C—3A

58

Free S0 2 . This is obtained by difference between the per cent, total and per cent, combined S0 2 . Loosely combined S0 2 . A portion of the S0 2 combined with the dissolved ligneous materials in sulphite liquor is but very loosely combined, presumably with aldehyde groups, and may be hydrolyzed off with NaOH. This is the basis of tho quantitative estimation of S0 2 in this form. The method follows : After the titration of sulphite liquor for total S0 2 by iodine, the blue colour is destroyed by a drop of N/16 Na 2 S 2 0 3 (sodium thiosulphate). An excess, 10 cc. of 15-per-cent. NaOH is added, and the solution allowed to stand for twenty minutes, the solution then being made slightly acid with HCI, and again titrated with N/16 iodine. This titration gives tho per cent. S0 2 present as loosely combined S0 2 , each cubic centimetre of iodine being equivalent to 0-1 per cent. S0 2 . Determination of Total S0 2 . A method for the determination of S0 2 in all forms, or the total S0 2 content of sulphite cookingliquor, as worked out by Miller and Swanson, Forest Products Laboratory, is as follows : A 2 cc sample of the liquor is run into a 1 cc. portion of 15-per-cent. NaOH in a 35 mm. porcelain crucible. This is then absorbed in about 4 grm. Eschka mixture, and the crucible and contents dried at 105° C. The crucible is then placed in a Shimer crucible and heated in a stream of oxygen, at a dull red for one hour. The oxygen and sulphur gases pass through a ret-hot zone in the silica tube attached to the Shimer crucible, and out through an outlet-tube into a Drexel flask containing 5 cc of 15-per-cent. NaOH and 10 cc. saturated bromine water in 100 cc distilled water. After the combustion the porcelain crucible and contents are placed in a 600 cc beaker and allowed to cool. Then the contents of the Drexel flask are added, the flask being washed with hot water three times, the washings being added to the beaker-contents. The beaker is placed on a hotplate, and allowed to digest for half an hour. Then the contents are filtered into a 600 cc beaker, the residue and crucible being washed six times with hot water. The filtrate, about 400 cc, is made slightly acid with HCI, the excess bromine boiled off, and 10 cc of 10-per-cent. BaCL, solution pipetted into the solution. The BaS0 4 , precipitated, after digestion for one hour on a water bath, is separated by filtration into a weighed Gooch. After washing the precipitate several times with hot water the crucible and contents are dried and ignited, cooled, and weighed. From the weight of Ba.S0 4 obtained, the per cent. S0 3 is found thus : —- Wt. BaS0 4 X 0-27443 X 50 = per cent, S0 2 . or Wt. BaSO, X 0-1372 = per cent. S0 2 . Soluble Sulphates in Sulphite Liquor. In 250 cc Soxhlet flasks are placed about 55 cc of distilled water and 25 cc of 25-per-cent. hydrochloric acid (approximately equivalent to 16 cc of ordinary concentrated acid). The flasks are clamped in position under the condensers, and 25 cc of waste sulphite liquor, or raw sulphite acid, as the case may be, are pipetted into the two Soxhlet flasks, the second being corked immediately and connected to the generator. The condenser-stoppers are coated with collodion in order to ensure tight connections. The flasks containing the iodine solution and standard sodium thiosulphate arc connected to the condenser-outlet. Carbon dioxide is then passed through for about five minutes until all the air is displaced from the apparatus, thus obtaining an atmosphere of carbon dioxide in which very little or no oxidation of S0 2 to SO., takes place. At this stage the solutions in the Soxhlet flasks are slowly heated until they boil gently, and the boiling is continued until the experiment is finished. Any liquor vapourizing during heating is condensed back into the flask, thus maintaining almost constant concentration of the solution. The S0 2 evolved is swept out by the carbon dioxide, is cooled on escaping, and is finally absorbed in the iodine solution. At the end of the experiment the excess iodine is titrated with sodium thiosulphate of equivalent strength, using starch solution as an indicator. From the amount of iodine reduced by the evolved S0 2 the quantity of the latter is calculated. The end point of the reaction occurs when the iodine solution is no longer decolorized, or, better still, by bubbling tho escaping gas through a very weak iodine solution in a pill-plate in case the amount of S0 2 given off is not being determined. Ordinarily it takes from forty-five minutes to one hour to completely remove the S0 2 , this time varying according to the amount of S0 2 present, the rate at which the C0 2 is passed through the solution, and also according to the rate of heating of the latter. When all the S0 2 is expelled the Soxhlet flasks are disconnected, and the sulphate-free precipitate formed is filtered off. The filter is thoroughly washed with hot water, and the clear filtrate is neutralized with NH 4 OH and made slightly acid with HCI. It is diluted to approximately 400 cc, brought to a boll, and 10 cc of N/2 BaCl 2 are very slowly added. Boiling is continued until the BaSO,, precipitated becomes granular, and is then kept hot for from four to six hours longer. The BaS0 4 is then filtered and thoroughly washed with hot water until free from chloride. It is carefully ignited, allowed to cool, and then weighed. Lime as CaO in Sulphite Liquor. A 10 cc sample of the liquor * is pipetted into a 250 cc beaker, and is treated with 5 cc concentrated H 2 S0 4 and 10 cc concentrated HN0 3 , and heated until sulphur trioxidc (white) fumes appear. If the organic matter is not destroyed, more HN0 3 is added, and heat applied, until the solution appears clear. Then the solution is allowed to cool, is diluted to 150 cc and treated with

* Raw or finished acid may be treated as follows : Heat 10 c.c. in 250 o.e. beaker as above, but add 10 c.o. concentrated HCI instead of HNO 3 and H 2 SO 4 ; boil to expel SO», and proceed as above.