846 SODA. —Utilization

and hydrosulphides are formed, which are afterwardsmore or less oxidised to hyposulphites. Later onthe hyposulphite is decomposed into sulphur andsulphite, both of which are insoluble and lost inlixiviating; and this is, of course, equally the casewith the ultimate product, calcium sulphate, which,however, is always found in the mass. It is there-fore evident that the oxidation must not be carriedon any further than necessary, and this is done whenthe proportion of sulphides to that of hyposulphiteis as nearly as possible 2:1. In this case the reactionwith hydrochloric acid runs as follows :—

2CaS + CaS 2 0 3 + 6HC1 = 3CaCI 2 + 3H 2 0 + 4S.

The hydrosulphide, CaII 2 S 2 , and the bisulphide, CaS 2 ,or any other polysulphides, behave just like CaS,which, indeed, cannot exist as such in the liquor,being insoluble; but the reaction is shown for itfor the sake of simplicity. If in any liquor thesulphides prevail over the above proportion, therewill be an evolution of II 2 S; or if the hyposulphitesprevail, of S0 2 , both of them causing a nuisance aswell as a loss of sulphur. In practice it is notpossible to attain the above proportion exactly, andas more harm is caused by an excess of sulphidesthan by their deficiency, the proportion is keptslightly the latter way. It has been stated abovethat the anemometer serves for regulating the oxida-tion according to the tests of the liquors. Theanalysis of the latter is performed by Mond asfollows :—1. The hyposulphite is estimated as usualby a decinormal solution of iodine and starch, afterhaving first separated the sulphides by zinc chlorideand filtering. 2. To a certain quantity, say 3'2 c.c.of the liquor, a little starch solution is added, andthen a standard solution of iodine, till it turns blue ;the quantity is noted; the liquor is then againdecolorized by a drop of sodium hyposulphite solu-tion, litmus is added, and a standard solution ofsoda is turned in till the liquor is neutral. Thefollowing reactions take place simultaneously:—

2(CaS 2 0 3 ) + 21 = Cal 2 + CaS 4 O 0 .

CaS 2 + 21 = Cal 2 + 2S.

CaIl 2 S 2 + 41 = Cal 2 + S 2 + 2III.

And subsequently the free III is disposed of thus:

2111 + Na 2 0 = 2NaI + H a O.

The standard soda thus shows directly the hydroiodicacid, and indirectly the equivalents of hydrosulphide ;the iodine used in the first titration is that ofhyposulphite, and that used in the second titration,deducting the two former numbers, that of sulphide(polysulphide). Using for both titrations 3'2 c.c. ofliquor, and decinormal solutions, and presumingthat the polysulphide is bisulphide only (this is,however, refuted by Staiilschmid’s researches, whofinds it to be mostly tetra- and penta-sulphide), wehave simply to add together the measures of iodineused in both titrations, subtract that of causticsoda, and divide the remainder by ten, in order tofind the total percentage of sulphur in the liquor;and from this the hydrochloric acid can be easilycalculated, as each 32 of sulphur require 30'5 of IIC1.

or Black-ash Waste.

When it is only required to see whether the propor-tion is a favourable one for decomposition, it needonly be noticed whether the measure of iodine usedafter precipitating the sulphides by zinc solution tothat used for the original liquor is nearly 1 : 5, andit ought to be rather below than above 5; that is,there ought to be rather less than mare than 2equivalents of sulphide for one of hyposulphite.Mond believes that the decomposition of calciumhyposulphite is not as simple as usually explained,viz.—

CaS 2 0 3 + 2IICl = CaCl 2 + S0 2 + II 2 0 + S;

but that a great deal of it is decomposed with theformation of calcium trithionate, thus :

SCaS 2 0 3 + 6IIC1 = 3CaCI 2 + 3H 2 0 + 2CaS 3 0 6 + 4S.

On boiling the calcium trithionate is decomposed intocalcium sulphate, sulphur, and sulphurous acid; thelatter re-forming trithionate and so forth, till nothingbut sulphur and calcium sulphate remain besides cal-cium chloride and water. Staiilsciimid’s researchesbear out Mond’s statements in this case. This meansa loss of acids and sulphur, as calcium sulphate isworthless, and for this reason Mond rejects Schaff-ner’s method of decomposing the yellow' liquors,and rather runs the latter into a large wooden tubat the same time with common weak muriatic acidfrom the post-condensers (wash-towers), just suffi-cient to decompose them; if the proportion ofsulphides be rather less than two to one hyposul-phite, if the temperature be kept at 140° to 150° C.,and if the liquor is thoroughly agitated by a mech-anical stirrer, no gas whatever is evolved, the liquorkeeping the slight excess of S0 2 in solution, and tiiesulphur is precipitated free from ferrous sulphide.By taking a sample from time to time, and observingwhether it is of the proper colour, the workman canjudge whether there is not too little acid, and lietries whether there is not too much either, bypouring a little yellow liquor into the samples,which ought very soon to produce a grey colourthrough formation of FeS; when too much yellowliquor is used before this grey colour appears, theflow of acid must be checked. The tub is coveredin and connected with the chimney, to convey a\yayany occasional escape of gas. It cannot be deniedthat it is not quite easy, in ordinary work, to workaltogether without evolution of sulphuretted hydro-gen, especially in commencing the process, sinceworkmen cannot always be trusted to be sufficientlycareful; and as Sciiaffner’s apparatus does not allow'any such escape under any circumstances, it W’ouldappear as if its use were advisable, even if, as Mondasserts and as Stahlsciimid has confirmed, ratherless sulphur is yielded by it than by the decomposi-tion on his principle—at least in such cases whereany escaping gas would cause complaints in theimmediate neighbourhood. Schaffner has, how-ever, always maintained that no trithionate andconsequently no calcium sulphate is formed, if theoperation is carried out exactly as prescribed by him,and Lunge’s researches have proved this to be the