C. L. LU K E
Bell Telephone Laboratories, In c ., New York, N. Y.
A n e w v o lu m e tr ic m e th o d h a s b e e n de
veloped fo r th e d e t e r m in a tio n o f s u lfa te s u lfu r . T h e s u lfa te is re d u ce d to su lfid e b y t r e a t m e n t w it h h y d r io d ic a c id a n d th e h y d ro g e n su lfid e is d is tille d off a n d t itr a te d io d o m e lr ic a lly . T h e n e w m e t h o d h a s b e e n a p p lie d to t h e d e t e r m in a tio n o f t o t a l s u l
f u r i n n a t u r a l a n d s y n th e tic ru b b e r .
W
IT H the recent increase in research on natural and synthetic rubber there has arisen a need for a rapid method for the determination of total sulfur in rubber samples where only a semimicrosample can be spared for analysis.
The usual methods in which the rubber is oxidized by di
gestion with acid, followed in some cases by fusion to complete the oxidation, and the sulfur is precipitated and weighed as barium sulfate, yield good results on semimicrosamples but are time-consuming because quantitative precipitation of the sulfate cannot be attained unless the solution is allowed to stand overnight before filtration. Several volumetric meth
ods, most of which depend upon the precipitation of the sulfur as barium sulfate, have been described, but none is very satis
factory in the determination of small amounts of sulfur.
St. Lorant (2) has proposed a volumetric method for the determination of microsamples of sulfate, in which the sulfate is reduced to sulfide with hydriodic acid, and the hydrogen sulfide is distilled off and determined colorimetrically. The author has recently developed a modification of this method which has proved to be very satisfactory in the analysis for sulfur in small samples of rubber. The procedure consists essentially of oxidation of the rubber in the usual manner, reduction of the sulfate to sulfide, separation of the hydrogen sulfide by distillation, and titration of the latter iodometri- cally.
The new method is very rapid (20 to 25 minutes for the distillation and titration) and is not subject to the errors of
adsorption, occlusion, and solubility encountered in the gravimetric barium sulfate method, or to difficulties in de
tection of the end point encountered in some of the volu
metric methods. I t possesses the distinct advantage over some of the gravimetric methods, that it is applicable to the analysis of rubber containing barium, lead, and calcium.
The only serious objections to the method from the stand
point of general use are that hydriodic acid is expensive and that the applicability of the method is somewhat limited because of the fact that no more than about 5 mg. of sulfur can be handled conveniently. This means that the sample size must be small when analyzing rubber high in sulfur;
and when small samples are used there is always the danger of error due to unequal distribution of sulfur in the sample.
A p p a r a tu s
The apparatus is shown in Figure 1. It consists of a 125-ml.
Erlenmeyer flask with standard taper 24/40 joint, a distillation head with capillary pressure regulator, and a 300-ml. Erlenmeyer flask with standard taper 24/40 joint. The capillary tubing is 2 mm. in inside diameter and extends to within 2 to 4 mm. from the bottom of the 125-ml. flask.
R e a g e n ts
N i t r i c A c id - Z in c O x id e - B r o m in e M i x t u r e . Dissolve 10 rams of zinc oxide in 100 ml. of nitric acid and saturate with romine.
A c id M i x t u r e f o r D i s t i l l a t i o n . Place 160 ml. of hydriodic acid (specific gravity 1.70), ICO ml. of hydrochloric acid, and 45 ml. of hypophosphorous acid (50 per cent) in a 500-ml. Erlen
meyer flask. (Ii the hydriodic acid contains hypophosphorous acid as a preservative use only 40 ml. of hypophosphorous acid.) Add a few grains of silicon carbide and boil vigorously without cover for 5 minutes. Cool in an ice bath to room temperature.
Keep stoppered in a 500-ml. brown glass-stoppered bottle to avoid oxidation of the hydriodic acid.
A m m o n ia c a l C a d m iu m C h l o r i d e S o l u t i o n . Dissolve 10 grams of cadmium chloride dihydrate in water. Add 500 ml.
of ammonium hydroxide and dilute to 5 liters.
S t a n d a r d P o t a s s iu m I o d a t e S o l u t i o n (0.01 N). Recrystal- lize c. p. potassium iodate from water twice and dry at 180 0 C . to constant weight. Weigh 0.7134 gram of the pure potassium iodate and dissolve in water. Add 2 grams of sodium hydroxide and then 10 grams of potassium iodide (free from potassium
A N A L Y T I C A L E D I T I O N 603
Fi g u r e 1
iodate). After complete solution of all salts adjust to room temperature and dilute to 2 liters in a volumetric flask.
S t a n d a r d S o d iu m T h i o s u l f a t e S o l u t i o n (0.01 N). Dis
solve about 5 grams of sodium thiosulfate pentahydrate in 2 liters of freshly boiled and cooled distilled water. Store in a clean Pyrex bottle. To standardize this solution, pipet 25 ml. of the standard potassium iodate solution into a 300-ml. Erlenmeyer flask. Add 200 ml. of water and 20 ml. of hydrochloric acid.
Allow to stand 1 minute and then titrate with the thiosulfate solution. As the end point is approached add 10 ml. of 0.2 per cent starch solution and titrate carefully until the solution is colorless. The thiosulfate solution should be restandardized every 15 days.
S t a r c h S o l u t i o n . Add a cold aqueous suspension of 2 grams of soluble starch to 1 liter of boiling water. Cool to room tem
perature and store in a clean bottle.
P ro c e d u re
Place 0.050 to 0.100 gram of the rubber sample, which has been either “crumbed” or cut into as small pieces as possible, in a 125- ml. Erlenmeyer flask with ground-
glass joint. (The method as written r, , ,, . rrTr. , . , T o r.
is applicable to the analysis of semi- [(Ml. of M O ;___ ml. of is a:S20;
until most of the organic matter is destroyed and the acid is all but expelled. With difficultly oxidizable samples such as Buna S and Butyl rubber it may be necessary to add more fuming nitric acid and take down to near dryness 2 or 3 times before enough of the sample has been oxidized to make it safe to continue. Fi
nally, remove the cover and bake over aTirrill flame until the zinc nitrate is converted to zinc oxide and most of the brown fumes are expelled. Return the flask to the hot plate to cool and to allow the remaining fumes to be expelled. Cool to 30 ° C. Wash down the sides of the flask with 5 ml. of hydrochloric acid and boil without cover to expel about half of the acid. It is e&^ntial that all nitrates be expelled in order to avoid interference in the titra
tion described below. Cool to room temperature and reserve and boil gently at a hot-plate temperature of 180° C. to 200° C.
until white perchloric acid fumes appear. During this boiling it is essential that the flask be completely covered with a watch glass. With difficultly oxidizable material such as Butyl rubber considerable amounts of organic material may remain after ex
pulsion of the nitric acid. If this occurs, the oxidation with perchloric acid should be allowed to proceed only to the point where the sample begins to char. At this point add more fuming nitric acid and repeat the evaporation. Repeat if necessary.
Finally,’when most of the organic matter is destroyed, the sample can safely be heated until fumes of perchloric acid appear without danger of ignition or explosion. Fume vigorously on a flame without cover to expel all but about 2 ml. of the acid. Cool to room temperature and reserve for the distillation.
After oxidation of the sample, add about 5 grains of silicon carbide (12-mesh grains that have been boiled in hydrochloric acid) to prevent bumping, and 35 ml. of the acid mixture and immediately cap with the distillation head. Place on a hot plate with surface temperature of 375° C. to 400° C. and boil for 5 min
utes, catching the distillate in 150 ml. of ammoniacal cadmium chloride solution in a 300-ml. Erlenmeyer flask with ground-glass joint. The boiling should be timed from the moment fumes of ammonium chloride are first seen in the 300-ml. flask.
Remove the small flask from the plate and detach the distilla
tion head. (If difficulty is encountered in removing the head, cool the joint and then run hot water on it.) Cool the distillate rapidly in an ice bath to 10° C. (In the analysis of samples containing over about 5 mg. of sulfur, add an excess of potassium iodate to a 125-ml. flask containing 20 ml. of hydrochloric acid and 20 ml. of water. Pour this solution in one stroke into the flask containing the sample. Stopper immediately and shake to oxidize all the sulfide in the flask. Wash the remains of the iodine into the large flask and back-titrate with thiosulfate as directed.)
Add 20 ml. of hydrochloric acid and swirl once to mix thor
oughly. Titrate immediately and rapidly with potassium iodate solution, using a rapidly flowing buret— (i. e., a 50-ml. buret which will empty by gravity in 60 to 70 seconds)—until an excess of about 1 or 2 ml. has been added, as indicated by the yellow color.
Cap immediately with a ground-glass stopper and shake vigor
ously to entrap any hydrogen sulfide in the atmosphere in the flask. Allow the potassium iodate buret to stand 1 minute after the titration before reading it. Add 10 ml. of starch solution and titrate with standard sodium thiosulfate solution (0.01 N).
Carry a blank through the whole procedure. The blank should not be greater than 0.1 ml. of potassium iodate solution (0.01 N):
X KIO» factor) — blank] X 0.01603 microsamples of rubber. If an analy
sis of larger samples is desired ap
propriate increases in the amounts of reagents must be made.
It is best, however, to limit the amount of sulfur in the sample to 5 mg. or less.)
(When highly saturated material such as Butyl rubber is to be analyzed, it is desirable to limit the sample size to 0.05 to 0.10 gram in order to decrease the time of oxidation and danger of explosion with perchloric acid. In general, oxidation of diffi
cultly oxidizable material is more rapid and convenient with the perchloric acid method than with the A. S. T. M. method.)
Proceed by either of the following methods:
A. S. T. M . M e t h o d . Add 4 ml. of nitric acid-zinc oxide- bromine mixture. Cover and heat on a steam bath or low-tem- perature hot plate to decompose the sample. When rapid solu
tion ceases add 3 ml. of fuming nitric acid. Cover, and, while swirling the flask to prevent ignition of the sample, heat on a hot plate with surface temperature maintained at 180° to 200° C.
When danger of ignition is past, allow the solution to boil gently
Sample weight in grams = per cent sulfur
D is c u s s io n
The proposed volumetric method for the determination of sulfate yields satisfactory results when the amount of sulfur to be determined is less than about 10 mg. although slightly low values are obtained when the amount of sulfur present exceeds about 1 mg. (see Table I). W ith quantities greater than about 5 mg. special precautions must be taken to avoid loss of hydrogen sulfide during the titration.
Low results are obtained if the hydrogen sulfide is allowed to escape during the distillation or titration, if the temperature of distillation is too low, if the concentration of hydriodic acid is too low, and if nitrates in quantities greater than traces are present at the distillation. High results may be obtained
604 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Titrated with K IO i using CC1* as indicator.
1 No H CiO , present in acid mixture.
0.2 gram of B aCIj.2H iO present.
0.05 gram of PbCIi present.
Different K1SO1 solution used.
unless a blank correction for the reagents is made. Because of the danger of exceptionally heavy contamination-of the hypophesphorous acid with sulfur compounds, it is necessary to resort to a preliminary distillation of the acid mixture in order to reduce the blank to a reasonable figure.
In preliminary work the usual distillation apparatus for the evolution method for sulfur in steel was used. This proved unsatisfactory because the variation in the diameter of the neoprene rubber stoppers and the neck of the Erlen
meyer flasks was so great that the distance between the bottom of the flask and the end of the capillary pressure regulator could not be conveniently controlled. In the apparatus finally chosen the rubber stopper was replaced by a standard taper ground-glass joint.
When hydrogen sulfide is titrated directly with standard iodine using starch as indicator the end point is very poor, owing to the appearance, in the solution, of a permanent reddish color wliich obscures the blue of the end point. The color is probably caused by the adsorption of some compound of iodine and sulfur on the starch molecule. As evidence of this it is not*d that very little free sulfur appears during the titration of those solutions which contain starch. Experiments indicated that the red color is more pronounced the colder the solution and the greater the salt concentration, especially if the salt is an iodide. It was found that in the titration of hydrogen sulfide in solutions containing but small quantities of salts the red color could be almost completely eliminated by adding about 0.5 mg. of mercuric iodide or mercuric chloride to the solution or by titrating at 35° C. to 40° C. Un
fortunately, the addition of mercury caused the results to be slightly low, probably because of the conversion of part or all of tlie mercury to sulfide. Because of this, and the fact that the use of mercury salts is not very effective in the presence of appreciable quantities of salts, the problem was not considered further.
The best method for titrating hydrogen sulfide is that in which an excess of iodine is added, followed by back-titration with thio- sulfate using starch as indicator. This method has the advantage that the end point is good and the danger of loss of hydrogen suifide can be greatly reduced by rapid overtitration with iodine using a fast flowing buret.
It will be seen in the procedure that no extra iodide is added to the solution previous to titration, as is sometimes done in direct titrations of sulfide with iodine. This addition is unnecessary and in fact is purposely omitted in the method because a con
siderable amount of hydriodic acid comes over with the hydrogen sulfide, and any further addition of iodide is detrimental to the starch-iodine end point.
The two gravimetric methods most often used for the determination of sulfur in rubber stocks which are free from barium and large amounts of lead and calcium are those of Wolesensky (3, hereinafter called the perchloric acid method), and Kratz, Flower, and Coolidge (1, hereinafter called the A. S. T. M . method). In the analysis of representative
samples of natural and synthetic rubber stocks these two methods give results which are in good agreement. In like manner it was found (see Table II) that results obtained by the proposed volumetric method and the perchloric acid method check very well, although as would be expected from the data in Table I, the volumetric method gives somewhat lower results.
In agreement with Wolesensky it was found that great care must be taken to prevent loss of sulfur during the diges
tion of rubber samples with nitric and perchloric acids. The loss is negligible if the sample is digested slowly in a long
necked Kjeldahl flask. When a small Erlenmeyer flask is used, however, the loss may amount to 1 or 2 per cent (when the new volumetric method is used for the final determina
tion), unless the digestion is slow and the flask is kept com
pletely covered with a watch glass until all organic matter is expelled. In the procedure an attempt has been made to minimize this loss by delaying the addition of the perchloric acid until most of the readily oxidizable material has been oxidized. After oxidation of the organic material the sample can be taken to copious fumes of perchloric acid without loss of sulfur, providing an excess of zinc is present. This sug
gests that the sulfur, which is lost during the digestion of the organic matter, has not been oxidized to sulfate. No loss of sulfur is encountered when the A. S. T. M . method is used.
Vol. 15, No. 9
A standard solution of potassium sulfate was prepared by dis
solving 0.6780 gram of the pure dry salt in water and diluting to 1 liter in a volumetric flask. Aliquot portions of the solution were evaporated to dryness in 125-ml. Erlenmeyer flasks. The sam
ples were then analyzed for sulfur as directed in the procedure, except that an acid mixture of the following proportions was used:
100 ml. of hydriodic acid (specific gravity 1.70), 100 ml. of hydro- cliloric acid, 25 ml. of hypophosphorous acid (50 per cent), and 25 ml. of perchloric acid (60 per cent). The results are shown in Table I.
Four representative types of rubber stocks (containing no barium, lead, or calcium) were analyzed for sulfur as directed in the procedure. Results were obtained using both the A. S. T. M . and the perchloric acid methods of oxidation.
The samples were also analyzed by the gravimetric per
chloric acid method. The results are given in Table II.
A c k n o w le d g m e n t
The author wishes to express his gratitude to B. L. Clarke and M . L. Selker of these laboratories, who read the manu