Quantitative Estimations of Minute Quantities of Reducible Sulfur PHILIPC. SCHERER, JR.,AND W. W. SWEET,Virginia Polytechnic Institute, Blacksburg, Va. any convenient length with a slight R A P I D and a c c u r a t e A METHOD for the determination of sulfur constriction 5 cm. from one end m e t h o d is required by has been worked out which, under rigorously formed by softening that portion various industries for the of the tube uniformly in a fine hot controlled conditions, will determine accurately estimation of minute quantities flame, and drawing slightly. The from 0.000001 to 0.000015 gram of sulfur. It end of the tube nearest the conof reducible sulfur in materials striction is rounded in the flame, can be used for rough estimation of sulfur up upon which they operate. This but the other end is left raw, as is particularly true of the pulp to 0.0001 gram and may be applied to the deterwhen cut. The constricted end is and paper industry for the proinserted through a well-rolled nonmination of reducible sulfur in pulp and paper. porous cork stopper so that it production of anti-tarnishing tissue This method should be applicable to the estijects through the stop er about for wrapping silver-ware. The 0.5 cm. Before each leterminamation of sulfur, reducible by metallic aluminum rayon industry is also interested tion a fresh small wisp of Bauer and hydrochloric acid, in rayon, blood, urine, in such a method for the control and Black’s best grade of absorbent cotton is placed in the exof the desulfurization process in and other biological products, synthetic organic treme lower end of the tube. the production of finished viscompounds containing suIJur, and various comA complete assembly of the apcose rayon. paratus is shown in Figure 1. mercial products where the possible subsequent Several me t h o d s have been REAGENTS. Test strips, made evolution of hydrogen suljide may be detrimental by soaking sheets of Whatman’s proposed in the past. They all No. 40 filter paper in a solution of to its use. It will not determine sulfur present depend, as does this one, upon 1per cent lead acetate, pressing out as sulfates or other non-reducible forms. some quantitative color reaction the excess solution (water solution) of hvdrogen sulfide. Those of between sheets of dry filter paper, Yoe “(6),b u s h e l and Elston ( I ) , Sutermeister (4), Sheppard and hanging up in the dark to dry. When dry, the sheets are flat and cut into strips 5 mm. in width and of any conand Hudson (S),and the Technical Association of the Pulp and pressed venient length, and stored in cork-stoppered test tubes in the Paper Industry (6) are the only ones which have received any dark until required for use. practical application by the industries. The methods of Yoe, Distilled water, unless otherwise specified, is made by the and Sheppard and Hudson, which involve the production method of Kraus and Dexter ( 2 ) using ordinary distilled water of methylene blue, have the drawback of requiring several containing a small amount of potassium permanganate and sodium hydroxide as a starting point. It is always boiled to redays for the development of maximum color for very small move any dissolved oxygen immediately before using. quantities of sulfur. The methods of Drushel and Elston, Hydrochloric acid solution, made by diluting Baker’s c. P. Sutermeister, and the T. A. P. P. I., which estimate the analyzed concentrated hydrochloric acid, sulfur-free, to onequantity of sulfur from the stain produced on lead acetate third of the original volume with distilled water and kept in paper by passing the evolved gases through the test paper, tightly stoppered bottles when not in use. Sodium hydroxide solutions of various concentrations as reinvolve a source of considerable inaccuracy in that the stains quired, with any high-grade o. p. sodium hydroxide, which has produced tend to be spotty, owing t o variations in porosity been shown by this method t o be free from sulfur, made with of the lead acetate paper, thus introducing difficulty in distilled water, and containing 0.1 gram of hydroquinone per liter. Standard solution, made by diluting a 0.5 N solution (strength comparing them. determined by titration with standardized iodine solution) of sodium thiosulfate with distilled water so that it contains 0.000001 PRINCIPLES OF NEWMETHOD gram of sulfur per cc., and 5.0 grams of sodium hydroxide and The method of the authors depends upon the slow reduc- 0.1 gram of hydroquinone per liter, and kept in the dark except tion of the sulfur compounds by means of some metal which when required for use. Aluminum reductor strips, pure sheet aluminum cut into has a strong reducing action in the presence of hydrochloric strips 1.5 by 3.0 cm. and from 1.0 to 1.5 mm. thick. They are acid, and subsequent sweeping of the reduced sulfur com- cleaned just before using by placing them for 1 minute in boiling pounds, hydrogen sulfide, out of the container past a strip 1.0 per cent hydrochloric acid solution and then washing of dry lead acetate paper so that they meet the test strip thoroughly with distilled water. endwise. If the evolved gases contain the hydrogen sulfide Place the weighed sample or aliquot portion of a solution of in the same concentration, stains on the test strips of comparable length and color will be obtained for successive the sample in 30 cc. of 0.5 per cent sodium hydroxide solution and heat just below the boiling point for 10 minutes. determinations. Cool .to room temperature, add two reductor strips and 30 The reactions involved are: cc. of the hydrochloric acid solution, and stopper immediately 1. Reductions: with the cork containing the exit tube, which has been preNasSQs 2HC1 6[H] -+ 2NaCl HeS 3HzO viously steamed and dried and has had a test strip inserted NatSaOs 2HC1 8[H] +2NaCl 2H2S 3H20 so that the lower end rests on the constricted portion of the NazS 2HC1 --3 2NaC1 HzS tube. At the end of 2 hours, or when the reaction has ceased, AI 3HC1 +AlCla 3[H] unstopper the flask and shake out the test strip, and com11. Production of stain: pare with standards for from 0.000001 to 0.00001 gram of Pb(CzHaO2) HzS +PbS 2HCzHsOa sulfur, prepared by substituting from 1 to 10 cc. of the standard sulfur solution for an equal volume of the 0.5 per PROCEDURE cent sodium hydroxide solution in the above method. If APPARATUS.Container, 250-cc. Erlenmeyer Pyrex flask, sethe strips are to be kept for any length of time for reference, lected for symmetry of opening. Exit tube or test-strip holder, glass tube of 6-mm. bore and they should be protected from the light and atmosphere.
A
+
+
++ ++
+
+++ + +
++
103
104
ANALYTICAL EDITION
Fresh standard strips should be made up when the last set is not more than 2 weeks old. I n order to obtain comparable results, the above method must be adhered to strictly, as any variation from the above factors of quantity of reductor, acid concentration, and temperature gives rise to stains of indefinite color, length, and general appearance. This is shown by the following experimental observations, using 0.000010 gram of sulfur in each case. m m.-Rore < h r s Tube
\ FIGURE 1. COMPLETE ASSEMBLY OF APPARATUS
Vol. 4, No. 1
as reductors, using 1 gram of the metal per determination, and were found to contain 0.000008 to over 0.000035 gram of sulfur per gram of metal. Samples taken from the same package showed variations of up to 0.000003 gram of sulfur per gram of metal. Magnesium and calcium were tried as substitutes for aluminum and found to be easily obtained in a form free of sulfur, but they offer the difficulty of being so highly reactive that even in small quantities in the presence of known quantities of sulfur the hydrogen is evolved so rapidly that long stains of varying color and intensity for the same amount of standard are obtained. INFLUENCE OF HEAVYMETALSIN SAMPLE. Solutions of the chlorides of copper, nickel, and lead within the same range of dilution as the standard solution were placed in measured amounts in the presence of more than enough sulfur to form cupric, nickelic, or plumbic sulfide, as the case may be, and within the limits of error of observation were found in every case to retain exactly the chemically equivalent amount of sulfur required to form the sulfides mentioned above. RANGEOF APPLICATION.From 0.000001 to 0.000015 gram of sulfur comparable stains are obtained. Above this maximum, the amount of sulfur cannot a t best be estimated any closer than multiples of 0.000005 gram of sulfur, because of variations in color and intensity of the stain on each side of the same strip. Above 0.0001 gram of sulfur the results are worthless. USE OF ORDINARY DISTILLED WATER. The distilled water produced in the laboratory by a Barnstead still was found to contain 0.000002 gram of sulfur per 50 cc. according to this method.
EXPERIMENTAL PRACTICAL APPLICATIONS The standard method was applied to several different commercial pulps and papers and anti-tarnish tissues, with the following results: SAMPLE
QUANTITY OF REDUCTOR. 1. Placed one reductor strip in the flask in place of two. Stain equivalent to 0.000007 gram of sulfur and rather pale in color. 2. Used two reductor strips. Stain dark, with sharp boundaries and of the same length on both sides of the strip. 3. Used three reductor strips. Stains of good color and sharp boundaries, but in all of ten different trials they varied in length of stain from the equivalent of 0.000008 to 0.000009 gram of sulfur on one side of the strip, to 0.000001 or 0.000012 gram of sulfur on the other. 4. When more than three reductor strips were used, the stain ran out into a light streamer of varying length and color for two sides of any one strip. ACID CONC~NTRATION. Caused uniformly low results for amounts less than 25 cc. and uneven stains for more than 35 cc. of the acid called for. TEMPE~ATURE. A matter which will ordinarily be taken care of by the heat of solution of the aluminum but, if for any reason the temperature becomes so high that drops of water form in the tube at or above the constriction, lead chloride will be formed on the stain already present, making it impossible to obtain an accurate reading. When the cotton filter is left out, the above result is always obtained. EXPERIMENTAL RESULTS USE OF OTHERSTANDARDS.Solutions of sodium sulfide, sodium sulfite, and sodium bisulfite made up to contain 0.000001 gram of sulfur per cc. gave equivalent results to that of the chosen standard, but it was found that the sodium thiosulfate standard was much superior to the others from the point of view of keeping quality, all the others tending t o fall off in strength with time. USE OF OTHERMETALSFOR REDUCTORS.With zinc, none Free from sulfur is readily obtainable. Several different brands of c. P. reagent-grade zinc marked “sulfur-free” were tried
SI’LFUR
% A
B C D E F G H I
0.0004
0.0008 0.0012 0.0004 0.004 0.0008
0.0002 0.0008
0.0001
A, High-grade sulfite pulp, well bleached and having a mill treatment with hot 3 per cent caustic solution. B, Unbleached sulfite pulp. C, Same pulp as B, but having had a great deal of mechanical handling while wet. D, Very soft and absorbent sulfite pulp that has received a very thorough bleaching and treatment with strong caustic (strength unknown) in the mill. E, Unbleached kraft wrapping paper. F, Same pulp as was used for E after a thorough bleaching with soda bleach. G, H, and I, Tissues sold by three different manufacturers specifically for wrapping silver-ware.
LITERATURE CITED (1) Drushel, W. A., and Elston, C. M., Am. J. Sci., 42, 155 (1916). (2) Kraus, C. A., and W. B. Dexter, J. Am. Chem. SOC.,44, 2468 (1922). (3) Sheppard, S. E., and Hudson, J. H., IND.ENG.CHEM.,Anal. Ed., 2, 73 (1930). (4) Sutermeister, E., “The Chemistry of Pulp and Paper Making,” Wiley, 1929. (5) Tech. Assocn. Pulp and Paper Ind., Official Methods, 1928. (6) Yoe, C. H., “Photometric Chemical Analysis,” Vol. I, p. 375, Riley, 1928. RECEIVEDJune 29, 1931. From a thesis submitted by W. W. Sweet in partial fulfilment of the requirement8 for the degree of bachelor of scienoe.
