The Detection of Arsenic in Sulfur
Journal of Industrial & Engineering Chemistry
pine gave 0.96, 0.66, 0.43, and 0.80 per cent; sugar pine gave 0.55 and 0.35 per cent of galactan. It is believed t h a t t h e described procedure gave approximately correct results. A critical study of t h e whole galactan method is still much needed, however. DISCUSSION O F RESULTS It is believed t h a t analyses of coniferous woods made by t h e methods described account for all important constituents of those woods. The sum is in every case slightly over I O O per cent b u t not more so t
... u t not more so t h a n might be expected from the character of the methods employed. A variation of several tenths of a per cent between duplicate determinations is usual. The cellulose and lignin are probably always slightly overestimated on account of t h e practical impossibility of getting complete contact between all particles of t h e material and t h e attacking reagents. This difficulty could be partly overcome by better mechanical condition of t h e wood. The possibility of overlapping of values due t o certain constituents being included in more t h a n one determination has received careful consideration throughout these investigations. It is believed t h a t such double estimations have been largely avoided. Most of the proximate groups can be shown t o be free from any of the constituents contained in the other groups. It has already been pointed out t h a t this is true in t h e case of cellulose and lignin. Mannan and galactan determinations depend upon reactions too specific t o admit of the inclusion of other constituents. By improving the mechanical condition of the sample in respect t o fineness, better results should be obtained in all the determinations. Some of the determinations, particularly t h a t of galactan, could probably be made more accurate and reliable. These changes would no doubt give a considerably better summation and a consequently more satisfactory accounting of the constituents of the coniferous woods. S U 14 MA R Y I-An improved procedure is described for the summative analysis of coniferous woods. Methods are given for t h e estimation of the following constituents: loss on drying, benzene extract, alcohol extract, cellulose, lignin, soluble pentosans, mannan, and galactan. 2---By t h e omission of preliminary hydrolysis, more reliable results are obtained in the cellulose and lignin determinations t h a n by methods previously used. 3---Soluble pentosans are determined in the chlorination liquors, mannan and galactan on separate portions of the original material. Improvements in t h e galactan determination are described. 4--Complete analyses of redwood, yellow pine, and sugar pine are carried out by these methods. A summation of slightly over I O O per cent is obtained in every case. 5--The results indicate t h a t overlapping of t h e proximate groups, i. e . , partial inclusion of any constituent in more t h a n one group, has been largely avoided. 6--Analyses by the proposed scheme probably A certain sample of sulfur was reported t o contain arsenic. It had been tested by a method first devised by Schaeppil and later accepted b y many authorities.2 We found t h a t the sulfur contained no trace of arsenic, b u t t h a t when i t was tested by Schaeppi's method, a decided qualitative test for arsenic was obtained. On further investigation it was found t h a t this method contains a fundamental error which makes it useless as a test for small quantities of arsenic. The method as described by Schaeppi consists essentially in first digesting the sulfur with dilute nitric acid for t h e purpose of removing calcium chloride, sulfate, and sulfide. The sulfur washed free from acid is now treated with dilute ammonia solution for 1 5 min. a t 7 o o t o 80' C. Schaeppi assumes t h a t any arsenic present in the sulfur must exist as sulfide, mainly arsenious sulfide, and will be dissolved b y t h e ammonia. The arsenic is then estimated indirectly through determination of the amount of sulfide by precipitation with silver nitrate in neutral solution. "For very rough estimations, i t is sufficient t o treat I O g. of finely ground sulfur with nitric acid, t o extract with ammonia, and t o add silver nitrate. From the intensity of the color or the quantity of t h e precipitate of silver sulfide, i t may be judged whether the sulfur is approximately free from arsenic or is strongly contaminated."3 Schaeppi endeavored t o test the accuracy of his method by dissolving known amounts of pure arsenious sulfide in ammonia solution, carefully neutralizing with nitric acid, and titrating with decinormal silver nitrate using chromate as indicator. His results showed fairly good agreement between t h e quantities of arsenic taken and those found. However, his results would have 1 Chem. I n d . , 4 (1881). 409.