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ANALYTICAL EDITION
Vol. 1, No. 1
Automatic Apparatus for the Determination of Small Concentrations of Sulfur Dioxide in Air-11' Moyer D. Thomas and John N. Abersold DEPARTMENT OF AGRICULTURAL RESEARCH, AMERICAN SXELTING $2 REFINING COYPANY, SALT LAKECITY, UTAH
RECENT paper from this laboratory2 described electrodes and a continuous record of the conductivity can a continuous method for determining small concentra- be obtained by means of a Leeds and Northrup recording tions of sulfur dioxide in air, which is automatic to the Wheatstone bridge. The bridge has a range from 125 t o point a t which the solutions are discharged from the machine 40,000 ohms. The cell constant is about 0.06 reciprocal ready for titration. In order to make the method completely centimeter. Temperature compensation has been accomautomatic, it is necessary to measure the change in some plished during the past season by a rheostat and scale on the property of the absorbing solutiqn by a recording device. recorder, because a compensating cell sufficiently constant The absorbent employed, containing 0.2 per cent potassium for use with the dilute solutions employed was not available. iodide and 0.1 per cent starch together with the necessary The writers propose to construct such a cell. T o obtain a iodine, offers serious difficulties to the accomplishment of complete record of each absorption a recorder for each abthis purpose. The intensity of starch-iodine color might sorber is necessary, but if a part of the record can be omitted, be measured, but the range of iodine concentration which is or if the ga,s sampling is interrupted for 20 to 30 seconds after the completion of an absorpaccompanied by an apprecition, to allow the recorder able color change is too small The use of slightly acidulated distilled water containtime' to return to its initial to be useful. Since the abing 0.003 to 0.006 per cent hydrogen peroxide is proposed p o s i t i o n , a s w i t c h can sorption of sulfur dioxide as an absorbent for traces of sulfur dioxide in air to rereadily be installed so that produces one e q u i v a l e n t place the iodine solution previously used for this purone recorder will serve both each of sulfuric acid and hypose. The absorption of the gas in the peroxide soluabsorbers. driodic acid, it was thought tion can be followed either by titration with alkali using that if the potassium iodide Preparation of Reagent methyl red as an indicator or by a conductivity method. i n t h e s o l u t i o n could be A study of the method shows that it is fully as reliable The reagent consists of e l i m i n a t e d a conductivity as the iodine method in the absence of sulfur trioxide, distilled water to which is method or an iodine elecand permits the construction of apparatus which is added 0.1 to 0.2 cc. 30 per trode could be employed. cent hydrogen peroxide and completely automatic. It was impossible, however, 0.3 cc. 0.1 N sulfuric acid to omit the salt from the abper liter. The solution is s o r b e n t without rendering the iodine quite volatile. iimilarly, it was not practicable to aspirated for a few minutes with pure air to adjust the carbon measure the hydrogen-ion concentration because of the reac- dioxide equilibrium. One hundred cubic centimeters of this tion between iodine and both the hydrogen electrode and the solution will take care of 2 gram-molecular volumes of air quinhydrone electrode. containing 50 to 100 p. p. m. sulfur dioxide. The recorder is calibrated by adding known amounts of sulfuric acid to the Hydrogen Peroxide as Absorbent solutions in the absorbers and aspirating with pure air, as in Because of these difficulties another absorbent was sought. the regular operation of the machine. The addition of the acid in the preparation of the reagent I € was observed that distilled water is an excellent absorbent for appreciable amounts of the sulfur dioxide. For example, serves two purposes. Carbonates are decomposed,,permitting a t 5 p. p. in. practically all of the sulfur dioxide can be removed a rapid adjustment of the carbon dioxide equilibrium. This from 30 liters of air aspirated a t 15 liters per minute. At is essential because the recorder may show an increased 50 p. p. m. only4 per cent of the gas reaches a second absorber resistance when the absorption commences, corresponding under these conditions, but about 25 per cent of the absorbed to an error as great as 0.5 p. p. m. sulfur dioxide if distilled material is oxidized. For analytical purposes it is, of course, water which has been standing in glass bottles for a week or necessary either to prevent this oxidation or carry it to com- two is used without this preliminary treatment. If the pletion. The second course is easily accomplished by the concentration of the sulfur dioxide in the sample is constant, use of hydrogen peroxide. This is an ideal oxidant, because it is possible to correct the recorder reading for this effect it is a non-electrolyte which reacts rapidly with sulfur dioxide. by noting the slope of the absorption curve and the time of Thus the liquid remains an efficient absorbent for larger sampling. The acid also reduces the resistance of the sohamounts of the gas, and the conductivity of the solution can tion to less than 8000 ohms. At higher resistances the recorder is somewhat insensitive to added acid. I n the be employed for quantitative measurements. range between 8000 and 1500 ohms, representing 0 to 5 p. p. m. Modifications in Apparatus sulfur dioxide under the conditions of operation, the instruI n applying the results of this preliminary work only slight ment responds linearly a t the rate of 0.2 p. p. m. sulfur dioxide modifications of the apparatus previously described had to per scale division. At lower resistances the response of the be made. The discharge valve of each absorber is placed on recorder gradually diminishes, until a t 150 ohms, representing the outside of the vessel and the platinum electrodes, mounted about 50 p. p. m. sulfur dioxide, the rate is only 2 p. p. m. per on the end of a glass tube, are inserted near the bottom of the scale division. abfiorber about 1 cm. below the circular bubbler. In this Titration position the gas bubbles do not come in contact with the As a check on the recorder readings the aspirated solutions 1 Received November 19, 1928. may be titrated with sodium hydroxide using methyl red a s 2 Thomas and Cross, IND. ENG.CHEM., 20, 645 (1928).
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IhTDUSTRIAL A N D ENGINEERING CHEMISTRY
January 15, 1929
an indicator. A definite amount of the indicator-for example, 0.5 ec. of 0.02 per cent solution, the reaction of which has been adjusted to approximate neutrality-is added to the solution before titration and the end point determined by matching with the faint pink color of a standard solution. This titration is very readily carried out and the end point is extremely permanent and sensitive, 0.1 cc. of 0.002 N alkali being sufficient to cause a definite color change. A blank titration must also be made. Table I-Sulfur Dioxide in Mixtures of Sulfur Dioxide a n d Air a s Indicated by Hydrogen Peroxide a n d Iodine Methods
sox CONCENTRATION
DIFFERENCE BETWEEN CONDUCTIVITY METHOD AND: TION METHOD
ConducAcid tivity titration Iodine ~
1
1 1 1 1 6 15 20 20 6 15 8 12 4
--
0.37 0.39 0.40 0.69 0.93 0.35 1.36 1.71 3.19 6.41 13.58 24.1 36.4 58.3
0.35 0.37 0.41 0.71 0.90 0.30
...
1.61 2.90 6.40 13.55 24.3 37.2 59.1
0.38 0.38 0.39 0.69 0.88 0.35 1.44 1.75 3.24 6.43 13.40 24.4 35.0 56.4
Average
Maximum
0.05
0.10
0: i7
0: 40
0.30 0.07 0.20 0.2 0.8 0.8
0.50 0.10 0.50 0.3 1.7 1.2
I
second absorber placed in series with the first having indicated that this amount of gas escaped absorption. At 15 p. p. m. less than 1 per cent of the gas reached a second absorber. Advantages of New Method The new method has several advantages over the iodine method. The solutions are easier to prepare and the absorption is as rapid and complete as with the iodine method. It is not necessary to adjust the strength of the oxidant to correspond to the particular concentrations of sulfur dioxide with which it is desired to work. The data give the amount of gas absorbed directly and not indirectly through the decrease in concentration of another constituent of the system.
IODINE METHOD
Measurement of Both SOz and SOs
Aver- Maxiage mum
0.00 0.07 0.04 0.06 0.14 0.27 0.3 1.4 1.9
0.03 0.10 0.09 0.10 0.25 0.5 0.7 2.2 2.7
Comparison of Methods The new method has been checked against the iodine method, previously described, by analyzing uniform gas mixtures simultaneously with both procedures. The results are given in Table I and ,show a satisfactory concordance over a range from less than 0.4 p. p. m. to 60 p. p. m. The five single det,erminations represent large samples of gas, 120 t o 200 liters; the other values are the mean of a number of consecutive analyses of a uniform gas mixture carried out according to the regular procedure. At the highest concentrations the results are probably low to the extent of 2.5 per cent, a
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If it is desired to measure both sulfur dioxide and sulfur trioxide in a gas mixture, the two methods could be conveniently applied to this end-the iodine method to measure the sulfur dioxide and the hydrogen peroxide to measure the total acidity. As a general rule, however, the amount of sulfur trioxide is inappreciable compared with the sulfur dioxide, and can be neglected. Application in the Field The method can doubtless be applied to the problem of the continuous measurement of the concentration of sulfur dioxide in the field. Only slight modifications would be necessary to permit the apparatus to operate for 1 or 2 days without any attention. The period of sampling could conveniently be increased to about 10 to 20 minutes since concentrations greater than 5 p. p. m. would seldom be encountered. The final adjustment of the carbon dioxide equilibrium, which is always accomplished in less than half a minute, would then have a negligible effect on the results. It would also be desirable to have a record of the gas volumes and liquid volumes, even though these values are constant. A thermograph and barograph would complete the necessary equipment to permit the calculation of the sulfur dioxide concentrations.
Continuous Laboratory Extractor for Liquids' Alfred W. Francis ARTHUR D. LITTLE,INC., CAMBRIDGE, MASS.
ONTINUOUS extractors for liquids with volatile immiscible solvents have been described in the literature,2 but most of them'require either elaborate glass blowing, or else, since fairly large volumes of solution are used, the extraction is very slow. A simple apparatus was devised for the analytical extraction of liquids. Two forms are illustrated ( A ) for solvents lighter than the extracted solution, and ( B )for heavy solvents. Each is made from a pipet of any desired size with the stems bent and connections sealed, as shown. In ( A ) the bulb of the pipet is filled four-fifths full with glass beads before bending the upper stem. The bulb in ( B ) can be filled completely afterwards. The sample, about half the capacity of the bulb, is pipetted into the bulb, before the solvent in ( A ) and after it in ( B ) . Enough solvent is used to fill the rest of the bulb and tubes and have 20 to 30 cc. in the receiving flask. The solvent is distilled by means of a hot plate through the upper tube which is bent downward to avoid excessive r d u x condensation. After dropping back from the condenser, the solvent bubbles through the
C
1 Received November 9, 1928. * Fiske, A m . Chem. J., 41, 510 (1909): Berlin, German Patent 251,459 (June 24, 1911), see C. A., 7 , 7 (1913);Watkins, IND. ENG. CHEM.,il, 612 (1925).
sample in the bulb, up in ( A ) or down in ( B ) ,and overflows back into the receiver. If the extract can be titrated directly, as in the case of acetic acid, it is convenient to provide a buret and follow the extraction as it proceeds. This permits extrapolation to infinite time with great accuracy. An extraction of acetic acid from 50 cc. of salt solution with ether removed half the total in 8 minutes, half the remainder in the next 8 minutes, etc., so that in an hour the extraction was practically complete.
b-----d A
