The Determination of Sulfur Dioxide

benzol derivatives present. IV·—Lignite raw oils will give valuable solvents, burning oils, engine fuel oils, and lubricating oils. V— About seve...
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OCt., 1917

11-About j . 5 per cent of t h e coal may be obtained as raw oils. 111-These raw oils are more similar t o petroleum a n d shale oil t h a n t o coal t a r , there being no benzol or benzol derivatives present. IV-Lignite raw oils will give valuable solvents, burning oils, engine fuel oils, and lubricating oils. V-About seven pounds of paraffin wax per t o n of coal may be obtained. VI-The gas resulting from t h e low temperature distillation of lignites is of low heat value a n d is small in volume. VII-Small quantities of ammonia can be obtained from lignite t a r water. VIII-The residue from lignite distillation is a valuable fuel having a calorific value of over 12,000 B. t. u. per lb. IX-Due t o t h e small unit used i t is impossible to determine t h e commercial feasibility of lignite distillation, b u t sufficient d a t a have been obtained t o warrant a semi-commercial test t o be made upon t h e Tono lignites. LABORATORY OF








Received May 31, 1917

The accurate determination of sulfur dioxide in gas mixtures is a matter of great importance a t the present time. T h e control of sulfuric acid plants, t h e investigation of air charged with fumes from smelters, t h e regulation of community nuisances from high pressure sulfuric acid concentration in unreasonable proximity t o residential districts, a n d t h e analysis of t h e products from t h e explosion of gunpowder are some of t h e cases where such a determination is necessary. Since t h e sulfur dioxide is nearly always accompanied by sulfur trioxide, those methods which oxidize t h e dioxide a n d weigh i t as barium sulfate are not suited t o this kind of work. The nature of such investigations generally requires a volume of determinations a n d therefore speed of method is strongly desired. T h e titration of t h e sulfur dioxide with standard iodine solution has been developed into a satisfactory method by t h e “Selby Smelter Commission” a n d is described in t h e Bureau of Mines Bull. 98. The objections t o this method are t h a t i t uses a n iodine solution which must be frequently standardized a n d is so sensitive t o t h e action of light t h a t a blank must be run during t h e determination. Potassium permanganate is known t o be a n oxidizing medium for sulfurous acid. Since its solution is more stable t h a n t h a t of iodine, a n d can be used without a n indicator, i t is a more inviting reagent t o use. I n applying permanganate, however, it was found t h a t thereaction did not give results which would beexpected if t h e sulfurous acid were oxidized completely t o sulfuric acid. There was fair agreement among t h e


results themselves, however. Dymond and Hughes,’ who studied this reaction exhaustively, found t h a t a part of t h e sulfurous acid was oxidized t o t h e dithionate accordingly t o t h e equation 17H2SOa 6KMn04 = 2K2S206 K t S O l + 6MnS04 6H2SOa 11H20. This explained our failure t o get results in accordance with a complete oxidation t o sulfuric acid. They also found t h a t this reaction is not modified b y t h e concentration of sulfuric acid present, t h e temperature, or b y t h e dilution of t h e solution. Our results confirmed all of this, except t h a t t h e concentration of t h e sulfuric acid must be within certain wide limits, a n d there must always be a n excess of permanganate present. A very considerable amount of. experimenting was necessary before t h e proper conditions for making titrations were obtained. It is necessary t o record here only t h e conditions which were finally found t o give proper results. It was found t h a t 0.005 N potassium permanganate was t h e best strength t o use. Under proper conditions t h e effect of one drop could be observed, a n d t h e low normality made a high degree of accuracy possible. One drop of such a solution corresponds t o 0.000009 g. SOz. Should t h e concentration of sulfur dioxide be very large a stronger solution should be used. The‘solution was prepared by diluting t h e laboratory stock solution, and, after standing for several days, was standardized. Satisfactory results were obtained b y using thiosulfate, b u t in order t o duplicate more, nearly t h e conditions of t h e actual analysis sodium sulfite was used. Both Dymond a n d Hughes, a n d t h e Selby Smelter Commission used sodium sulfite for their standard. The sodium sulfite was purified b y a series of recrystallizations from water, after which i t was dried. Weighed quantities were dissolved in water a n d definite portions were pipetted out a n d titrated. Measured volumes of pure sulfur dioxide gas were also dissolved in water and portions were titrated a n d found t o check t h e sulfite method as did other methods of standardizing. As was stated above, t h e permanganate must always be present in excess. For this reason i t was impossible t o titrate t h e sulfurous acid directly. Recourse was had t o t h e scheme of t h e Selby Smelter Commission. A certain amount of t h e permanganate was run into dilute sulfuric acid solution, and, after mixing, was divided into two equal parts. T h e sulfur dioxide was dissolved in one of t h e parts and standard permanganate was then added from a burette until t h e color again matched t h a t of t h e other portion. Of course, such a n amount of permanganate was added as would oxidize t h e sulfur dioxide a n d still be in excses. Experiments showed t h a t t h e best color t o match was produced b y adding I O cc. of approximately 0.005 N permanganate t o 490 cc. of water. It was observed t h a t after t h e permanganate was reduced, t h e color on back titration did not exactly match t h e original, b u t h a d a slightly redder tinge. If, however,




. Chcm. SOC.,71, 314.






the solution was reduced a n d oxidized once or twice, before dividing t h e solution, then t h e color could be easily matched. I n fact, when a series of analyses are made it is best t o mix t h e two portions after each determination, divide t h e m again, and use t h e same portion over and over for color check. The solution should contain from 2 j t o j o cc. of approximately 2 N sulfuric acid. Less t h a n this amount gives a reddish colored solution which is difficult t o match; more t h a n this amount acts on t h e permanganate. Even when large amounts of sulfur trioxide accompany t h e sulfur dioxide it is generally safe t o use 2 5 cc. of t h e double normal acid. T h e reaction is complete in t h e cold a n d t h e titration is made a t ordinary temperature. It was found t h a t merely shaking t h e permanganate solution in t h e sample bottle, free from sulfur dioxide, caused no loss of color; hence i t was not necessary t o run a blank b u t merely t o match color accurately. A little practice in color matching brings accuracy a n d speed. T h e apparatus used was similar t o t h a t described in t h e Bureau of Mines Bull. 98, b u t since no blank was necessary, less apparatus was required; some of t h e parts were also simplified. APPARATCS

One 24-liter, or larger, bottle such as a carboy provided with a two-holed, properly cleaned, rubber stopper containing a large stopcock a n d a plug, is used as a sample bottle. Two joo-cc. glass bottles of uniform clear glass are necessary for titrating. T o facilitate t h e color matching, these should be free from waves. One of t h e bottles should be provided with a two-holed rubber stopper containing a t u b e which will reach t o t h e bott o m of t h e bottle. A white background is necessary for titrating and is provided b y a box with a partition, t o prevent color reflection, t h e inside being painted white. One 1000-cc. bottle is needed for diluting a n d mixing the permanganate solution. A 25-cc. burette is best, since t h e readings can be made with greater accuracy. Two Nessler tubes should be provided for very accurate work. A suction p u m p is necessary for evacuating t h e sample bottle and a manometer tube or gauge is required t o obtain t h e amount of evacuation. METHOD OF P R O C E D U R E

T h e large sample bottle is evacuated a n d t h e pressure within t h e bottle is noted a n d t h e temperature taken. When t h e sample is t o be taken, t h e end of t h e stopcock is p u t in communication with t h e gases t o be analyzed a n d t h e stopcock opened. T h e vessel is t h e n closed a n d t h e temperature a n d t h e barometric pressure are noted. F r o m this d a t a t h e volume of t h e sample can be calculated. About 4 7 5 cc. of water are placed in t h e 1000-cc. bottle a n d 30 cc. of 2 N sulfuric acid are added, after which I O cc. of t h e recently standardized 0.005 N permanganate are run in. After mixing, t h e solution is divided about equally into t h e two 500-cc. bottles.

Vol. 9, No.


Sodium sulfite solution, or sulfurous acid, is added t o one of t h e bottles until t h e color is very faint, after which t h e color is roughly restored with permanganate. The solutions are now mixed a n d again 6;vided. The burette 'is filled with standard permanganate solution a n d t h e position of t h e meniscus is noted. Such a n amount of t h e solution is added t o one of t h e bottles as will prevent t h e sulfurous acid from completely decolorizing t h e solution if this is likely t o happen. The two-holed rubber stopper a n d t u b e are now placed in t h e permanganate bottle. T h e gas sample bottle is inverted and t h e end of its stopcock pushed just through t h e free hole of t h e permanganate stopper. On opening t h e stopcock, removing t h e plug and inverting t h e sample bottle, t h e solution runs into t h e same. If t h e stopcock t u b e be bent a t a right angle t h e sample bottle can be laid on its side and t h e liquid run b y swinging t h e permanganate bottle through a half circle. After agitating t h e sample bottle a n d its contents for some time, t h e solution is r u n back into t h e small bottle and is titrated with permanganate until t h e color matches t h e other portion of t h e solution. It is again run into t h e sample bottle as before a n d then r u n out a n d matched closely with t h e other portion. This matching should be done against a white background, and for very close work portions can be compared in Nessler tubes. The total amount of permanganate run from t h e burette gives t h e amount of sulfur dioxide in t h e sample. T h e details of manipulation are t h e same as those so thoroughly worked out for t h e iodine method by t h e Selby Commission (LOG. cil). The following results were obtained by using known amounts of gas, or solutions of known sulfur dioxide content. These results are typical of a large number of analyses made. GRAMSSOP PRESENT 0.001234 0.002468 0.000370 0.001897 0,001379


No. 1 0.001120 0.002314 0.000373 0.001866 0.001381

0.001306 0.002407 0.000373 0.001875 0.001388

0.001230 0.002407 0.000373 0.001904 0.001381

0.001306 0.002388

.... .... .... The accuracy of this permanganate method is good, when t h e dilution of t h e gas is considered, and i t compares favorably with t h e best results obtained b y other methods. It was found very satisfactory for t h e determination of sulfur dioxide in t h e atmosphere about t h e plant in which we were interested. CONCLUSIONS

This work has shown t h a t permanganate is t o be preferred t o iodine for SO2 determination for t h e following reasons: I-It gives as great accuracy. 11-It is as easy t o prepare. 111-It can be more easily manipulated. IV-It can be operated on small traces of SO2 as well as large amounts. V-It is more stable t o light. VI-It gives a color as easy or easier t o match t h a n t h e starch-iodine end-point. VII-It requires no simultaneous blank, and hence less apparatus t o be transported in t h e field. LABORATORY OF INDUSTRIAL CHEMISTRY OHIO STATE UNIVERSITY. COLUMBUS

OCt., 1917



Various methods' for t h e separation of aluminum from iron have been proposed. None of these, however, have found extensive use in analysis. As t h e method of separation herein proposed is based on t h e use of organic solvents, only such processes as have a direct bearing on this method will be discussed in this paper. It had been noted by Gladysz2 t h a t hydrous aluminum chloride was very slightly soluble in strong hydrochloric acid, while ferric chloride was readily soluble in t h a t medium. Gooch a n d Havens3 utilized t h e observation of Gladysz, b u t introduced t h e use of ether t o reduce t h e solubility of aluminum chloride. They precipitated this salt from a mixture of equal parts of concentrated hydrochloric acid a n d ether saturated with hydrochloric acid gas a t I 5 O C. It appears t h a t t h e method of Gooch a n d Havens is t h e only one on record which utilizes a n y organic solvent (one-half aqueous hydrochloric acid a n d onehalf ether). Frankforter4 in studying t h e action of aluminum chloride on aliphatic ethers found t h a t t h e presence of moisture caused precipitation of a compound of this salt with water and hydrochloric acid from a n ether solution of t h e anhydrous aluminum chloride. This precipitate, however, was not constant in composition. H e utilized this reaction as a qualitative test for water in ether. T h e method proposed in this paper, like t h e Gooch a n d Havens method, depends on t h e insolubility of hydrated aluminum chloride in solvents as contrasted with t h a t of iron, b u t may be more closely compared with t h e qualitative observation of Frankforter, as i t neither uses a mixture of water and ether, nor requires saturation of t h e medium with hydrochloric acid gas. I t is primarily t h e latter feature in t h e Gooch a n d Havens method which is troublesome a n d objectionable for ordinary laboratory procedure, as in each determination, hydrochloric acid gas must be passed through t h e solution a t low temperature ( I S " C.) t o saturation (a point rather difficult t o determine with certainty) a n d t h e precipitate washed with ether-water mixture, itself saturated with HC1 gas. I n t h e proposed method, t h e medium is entirely organic solvent, except for traces of moisture introduced b y t h e solvents. The essentials of t h e method are as follows: The dried mixed chlorides of aluminum and iro? are taken u p in a small amount of hydrochloric acid alcohol solution and evaporated t o crystallization of t h e salts. T h e residue is then again acidified with alcoholic HC1. Ether (U. S. P.), which contains a trace of water, is gradually added, and t h e aluminum 1

W. W. Scott, "Standard Methods of Chemical Analysis," 1917,

pp. 4-5. 2 J


Ber., 16 (1883). 447. A m . J . S c i , [4] 11 (1896), 416. J. A m . Chem. SOC.,87 (1915). 2560-7.


is precipitated as a hydrated chloride, varying i n composition, while t h e iron chloride remains in solution. EXPERIMENT A L E F F E C T O F MOISTURE-It was observed in preliminary experiments t h a t anhydrous aluminum chloride was readily soluble in absolute alcohol and absolute ether, b u t t h a t t h e presence of a trace of moisture, introduced as such or by means of wet alcohol or wet ether, caused a n immediate precipitate. An excessive amount of moisture, however, invariably caused t h e formation of a second layer of a viscous gelatinous mass. It was necessary, therefore, t o determine t h e limits of water tolerance and t h e best means of introducing t h a t water. A series of experiments was performed in which water was introduced in varying amounts using, at first, strong aqueous HC1, as t h a t acid was found necessary t o keep t h e iron in solution. This manner of introducing t h e water was found impracticable as t h e amounts of HC1 necessary caused, in many cases, a n excessive amount of water t o be introduced. An alcoholic solution of HC1 was, therefore, prepared a n d water was introduced b y means of t h e ether used. The following facts were observed: I-When a n insufficient amount of water was present as obtained b y dissolving t h e salt in a small quantity of alcohol a n d using largely absolute ether, t h e precipitate came down in a n exceedingly finely divided condition, colloidal or semi-colloidal, practically impossible of filtration. When filtered, t h e precipitate was exceedingly hygroscopic, behaving somewhat like finely powdered (dried) calcium chloride: t h e physical s t a t e of t h e precipitate was such t h a t a clean separation from t h e iron would be impossible even if t h e precipitation were complete. It was, in fact, found t h a t u p t o a n alcoholic content of about 4 per cent, t h e precipitation was complete, when t h e suspension was filtered several times. a-When a n excessive amount of water was present, which depended on t h e amount of salt present t o take care of i t a n d t h e amount of water introduced with t h e ether, i t was found t h a t a gelatinous layer of t h e salt was formed. EFFECT O F ALc0HoL-h excessive amount of alcohol had a tendency similar t o t h a t observed in t h e case of insufficient water, namely, t o precipitate t h e aluminum chloride in a very finely divided state. I n addition, t h e precipitation is incomplete when alcohol is present .beyond certain limits, depending upon t h e amount of water present. E F F E C T O F H Y D R O C H L O R I C ACID-This reagent was introduced in t h e form of t h e alcoholic solution. Aside from t h e fact t h a t HC1 was required t o keep t h e iron in solution, i t was found t h a t a slight acidity was essential for two reasons: ( I ) t o make t h e precipitation complete, ( 2 ) so t o modify t h e physical state of t h e precipitate as t o render it easy t o filter. On t h e other hand, excessive amounts of acid seemed t o tend toward formation of gelatinous salt layer as in the case of excessive water.