ANALYTICAL
VOLUME5 NUMBER 2
EDITION
Industrial Chemistry AND E N G I N E E R I N G
MARCH15, 1933
A M E R I C A NCHEMICAL SOCIETY HARRISON E. HOWE,EDITOR
P U B L I S H E D B Y THE
Analysis of Refrigeration-Grade Liquid Sulfur Dioxide Official Method of the Sulfur Dioxide Committee, Compressed Gas Manufacturers' Associa tion Introduction F. A. EUSTIS Chairman, Sulfur Dioxide Committee, Compressed Gas Manufacturers' Association
T
HE experience of the refrigeration industry has demonstrated that extreme refinement in the purity of liquid sulfur dioxide that is to be used as a refrigerant in auto-
matic refrigerating machines is commercially very important. Obviously, therefore, it has become important to determine reliably the exact degree of purity of this product, in which all impurities occur only in very minute quantities. Accuracy of analysis of from one to two parts per million is required. The manufacturers of refrigeration-grade liquid sulfur dioxide, wishing to avoid controversies in this matter of the purity of their products, created a special committee of chemists to draft a standard procedure for the analysis of liquid sulfur dioxide. The chairman of this committee was F. B. Downing of E. I. du Pont de Nemours & Co., Inc., Wilmington, Del., the other members being L. S. Larsen of Ansul Chemical Co., Marinette, Wis., and A. K. Scribner of the Virginia Smelting Co., West Norfolk, Va. The committee considered all the data that had been collected by the manufacturers represented, and after a year's work completed its draft of a standard method of analysis. This was submitted to the full Sulfur Dioxide Committee of the Compressed Gas Manufacturers' Association, which was aided by a representative of the Calco Chemical Co., who was not a member of the association. The report was approved with very minor changes, and then referred to the Governing Board of the Compressed Gas Manufacturers' Association, which approved it as the tentative standard of the association. The manufacturers regard it as the standard method to be used for all control analyses. The method is given in detail below. MOISTURE Sulfur dioxide for the refrigeration industry is usually guaranteed to contain less than 50 parts per million of moisture, while the actual moisture content may be as low as 5 parts per million in some cases. This represents from 0.0025
to 0.025 gram per pound of product (453.59 grams). The accurate determination of this small amount of moisture (1, ,2) presents difficulties, in that the detailed technic and the exclusion of outside moisture must be carefully controlled to obtain correct results. The method in outline is simple; a sample of liquid sulfur dioxide is drawn from the cylinder or drum in question and allowed to evaporate through a tared phosphorus pentoxide tube whose gain in weight is taken as the moisture content of the sample. Under the best conditions, the experimental error is 0.0002 per cent absolute. However, samples are usually taken outof-doors and the absolute humidity of the atmosphere has such a n effect that during damp or rainy weather it is practically impossible to obtain a representative sample. When the relative humidity of the atmosphere is above 50 per cent a t 68" F. (20" C.) or over 3.78 grains per cubic foot, results are very apt to be higher than the actual in spite of all precautions. REAGENTS AND APPARATUS. The reagents used are phosphoric anhydride (pentoxide) in powder form and 95 per cent sulfuric acid. The apparatus consists of sample transfer fittings, sample container, heat exchanger, and phosphorus pentoxide drying tubes. Auxiliary pieces of apparatus are a sulfuric acid bubbler, sulfuric acid scrubber, phosphorus pentoxide tower, and aspirator or other source of suction. Two types of fittings may be used. A and SAMPLE FITTINGS. B, Figure 1, illustrate the first type, which is more readily available for occasional use. C illustrates the second type, which is recommended for regular use. It consists of a T connecting to the drums or cylinder valve with a union and equipped with a valve on each branch. Construct this using Prestolite Y-type valves for small cylinders. Dry all fittings at 105' t o 120' C. and use while hot. Dr the valve on the container with a blow torch or other means of {eating and then connect fittings A or B t o this valve without passing liquid sulfur dioxide through the fitting. Finally place the moisture tube on the rubber stopper and draw the sample. The T fitting C does not requirk a dryin of the drum or cylinder valve. Merely flush this valve and t f e T fitting C through one of the Y valves and finally collect the sample in the moisture tube through the other valve. SAMPLE CONTAINER.For large samples use a'half-liter suction flask with a calibration mark at the 500-cc. point; for small samples use a special cylindrical tube of 200 cc. capacity calibrated for 100 cc. and having a tip at its base of 1 cc. capacity calibrated t o 0.01 cc. at its lowest ortion. Dry the containers at 120" C? Since ordinary oven-drying leaves moisture in the air in the flask and on the surface of the
78
Vol. 5 , No. 2
ANALYTICAL EDITION
glass, final drying is b means of the preliminary run described below. Repeated anaryses may be made using the same container without cleaning between runs, rovided air is excluded a t all times. Sample containers that Rave not been used for several days should be treated as wet. HEATEXCHANGER. A preheater is placed between the sample container and the phosphorus pentoxide tubes to prevent cooling of the tared absorption tube. For 190-cc. or 500-cc. samples evaporated slowly, use 1 foot (30.48 cm.) of glass or brass tubing. For 500-cc. samples evaporated at the maximum rate, use a 6-foot (183-cm.) helix. The tubing need not be larger than 5 mm. inside diameter.
SAMPLING.Two sizes of samples may be used: the 500-cc. sam le for the most accurate work, such as for 2000-lb. drums and settEment of disputes, and the 100-cc. sample for local control work and for the analysis of small cylinders when there is no disagreement. But, because of the possibilities of a larger experimental error in the case of small samples, particularly in times of high humidity, rejections should be made only on the basis of 500-cc. samples. Flush and carefully dry the drum or cylinder valve and attach the dry fitting A or B. If fitting C is used, connect it direct1 to the drum valve and flush both drum valve and fitting througz one valve of the T. Draw the sample directly from the original drum or cylinder into the especially prepared sample %fU88fZ37WPfE container used for the analysis. The use of a small cylinder to transfer material from the orininal container to the laboratorv is not permissible. PREPARATION OF APPARATUS.Before s t a r t i n g the actual determinations, equilibrate the apparatus by collecting a full-size sample of r e f r i g e r a tion-grade sulfur dioxide (using the sampling connections and dried container), and evaporating it through the assembled absorption train. This is n e c e s s a r y to complete the drying of the sample container, to remove all air from the phosphorus pentoxide tubes, and to dry tubing connections. In t r a n s f e r r i n g the sample container from the c y l i n d e r or d r u m to the a p p a r a t u s , k e e p it stoppered but vented through a Bunsen valve. At the end of the preliminary run, immediately close all stopcocks, beginning nearest the sample container, and then disconnect the absorption train. After disconnecting, the samDle container and the preheater tubing, dose both 'Jecurely. Weigh the absorption tubes preparatory to the moisture determination. The Fleming jars must FIGURE1. SAMPLING TUBES be weighed filled with sulfur dioxide. The Swartz tubes may be weighed filled with sulfur d i o x i d e The preheater is oven-dried and then finally dried in the pre- or they may be swept with air. If this is desired, sweep for liminary run. Keep it closed when not in use by means of glass 3 minutes with phosphorus pentoxide-dried air a t the rate of 2 liters per minute and weigh. When weighing the absorption or wooden plugs. DRYING TUBES. Two types of drying tube may be used: the tubes, use a duplicate counterpoise having one end open to the Fleming phosphorus pentoxide absorption jar (No. 3890 in A. H. Thomas catalog). or a 4-inch (IO-cm.) Swartz U-tube. The Fleming jar is betteradapted to 500-cc. samples because of the greater number of runs that may be made before refilling. The Swartz tube is better ada ted to 100-cc. samples and throughout its life is free from channeEng and has low resistance to flow of gas. Pack the Fleming jar with a mixture prepared by shaking two parts by volume of powdered hosphorus pentoxide with one part of Gooch asbestos fiber whicf has been dried a t 140' C. This mixture is light, porous, and has long life with minimum tendency to channel. Pack the reagent loosely in the jar on a 0.25-inch (0.6-cm.) layer of glass wool. Prepare a second phosphorus pentoxide tube like the first and place it in the absorption train. All moisture is collected in the first tube. Any change in the second tube is therefore used, with a change of sign, as a correction on the weight of the first tube. A correction for temperature, barometric pressure, and surface moisture is thus obtained automatically. Pack the Swartz tube with glass beads coated with hosphorus pentoxide. Use beads of not more than 0.125-incf (0.3-cm.) diameter and allow them to stand in the open air for a few hours so that the moisture on the glass surfaces is in equilibrium with the air. Then lace them in a stoppered bottle and add finely powdered hosptorus pentoxide in portions with shaking until the surfaces lave taken up all that will stick t o them. Fill the Swartz tube with these beads, using a lug of glass wool a t each end. Prepare a duplicate tube witgout phosphorus pentoxide to be used as a counterpoise. Weigh this tube, whether of Fleming or FIGURE 2. APPARATUS FOR DETERMINATION OF NONCONDENSwartz type, open to the air so that it is at the prevailing baroSABLE GASESIN LIQUIDSULFURDIOXIDE metric pressure. On tests that are only run an hour, brass 4 . 100-cc. gas buret E . Leveling bulb weights have been found to be satisfactory. 5. 3-way sto cook E". Rubber tube C. Cup for ajdition of potassium U H Pinchcocks The moisture in refrigeration-grade sulfur dioxide is quantitahydroxide i , J : Open ends of rubber tubing tively removed by one phosphorus pentoxide absorption tube. D. Connection for sulfur dioxide K. Glass slght tube AUXILIARY APPARATUS.For the purpose of judging the rate inlet of evolution of the sulfur dioxide, a sulfuric acid bubbler may be air. Also weigh the second Fleming jar used as a compensating placed at the end of the train. A 15-cc. pipet cut off just below the bulb and immersed in 15 cc. of concentrated sulfurlc acid in tube. Equalize the pressure within all analytical phosphorus pentoxide tubes with the prevailing atmospheric pressure before a rubber-stoppered bottle makes a safe bubbler. The Swartz tube may be weighed after sweeping with dry air closing them for weighing. PROCEDURE. Reassemble the absorption train. Collect the if desired. To accomplish this, sweep the tube for 3 minutes with 2 liters of air per minute. Set up a train as follows: A sulfuric sample directly in the sample container, observing all the preacid scrubber, a phosphorus pentoxide tower, the Swartz tube, cautions given above, and immediately transfer it to the laboraand then a manometer connected to an aspirator or other source tory and connect it to the absorption train. Regulate the rate of evaporation of the sulfur dioxide by cauof suction.
-
"t"
March 15. 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
tiously using a bath of lukewarm water. Allow at least 45 minutes for the evaporation of 100-cc. samples, and a t least 1.5 hours for 500-cc. samples, when using the Swartz U-tubes. The Fleming jar requires 45 minutes for 100-cc. samples and a t least 4 hours for 500-cc. samples. The rate of evaporation is limited by the back pressure of the absorption train and the danger of blowing out stoppers. At the close of the evaporation, shut off the stopcocks in the same manner as a t the end of the preliminary run and weigh the tubes, using the tare tube as counterpoise. 100 M 1.48 X cc. sample = % moisture
Where 1.46 is the specific gravity of sulfur dioxide at its boiling point a t atmospheric pressure, M is the gain in weight of the first phosphorus pentoxide tube less the increase or plus the decrease in the second or compensating phosphorus pentoxide tube.
RESIDUE TEST The residue left by the evaporation of refrigeration-grade sulfur dioxide is too small for a quantitative determination to be practicable. Hence, i t is determined microscopically and compared with a standard. PROCEDURE. Use a 200-cc. cylindrical sample container having a 1-cc. tip which at its lowest portion is graduated to 0.01 cc. Clean this container with cleaning solution, rinse with distilled water, and dry at 120' C. The glass surface should be bright and free of all film. Wipe the cylinder valve and flush. Draw a 100cc. sample without using any sample connection. Stopper the container either with rubber stopper and Bunsen valve or with a plug of cotton wool. Evaporate to dryness and compare with standard. SULFURICACID After evaporation of the sulfur dioxide and careful removal of all sulfur dioxide vapor, as shown by iodine test, the sulfuric acid is titrated with 0.01 N caustic. Use a 125-cc. Erlenmeyer flask, with a calibraPROCEDURE. tion mark at the 100-cc. point. Clean with chromic acid cleaning solution, wash out thoroughly, and dry at 120' C. Evaporate a 100-cc. sample, using a plug of cotton wool to stopper the flask. After the evaporation, remove sulfur dioxide vapor by connecting a Gooch crucible adapter to suction and applying the rubber end to the flask, then breaking the connection. Repeat fifteen times until no odor of sulfur dioxide remains and a drop of 0.01 N iodine added to the flask is not decolorized. Prepare water for the titration by adding 2 drops of methyl red and adjusting by means of either 0.01 N sodium hydroxide or 0.01 N hydrochloric acid to the exact neutral point. Add 25 cc. of this water to the flask and titrate with 0.01 N sodium hydroxide. cc. X N X 0.04904 X 100 = cc. of SO, X 1.46
%
79
NONCONDENSABLE GASES The 100-cc. sample of sulfur dioxide gas is collected in a 100-cc. gas buret and potassium hydroxide added. The residual gas is read as noncondensable gases. The results for small amounts of noncondensable gases are read directly to 0.02 per cent and may be estimated t o 0.005 per cent. REAQEST. 30 er cent potassium hydroxide solution. APPARATUS.T i e buret is similar to Eimer and Amend, No. 28940, except that it is of 100 cc. capacity and the upper tip is graduated in 0.02 cc. down to the 0.2-cc. mark. PROCEDURE. With pinchcock H closed, open the cock between A and C, and raise the mercury leveling bottle until the mercury just comes into the cup C. Close the stopcock B, leaving a small globule of mercury in the cup. Place the cylinder containing the liquid sulfur dioxide on its side so that the ramshorn of the lower valve is in the liquid phase. Connect the apparatus as shown in the diagram. With the spring pinchcock G open, the cylinder valve is cautious1 opened until liquid sulfur dioxide runs out at the tube and I . &ome analysts find it more convenient to substitute the use of one foot for pinchcock G . ) The pinchcock H is now o ened, the buret stopcock turned into the position shown in the iagram, and the pinchcock G and the cylinder valve closed. When the liquid sulfur dioxide has vaporized as indicated by the globule of mercury, the cylinder valve is opened to allow liquid sulfur dioxide to escape into the tubing only as fast as it will vaporize. Allow the gas to sweep out the air in the a aratus, which should be accomplished in 2 or 3 minutes. wft! pinchcock G closed, quickly turn stopcock B so that it is open between the cylinder and the buret A , lower the leveling bulb and fXl the buret with sulfur dioxide as. Close pinchcock H and open pinchcock G. Turn stopcoca B so that it is open between A and C and run this buret full of gas to waste through cup C, as it is used only to treat with any small amount of potassium hydroxide left from a previous determination. Refill the buret in the same manner as before with exactly 100 cc. of sulfur dioxide under atmospheric pressure. Close pinchcock H and turn stopcock into the position shown in the diagram. Add through cup C about 15 to 20 cc. of the potassium hydroxide solution, being careful to exclude all air. After the contraction has ceased, bring the level of the mercury in the bulb to that in the buret and read the volume of residual gas. eo. of residual gas =
% noncondenaable gases in liquid phase
LITERATURE CITED (1)
Flenner, A. L., and Caverly, W. R., Refrigerating Eng.,21, No. 5, 344 (1931).
(2) Scribner, A. K., IND. ENQ.CHEM.,Anal. Ed.. 3, 255 (1931). RECEIVED November 3, 1932.
Note on Washing the Potassium Cobaltinitrite Precipitate W. E. THRUN,Valparaiso University, Valparaiso, Ind.
T
HE precipitate of potassium cobaltinitrite obtained in
of sulfuric acid and the oxidizing agent, the precipitate
micromethods (1-4) for determining potassium becomes increasingly flocculent upon washing with water. It is therefore usually washed in centrifuge tubes by siphoning. It was found that when the precipitate is washed with a dilute solution of aluminum sulfate (1 per cent) it will be much less flocculent, so that it can be washed by the usual method of drainage after inversion. Three washings with 3 cc. of solution are usually sufficient. If the supernatant liquid from the second washing is not nearly colorless, a fourth washing should be performed. It was also found t h a t more consistent results can be obtained when the precipitate is to be titrated indirectly with solutions of permanganate or ceric sulfate if, upon addition
is thoroughly stirred for a while when the tube containing the reacting substances is placed in boiling water. Because of the instability of nitrous acid ( I ) , ceric sulfate should be a better oxidizing agent for this purpose than permanganate, as it furnishes a positive ion to oxidize a negative ion.
LITERATURE CITED (1) Jacobs and Hoffman, J. Biol. Chem., 93, 685 (1931). (2) Kramer and Tisdall, I b i d . , 46, 339 (1921). (3) Leulier, Velluz, and Griffon, Bul. SOC. chim. biol., 10, 1238 (1928). (4) Tisoher, Biochern. Z., 238, 148 (1931). RECEIVED Auaust
11, 1932.
