Determination of Total Sulfur in Rubber C. L. LUKE Bell Telephone Laboratories, Inc., N e w York,
N. Y.
Heat the acid solution until fumes of ammonium chloride begin to appear over the ammoniacal solution and then continue for 10 minutes longer. Remove the dist.illing flask and detach the distillation head a t once. Transfer the traces of cadmium sulfide from the condenser tube of the distillation head to the ammoniacal solution with the aid of a policeman and wash bottle. Buret or pipet enough standard 0.1 N potassium iodate solu.tion to provide a 5- to 25-m1. excess over that re uired for oxidation of the sulfide, to a 500-ml. iodine flask. A%d 25 ml. of hydrochloric acid to the iodate in the flask and wash down the sides with water. Immediately pour the ammoniacal solution into the flask and transfer all the sulfide with the aid of a policeman and wash bottle. (If the starch-iodine color disappears, showing that too little iodate has been used, immediately add more iodate from a buret to provide about a 5-ml. excess.) Immediately stopper and shake vigorously to entrap any sulfide in the atmosphere in the flask. Titrate with standard 0.1 N sodium thiosulfate solution. Run a blank through the whole procedure, using 5 ml. of potassium iodate solution for the oxidation of the sulfide. If (ml. of K103 - ml. of NanSzOsX KIOa factor) = A and (ml. of KlOs for blank ml. NazSnOs for blank X KIOa factor) = B.
The range of the hydriodic acid reduction-iodometric titration method for the determination of sulfur in semimicrosamples of rubber has been extended to provide for the analysis of macrosamples containing as much as 100 mg. OF sulfur.
I
Tu' A recent article ( I ) , the author described a rapid method for the determination of total sulfur in small samples of rubber where the sulfur present did not exceed about 5 mg. This method has proved to be so rapid and convenient that it has appeared desirable to extend its applicability to larger quantities of sulfur. Experiments have shown that this can be done very easily by making a few simple changes in technique. REAGENTS
NITRIC ACID-ZIXCOXIDE-BROMINE MIXTURE. Dissolve 20 granis of zinc oxide in 100 ml. of nitric acid and saturate with bromine. ACIDMIXTUREFOR DISTILLATION. Place 320 ml. of hydriodic acid (specific gravity 1.70), 320 ml. of hydrochloric acid, and 90 ml. of hypophosphorous acid (50%) in a 1-liter Erlenmeyer flask. 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 brown glass-stoppered bottle to avoid oxidation of the hydriodic acid. AMMONIACAL CADMIUM CHLORIDESOLUTION.Dissolve 20 grams of cadmium chloride dihydrate in water. Add 200 ml. of ammonium hydroxide and 150 ml. of 1% starch solution and dilute to 2 liters. NITROGEK.Commercial "prepurified" lecture bottle tank nitrogen. STANDARD POTASSIUM IODATE SOLUTION (0.1 N ) . Recrystallize C.P. potassium iodate from water twice and dry a t 180" C. to constant weight. Weigh 7.134 grams of the pure potassium iodate and dissolve in water. Add 2 grams of sodium hydroxide and then 50 grams of potassium iodide (free from potassium iodate). After complete solution of all salts adjust to room temperature and dilute to 2 liters in a volumetric flask. Dissolve ST.4NDARD SODIUM THIOSULF.4TE SOLETIOIL. (0.1 about 50 grams of sodium thiosulfate pentahydrate in 2 liters of freshly boiled and cooled distilled water. Storein a clean Pyrex bottle. To standardize this solution, pipet 25 ml. of the standard potassium iodate solution into a 300-ml. Erlenmeyer flask. Add 150 ml. of water and 20 ml. of hydrochloric acid and then titrate with the thiosulfate solution. As the end point is approached add 2 ml. of 1% starch solution and titrate carefully until the solution is colorless. STARCH SOLUTION.Add a cold aqueous suspension of 10 grams of soluble starch to 1 liter of boiling water. Cool to room temperature and store in a clean bottle.
-
then
Table No. 2
30 4
5
6b 7 8a 9
'
I. Determimation of Sulfur in Potassium SulFate b y the Proposed Method KIOs Used Sulfur Found
Sulfur Present
.Mg. 1
-
0.1603 ( A = per cent sulfur sample weight in grams
no
5.0 5 10 10 10 10 25 50
0 0 0 0 0 0 0
M1. 5
6 25 50 10 10 10 25 50
Mg.
Error
Ma.
5 1 5.0
+o
5.1
+0.1 -0.0
10.0 10.0 9.9 10.0 25.0 50.0 49 8
on
2
1 kO.0
LO.0 -0.1
*o.o
tO.0 -0 0 -0.2
+o
2
x).
a 6
HNOa-ZnO-Bri mixture used, Distillate containing CdS cooled to 10' C. before titration.
DISCUSSION
I n the above method the distillation is performed at, a m w h loxer temperature than that previously used and gasem5 nitr& gen is used to sweep the hydrogen sulfide out of the flask. This procedure eliminates the danger of loss of sulfide caused by incomplete absorption in ammoniacal cadmium chloride solution when large quantities of hydrogen sulfide are suddenly distilled over. The method cannot be used on samples which contain barium (and presumably lead and calcium) because the insoluble sulfate is but slowly decomposed. The iodometric titration of large amounts of hydrogen sulfide suggested in the previous publication is not very sound, since t h e loss of a certain amount of iodine can hardly be avoided. Attempts to titrate the sulfide by addition of a measured excess of a standard Wijs solution (iodine monochloride in glacial acetic acid), followed by back-titration of the excess iodine, failed because high results were obtained, due presumably to partial oxidation of the sulfide to sulfate. When large amounts of sulfide are titrated iodometrically the precipitated sulfur may occlude appreciable amounts of iodine.
PROCEDURE
Dissolve 0.5 gram of the rubber sample (containing 5 to 100 mg. of sulfur) in 10 ml. of nitric acid-zinc oxide-bromine mixture and 15 ml. of fuming of nitric acid in a 200-ml. Erlenmeyer flask as described (1). After destruction of the organic matter and conversion of zinc nitrate to oxide add 10 ml. of hydrochloric acid and boil down to near dryness to expel all oxides of nitrogen. Repeat if necessary. Add 35 ml. of hydriodic acid mixture, and immediately cap with the distillation head. Place on a hot plate with surface temperatureof 170"to 200"C.with the condensertube dipping to within about 0.5 cm. from the bottom of a 300-ml. tallform beaker containing 150 ml. of ammoniacal cadmium chloride solution. Pass a slow stream of pure nitrogen through the solution by way of the capillary pressure regulator tube. Adjust the flow of nitrogen so that it escapes from the receiver solution a t a rate of about 3 bubbles per second. (The rate of flow is not critical, providing it is not so fast as to prevent complete absorption of the hydrogen sulfide by the receiver solution; or so slow as to permit the ammoniacal solution to be sucked back into the acid solution.) 298
May, 1945
ANALYTICAL EDITION
This error can be eliminated by dissolving the sulfur in carbon disulfide, but a better procedure is to keep the sulfur colloidally dispersed with the aid of starch. I t is essential that nitrogen acids be completely expelled before the distillation with hydriodic acid, otherwise sulfide will be oxidized to sulfur during the distillation and unstable end points will be encountered in the titrations with thiosulfate. A P P L I C A T I O N OF THE M E T H O D T O I N O R G A N I C A N A L Y S I S
The new method for the determination of sulfate (I) has proved very useful in t,he analysis for sulfur in chemicals and alloys where the usual methods fail. Following is an outline of the method used in these laboratories for the determination of 0.0005 to 0.0570 sulfur in molybdenum permalloy-Le., 7900 nickel, 17YG iron, and 4cc molybdenum. Dissolve 10 grams of the metal in 80 ml. of aqua regia. Evaporate to moist dryness to expel most of the excess acid. Add 35 ml. of hydrochloric acid and heat to dissolve all soluble salts. Add 25 ml. of formic acid and heat to destroy the nitric acid. Transfer the solution to a 200-ml. flask and reduce the volume to 50 to 60 mi. by boiling. Add 35 ml. of acid mixture and perform
299
the distillatiun a s tlirectcd i t i tlic proccdutv above, doublitig tiit: time or' distillntion (and al.50 the con(witritioti oi ammonium hydroxide i t 1 tlw rweivcr) iti ortlcr t o a=ure complcte expulJion oi the hydrogcri .sulfitle. Titr.ite !I$ dirrcted ( 1 ) . EXPERIMENTAL
A standard solution of potassium sulfate ma< prepared by dissolving 27.18 grams of the pure dry salt in water and diluting to 1 liter in a volumetric flask. Aliquot portions of the solution were evaporated to dryncss in 200-ml. Erlenmeyer flaski. Hydriodic acid mixture WAS added and the samples were then analyzed for sulfur as directed in the procedure. I n some instances, 10 ml. of nitric acid-zinc oxide-bromine mixture were added to the sulfate sample and the nitric acid and bromine were expelled before the distillation and titration (see Table I). ACKNOWLEDGMENT
The author is indebted to B. L. Clarke of the3e laboratories who read thp mnnuscript. LITERATURE CITED
(1) Luke, C. L..
ISD. ESG.CHEM., ASAL.ED.,15, 602 (1913).
Laboratory Corrosion Tests R. M. BURNS Bell Telephone Laboratories, Inc., N e w York,
A critical review of laboratory corrosion tests i s presented, with description of methods of measurement, preparation of test specimens, and discussion of tests and testing facilities.
A
CORIIOSIOS test is a procedure for determining the rate of
reaction between a metallic material and a nonmetallic element of the surrounding environment. The primary electrolytic reactions of the process consist in the anodic dissolution of metallic ions and the cathodic reduction of hydrogen ions, other mctallic ions, or nonmetallic elements. These reactions occur a t high velocity. Secondary reactions which depend upon composition, diffusion, convection, and solubility of components of the system proceed much more slowly and usually control the rate of the corrosion process. RIost environments present a multiplicity of factors which may affect corrosion reactions and it becomes necessary to recognize those which are of significrance in a given instance. Of compositional components there are two classes, those which retard and those which accelerate corosion. When these co-exist in a natural environment, as is usually the case, their relative proportions or activities may determine the character as well as the rate of corrosion. For example, when the ratio of retardant to accelerator is high there may be no corrosive attack; vvhen it is low there may b e general corrosion over the metal surface, while for int,ermediate values of this ratio severe pitting action may develop. Practical corrosion tests are often more concerned with the determination of rate of corrosion in localized areas than in the over-all corrosion rate of the metallic specimens. For example, tests are designed to measure pitting, loss of tensile strength, etc. The purposes for which laboratory corrosion tests are to be used may determine their general character. If a test is intended to be used as a control test to ascertain the degree of uniformity of successive lots of a given material it is desirable to provide close control and standardization of conditions and procedures. If on the other hand the purpose of a test is to determine the relative suitability of materials for a certain application, then the test conditions should be patterned as closely as possible after the conditions of service. It is obvious that there can be no universal corrosion te3t-
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that is, no single test by means of which the corrodibility of a metal in all environmcnt,s can be determincd. To appraise corrosion resistance it is first necessary to know precisely the character of environments to which the metal is to be exposed. With this knowledge it becomes possible to devise laboratory corrosion tests in which the variables are known and can be controlled. These tests may then be used to obtain preliminary information concerning the corrodibility of metallic materials or the efficacy of protective coatiiigs which, if confirmed by extensive exposure te;ts in natural environments, may be assumed to be dependable for engineering purposes. Laboratory corrosion tests in simulating natural environmental conditions are designed to provide for the exposure of metallic materials to controlled at,mo.pheric or liquid media. Commonly both gaseous and liquid phases and even contact with solids are included in a sinxle test. Changes in temperature, humidity, or other variables experienced in natural environments may be provided in the laboratory. It is the usual practice to increase the rapidity and to some extent the s-verity of the io-called weathering cycles in an effort to accelerate corrosion rate. FThile this is permissible Fvithin reason, it must be emphasized that the reliability of the result is inversely proportional to the amount of acceleration which is induced. Corrosion tests which employ severe conditions may produce a degree of disintegration of metals or protective coatings which bears no relationship to corrodibility under conditions of service. It is preferable to carry out corrosion studies under nearly natural conditions and to use sensitive measuring techniques to determine rate of reaction rather than to depend upon highly accelerated tests to furnish the larger effects which can be detected by less precise methods of measurement. M E T H O D S OF M E A S U R E M E N T
The rate of corrosion may be measured by a variety of means, most common of which is change in weight of the metallic specimen under study. I n laboratory atmospheric tests the products of corrosion are usually adherent and produce a gain in weight, while in immersion tests in liquids in which the corrosion products are soluble, the course of reaction is accompanied by loss of weight. Observation of rate of consumption of corroding reagent
