloride and that some molecular rearrangement might cause a more ready release of water present, a distillation determination was tried with 2 grams of powdered sodium oxalate added to 15 grams of sirup. The result was a quicker distillation and less blackening of the residue. Another test with 10 grams of sodium oxalate gave results that were even better. Inasmuch as blackstrap has a considerable quantity of suspended particles and sodium oxalate is only sparingly soluble, the question arose as to whether the improved results were due to the chemical or physical effect of the sodium oxalate. To determine if finely divided material would assist in the distillation, 10 grams of Filter-Cel were added. The result was rapid evolution of the water, 8
Vol. 1, No. 1
and after 4 hours’ heating the residue showed no indication whatever of blackening or decomposition. While a small amount of water continued to distil over, the major part was over in 1 hour and the slight additional amount a t the end of 3 hours made the total in close agreement with the percentage as indicated by the official method. As an indication of the effect of the Filter-Cel it was noted that the same rate of distillation of the toluene could be maintained with the oil bath at 127” C. as a t 143” C. without Filter-Cel. The advantages of the distillation method as set forth by Bidwell and Sterling were found to be true, especially the advantage of a 15-gram sample and a single weighing. The total time consumed is a t most 4 hours, during which the operator is occupied but a few minutes at the start. The most satisfactory manipulation is to put half of a roughly weighed 10-gram portion of dried Filter-Cel into a flask and then weigh off 15 grams of sirup into a capsule formed from a piece of thin waxed paper resting in a 15-cc. Gooch crucible. After weighing, the paper may be lifted out of the crucible and sirup and all dropped into the flask, after which the second half of the Filter-Cel is added and the distillation carried out as described by Bidwell and Sterling. Comparison of Distillation M e t h o d w i t h Official Method for D e t e r m i n a t i o n of Moisture in Sugar Sirups 3 HOURS’ VACUUM SAMPLE BATR FILTER-C&tDISTILLATIONOVEN INVERT
Grams A
Sand
1
B
Sand Oil Oil
1
Per cent
Per cent
Per cent
Oil
IU
Sterling, IND.END.CHEM.,17, 147 (1925).
Evaluation of Stibnite‘ I-De termination of Sulfur Wallace M. McNabb and E. C. Wagner UNIVERSITY ox PENNSYLVANIA, PHILADELPHIA, PA.
M
ETHODS for the analysis of stibnite intended for
use in primers were described in 1918 by Cushman,2 and were incorporated into the specifications of the United States Ordnance D e ~ a r t m e n t . ~I n the recommended method for sulfur this element is oxidized by action of bromine and glacial acetic acid, and after suitable further treatment, including removal of antimony by use of aluminum powder, it is precipitated and weighed as barium sulfate. This procedure, Cushman states, was adopted a t the Frankford Arsenal “as giving fairly quick and accurate results.” Elsewhere in the same article occurs the declaration: “All other methods for the determination of sulfur have been rejected as leading to inconsistent results.” This condemnation specifically includes fusion methods, evolution methods, and electrolytic methods, and is offered without supporting \data. Wofk on the analysis of stibnite in 1917 showed the apTplicability of the evolution method, the sulfur being disengaged as hydrogen sulfide, absorbed in ammoniacal cadmium solution, and determined iodometrically by an adaptation of the familiar procedure. There was noticed a constant Received July 11, 1928. Cushman, J. IND.ENG.CHEM.,10,376 (1918). a U. S. Army Ordnance Dept., Sfiec. 60-11-14(July 24,1923). 1
tendency for the evolution method to yield results for sulfur slightly lower than those obtained by bromine oxidation. Table I-Comparison
of Sulfur Determinations by Evolution a n d Bromine Oxidation Methods EVOLUTION METHOD BROMINE OXIDATION
% 21.14 21.15 21.14 21.10 Av. 21.13 21.72 21.72 21.70 21.76 21.73 Av. 2 1 . 7 3
% 21.38 21.28 21.40
Av. 2 1 . 3 4 21.77 21.78
Av. 21.78
The differences of 0.21 and 0.05 per cent appeared to be significant, and not accidental. As the bromine oxidation is clearly a method for total sulfur, and as the evolution method can determine only sulfide (and oxysulfide) sulfur, it was suspected that the variations might be due to the presence of free sulfur in the stibnite. To test the two methods with respect to their ability to indicate a distinction between total sulfur and sulfide sulfur, a specimen of artificial black antimony sulfide was analyzed for sulfur by both methods, and a 10-gram sample was extracted with carbon tetrachloride and the free sulfur weighed.
INDUSTRIAL AND ENGINEERING CHEMI8TRY
January 15, 1929
The differences between results by the two methods average 0.65 per cent, which is very nearly the amount of free sulfur found. That the bromine-acetic acid method yields results for the total sulfur of stibnite was shown further by trials using this procedure in comparison with the carbonate fusion method of F r e ~ e n i u s . ~The results for the first specimen of stibnite mentioned above were 21.38 and 21.31 per cent of sulfur whose 21.35 per cent mean is in excellent agreement with the 21.34 per cent found by bromine oxidation. The method of Fresenius, though found to be applicable (it is among those rejected by Cushman), is fully as inconvenient as the “official” method, and its results have the same significance. Table 11-Comparison of Results by Two Methods on Sample Known to Contain Free Sulfur BROMINE OXIDATION EVOLUTION METHODCCla EXTRACTION SULFUR SULFUR F R ESULFUR ~
%
% 26.16 26.17
Av. 26.17
25.48 25.53 25.53 25.62 25.45 Av. 25.52
% 0.70 0.74
Av. 0.72
Recently there have been investigated the actual presence of free sulfur in the Chinese stibnite used in primers, its influence upon the determination of sulfur by bromine oxidation, and also the amount of the error introduced by the small quantity of sulfate sulfur usually present. The evolution method was subjected to critical trial and improvement, as reported below. The determination of antimony will be considered in a later paper. The occurrence of free riultur in stibnite, though not mentioned by Cushman, apponrs upon inquiry to be fairly common. Specimens of stibnite have been described6 with free sulfur present, sometimes in well-formed crystals. The specimens of primer stibnite used in the present study all contained sulfur extractable by carbon tetrachloride. Below are described the methods employed, and the results obtained, in the examination of four lots of stibnite for sulfur by bromine oxidation and by evolution, for free sulfur by extraction with carbon tetrachloride, and for sulfate sulfur.
33
Determination of Sulfur by Bromine-Acetic Acid Method
The directions given by Cushman2 were used unaltered. This analysis, if started late in the afternoon and the oxidation allowed to proceed overnight, can scarcely be completed before late the following afternoon. The results (for total sulfur) are concordant, and appear to be good, though some question might be raised as to possible interference of the added aluminum with the precipitation of barium sulfate.6 Determination of Sulfur by Evolution Method’
The sample of stibnite is decomposed by hot concentrated hydrochloric acid in an evolution flask, whereby sulfur present as sulfide and oxysulfide is given off as hydrogen sulfide, while any free sulfur is unaffected. The hydrogen sulfide is boiled out, and with the aid of a stream of carbon dioxide (or hydrogen) is conveyed into a receiver containing ammoniacal cadmium solution. The precipitate of cadmium sulfide is filtered off, washed, and decomposed by hydrochloric acid in contact with a measured excess of standard iodine solution, of which the excess is finally titrated with standard thiosulfate solution. The acid liquid in the evolution flask contains the antimony as trichloride, and may be used for the iodometric determination of the metal. APPARATUS-The evolution apparatus recommended for analysis of strong oxidizing agents by Bunsen’s method8 may be used, as could also, no doubt, that designed and described by Scott.B The simpler arrangement shown in the sketch is fully as satisfactory, and may be assembled from ordinary laboratory apparatus.
Determination of Free Sulfur
Transfer 10 grams of powdered stibnite to an extraction thimble, and extract with purified carbon tetrachloride in a Soxhlet apparatus. After 6 to 8 hours evaporate the solvent and weigh any residue of sulfur. I n trials of this method the extractions were continued during a further interval, but no increase in the weight of the extraction flask occurred. The carbon tetrachloride was shown by blank tests to be quite free from non-volatile matter. Four specimens of stibnite thus examined yielded 0.077, 0.09, 0.107, and 0.065 per cent of free sulfur. The extracted material was in one case identified by oxidation with bromine in carbon tetrachloride and precipitation as barium sulfate, 0.060 per cent of the 0.065 per cent originally weighed being recovered in this form. Determination of Sulfate Sulfur
Boil 10 grams of the powdered stibnite for several niinutes with 100 ml. of distilled water, and filter. Acidify the filtrate with hydrochloric acid, add barium chloride solution, and after about 6 hours filter the precipitate on paper and determine as usual. The four stibnites examined yielded the following results for sulfate sulfur: 0.011, 0.012, 0.010, and 0.012 per cent. Treadwell-Hall, “Analytical Chemistry,” 6th ed., Vol. 11, p. 316. ‘Foshag, U. S. Geol. Survey, Bull. ‘796-8, p. 116; Eakle, 2. Kryst. 4
Mineral., 24,587 (1895).
Separatory funnel A contains concentrated hydrochloric acid. Flask B is charged with lumps of calcite covered with water. Drechsel bottle C contains a strong solution of pyrogallol and sodium bicarbonate, and D contains water. The small separatory funnel E is a t first empty. The decomposition flask F is a 250-ml. Erlenmeyer flask; G is a bulbed tube which serves as a partial condenser, to reduce the danger that antimony chloride may distil over. Receiver H i s a 250-ml. Erlenmeyer flask, charged with ammoniacal cadmium solution. It is surrounded by cold water contained in beaker I . A second receiver was present in most of the experimental trials, but was shown to be unnecessary. The stopper of the flask P is of rubber. The stream of carbon dioxide may be replaced by one of hydrogen, generated from zinc and hydrochloric or sulfuric acid. In this case washing bottle C must be charged with alkaline lead solution. 7
Treadwell-Hall, 09. cit., p. 403. Cf. Scott, “Standard Methods of Chemical Analysis,” Vol 1, p. 500
(1922). 8 Q
Wagner, IND. ENQ.CHEW.,16, 616 (1924). Scott, 09. cit., p. 501.
Vel. 1, No. 1
ANALYTICAL EDITION
34
There is required also a suction filter provided with a supporting cone. SOLUTIONS-Sodium Thiosulfate. A solution of about 0.12 N strength is most convenient. This solution is accurately and easily standardized as follows, using potassium iodate as primary standard?' Weigh out accurately 0.8918 gram of pure dry potassium iodate. The best c. P. grade is satisfactory, but is slightly improved by recrystallization from water. Dissolve the iodate in water, transfer to an accurate 250-ml. volumetric flask, and dilute to the mark a t 20' C. As required, withdraw measured portions of this exactly decinormal solution by means of a 25-ml. pipet calibrated against the flask. Transfer to a 250-ml. Erlenmeyer flask 2.5 grams of iodatefree potassium iodide, dissolve in 25 ml. of water, and add 5 ml. of 6 N hydrochloric acid. As promptly as possible introduce from a pipet 25 ml. of the decinormal iodate solution and a t once dilute the solution to 150 ml. Titrate the liberated iodine with the thiosulfate solution, using as indicator 5 ml. of 0.5 per cent solution of soluble starch. Conduct a blank titration in the same way, not omitting a like exposure of the acidified potassium iodide solution to the air before dcution. The blank titration is usually zero (no color with stmch), but it should always be made. lodine. A solution of about 0.11 N strength is most convenient--14 grams of iodine and 40 grams of iodate-free potassium iodide per liter." This solution is standardized against the thiosulfate under' the conditions of the analysis, as described later, Ammoniacal Cadmium Solution. Dissolve in 500 ml. of water 63 grams of cadmium chloride or 74 grams of the sulfate, add 1250 ml. of strong ammonia water (sp. gr. 0.90), and dilute to 2500 ml. Of this solution 50 ml. suffice for 0.184 gram of sulfur. Procedure-Use for the analysis material which has been ground and passed through a 100-mesh sieve. Weigh out 0.2 to0.25 gram of the stibnite,and transfer tothe flask F of the apparatus. Charge the receiver H with 100 ml. of ammoniacal cadmium solution and 50 ml. of water. Pass through the apparatus a stream of carbon dioxide (or hydrogen) until removal of air is assured; this requires about 5 minutes. Introduce 30 ml. of concentrated hydrochloric acid into the addition funnel E, and allow it to enter the flask. With the current of gas passing steadily, heat to very gentle boiling the acid in the decomposition flask, and continue the gentle boiling until the sample is decomposed; this requires 10 to 15 minutes. When decomposition is complete, remove the flame, and continue the passage of carbon dioxide for about 20 minutes longer. The receiver now contains all the sulfide sulfur of the sample in the form of cadmium sulfide. The liquid in the decomposition flask contains the antimony as trichloride, and may be used for the determination of this metal. Disconnect receiver H , remove the stopper and inlet tube, and wash the latter free from any adhering precipitate. Allow the cadmium sulfide to settle, and decant the liquid through a suction filter supported by a cone. The filter paper should be of good quality (not affected by dilute iodine solution); Munktell No. 00, of 90 mm. diameter, was used and is satisfactory. Wash the cadmium sulfide several times with water by decantation, and then transfer the precipitate to the filter. Wash further several times with water, applied in a strong stream so as to churn up the precipitate thoroughly each time. Finally apply suction strongly, SO that the precipitate becomes densely caked on the paper. Open the filter, and tear off and discard the portions which are entirely free from cadmium sulfide. Cf. Treadwell-Hall, 09. cit., p. 553. 11 Chapin, J . Am. Chem. Soc., 41, 357 (1919). 10
Place the filter and precipitate in a 600-ml. beaker, and cover with 200 ml. of water. Add from a pipet 50 ml. of standard iodine solution, and introduce gradually and with stirring 40 ml. of concentrated hydrochloric acid. Break up lumps of precipitate and continue gentle stirring until no undecomposed flocks of sulfide are visible when the liquid is allowed to come to rest and the beaker viewed from beneath. The liquid is now turbid due to separated sulfur, and must be decidedly brown or yellow due to excess iodine. Titrate the iodine with standard thiosulfate solution, adding 5 ml. of 0.5 per cent starch indicator when the yellow color becomes faint. The end point is the disappearance of the last tinge of blue-gray color. STANDARDIZATION O F IODINE SOLUTION-Transfer to a 600ml. beaker 200 ml. of water and half of a filter paper. With the pipet used before introduce 50 ml. of standard iodine solution, and then add gradually 40 ml. of concentrated hydrochloric acid. Allow to stand, with stirring; for the same time required to decompose the cadmium sulfide in the analysis, and titrate with standard thiosulfate. COMMENTS-(~)A complete blank analysis is an excellent precaution against error caused by impure reagents. One series of trials yielded persistently high results, which by an over-all blank were finally shown to be due to hydrogen sulfide present in the hydrochloric acid. (2) The treatment of the cadmium sulfide precipitate on the filter, as prescribed above, appears to be necessary. Failure to bring the precipitate to a dense and caked condition by final application of strong suction was associated with variable but high results, the error amounting occasionally to 0.8 per cent of sulfur. (3) Too vigorous or too prolonged boiling will result in some distillation of antimony chloride. This of course affects results for antimony, but has not been found definitely to influence those for sulfur. (4) Permanent absorption of iodine by a t e r paper under the conditions of analysis appears not to occur. Thus 50 ml. of iodine required, in absence of iilter paper, 43.02 and 43.05 ml. of thiosulfate. In presence of filter paper, and under the conditions described, 50 ml. of iodine required 43.05 and 43.01 ml. of thiosulfate. (5). The use of EL rubber stopper in the decomposition flask is permissible. Comparative trials using the rubber-stoppered flask and an all-glass evolution apparatus yielded identical results.
Analytical Results The results for four specimens of primer stibnite, examined by the methods discussed above, are given in Table 111. Table 111-Analytical Data EVOLUTIONCClr EXT. METHOD FREE SULFUR SULFUR [A) (B) . .
.--,
% 20.05
20.03 20.08 20.02 20.08 Av. 20.05
21.66 21.60 21.64 21.65 21.66 Av. 21.64
%
SULFATE TOTAL SULPUR SULFUR (Cl (A B C) . .
+ +
%
%
BROMINE OXIDATION TOTAL SULFUR
% 20.16 20.14 20.18
0.08
0.01
20.14
20.16
21.71 21.66 21.73 0.11
0.01
21.76
21.70
The results obtained by the evolution method fail to support Cushman's condemnation of this procedure. It is indicated to be a method for sulfide sulfur, whereas bromine
I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
January 15, 1929
oxidation yields results for total sulfur, including sulfide sulfur, sulfate sulfur (negligible), and free sulfur. The demonstration that primer stibnite contains free sulfur extractable with carbon tetrachloride seems to introduce a new consideration into the analysis of this mineral. If the amounts of free sulfur in the specimens examined may be considered representative, then the error admitted when this sulfur is calculated to SbzSs will in general be of the order of 0.4 per cent, and it could of course be much larger without detection by the “official” method. The presence of free sulfur in stibnite to be used in primers is probably not harmful, but the impropriety of calculating it to SbzSa is obvious. The sulfide sulfur of stibnite includes that present in sulfides of antimony, lead, iron, and perhaps arsenic, the last three being commonly present as impurities. According to Cushman,2 iron and lead sulfides are not objectionable impurities, and may even be used as fairly good substitutes for stibnite in primers. As regards rapidity and convenience, the evolution procedure is much superior to the “official” method, and permits
35
determination of both sulfur and antimony in a single sample, both analyses being iodometric. Summary Commercial stibnite of the quality required for primer manufacture is likely to contain free sulfur. This can be determined by extraction with carbon tetrachloride. The bromine-acetic acid method for determination of sulfur yields results for total sulfur, and not specifically for sulfide sulfur. Its results are therefore not a correct basis for calculation of the apparent Sbz& content of stibnite. The evolution procedure described above is recommended as a rapid and satisfactory method for determination of the sulfide sulfur of stibnite. Acknowledgment The writers are indebted to H. C. Pritham, director of laboratories of the Frankford Arsenal, for the specimens of primer stibnite used in this work, and to F. L. Hess, of the Bureau of Mines, for information regarding the occurrence of free sulfur in native stibnite.
Application of the Vacuum Tube in the Falling-Ball Method for Dark-Colored Solutions* E. M. Symmes a n d E. A. Lantz HERCULES EXPBRIMIBNTAL STATION, HBRCULES P O W D E R CO., KBNVIL,N. J.
FTEN it is impossible to observe the falling ball in viscosity measurements owing to the color of the solution being tested. This is especially true of cuprammonium solutions of cellulose. If the cellulose is dirty, the solution is usually opaque and even the strongest light cannot penetrate it. The following method has been successfully applied on this type of solution. It has the advantage of substituting steel balls easily duplicated in dimensions for the glass beads which are rather difficult to duplicate.
0
Apparatus The radio vacuum tube has been applied to this method. I The circuit (Figure 1) consists of a simple oscillating circuit, 1
Received September 20, 1928.
phones
Figure 1-Circuit
for Vacuum Tube
tlie constants of which are shown in the diagram. The coils of t
