TAYLOR : RESEARCHES ON BLEACHING POWDER. 2541CCLX.-Besearches on Bleaching Powder.By ROBERT LLEWELLYN TAYLOR.FOR the purpose of this investigation, it was first of all necessaryto devise a method for distinguishing between pure chlorine andhypochlorous acid, and, in a mixture of the two, finding theirproportions.The method used was to pass the gases through a known volumeof N/lO-sodium arsenite. The action of chlorine and of hypo-chlorous acid on sodium arsenite may be represented thus:(1) AqO, + 2C1, + 2H,O = A%O, + 4HC1.(2) As,O, + 2HOCl= As20, + 2HC1.It is plain that, for the same amount of arsenite oxidised, twiceas much hydrochloric acid (or chloride) is produced in the case ofchlorine as in the case of hypochlorous acid. The arsenite (whichmust always be in excess, so that it is not completely oxidised) isthen divided into two equal parts.In onehalf, the amount ofarsenite remaining unoxidised is determined by means of N / 10-iodine solution, and from this the amount oxidised is ascertained.I n the other half, the amount of chlorine is determined by meansof N/10-silver nitrate. In the latter determination the use of anindicator was very soon discarded, although, if a considerableproportion of the arsenite has been oxidised, the arsenate producedacts fairly well as an indicator, and may be used instead of addinga chromate to the solution. Much more trustworthy results, how-ever, are obtained by acidifying the solution with nitric acid,adding a little of the silver solution, boiling for a minute or two,and then filt'ering a portion of the liquid.A little more of thesilver solution is added, drop by drop, to the filtered portion, whic2542 TAYLOR : RESEARCHES ON BLEACHING POWDER.is then returned to the bulk, and this boiled and filtered again.This is continued until the filtered portion gives no further pre-cipitate with the silver solution. As described, the process seemstedious, but in practice it works very well, and the determinationof the chlorine can be made fairly rapidly. In this process, as thesolutions used are all decinormal, and therefore equal to each other,when the oxidising agent is pure chlorine, the amount of silvernitrate used (=amount of chloride present) is the same as theamount of arsenite oxidised.When hypochlorous acid is theoxidising agent, the chloride produced is only half the amount ofarsenite oxidised.The Action of Carbon Diodde on, Bleaching Powder.It is very frequently stated, and probably usually considered, thatcarbon dioxide simply liberates hypochlorous acid from bleachingpowder. A considerable number of observers, however, havepointed out that chlorine is produced when pure carbon dioxideacts on bleaching powder.Thus, Richards and Juncker (Dimgl. Polyt. J., 1874, 211, 31)state that dry bleaching powder is almost undecomposed by carbondioxide. I f about 10 per cent. of water is present, both chlorineand hypochlorous acid are produced.Wolters ( J . p r . Chem., 1874, [ii], 10, 128) refers to the liberationof chlorine by the action of carbon dioxide on bleaching powder,and Lunge and Schappi (Dingl.Polyt. J., 1889, 273, 63) state thatcarbon dioxide expels nearly the whole of the chlorine from bleachingpowder.Dreyfus (Bull. SOC. chim., 1884, [ii], 41, 600) found that carbondioxide has no action on calcium chloride, but, in the presence ofchlorine monoxide, either dry or in aqueous solution, it liberateschlorine.More recently this question, with others relating to bleachingpowder, has been investigated by von Tiesenholt ( J . p r . Chem.,1901, [ii], 63, 30; 1902, [ii], 512; 1906, [ii], 73, 301). Some ofhis conclusions will be referred t o later.In my first experiments, carbon dioxide, in its ordinary moistcondition, after being well washed with water, was passed through aU-tube containing bleaching powder and a sufficient amount ofglass wool to give a free passage for the gas.Carbon dioxide, asordinarily prepared by the action of hydrochloric acid on marble,invariably carries with it a little hydrochloric acid, and, in someof the experiments, this was removed by passing the gas throughglass wool wetted with a solution of silver nitrate, which acts quiteeffectually. When, however, t,he gas is well washed with water, thTAYLOR : RESEARCHES ON BLEACHING POWDER. 2543amount of hydrochloric acid accompanying it is not sufiicientseriously to affect the results. After passing through the bleachingpowder, the gas was passed through a Bunsen U-tube cont,ainingN / 10-sodium arsenite.A considerable number of experiments were made, and thefollowing is an example of ths results invariably obtained. TwentyC.C.of the N / 10-arsenite were used :I.Amount of arsenito oxidised ................................ 8-35 C.C.Amonnt of N/lO-silver nitrate used ........................ 8'4 C.C.It is plain from this that the sole product of the action of carbondioxide on bleaching powder is chlorine. The escaping gas has astrong odour of chlorine, and none at all of hypochlorous acid.The action proceeds very rapidly if the carbon dioxide is quitemoist.In some further experiments, the carbon dioxide was dried bypassing it over calcium chloride. The effect of drying the gas isthat the action becomes much slower, and, as the moisture usuallypresent in the bleaching powder is gradually carried away by thedry carbon dioxide, it soon becomes extremely slow. When thispoint has been reached, the instantaneous acceleration of the actionwhen the drying tube is removed is very striking.The followingthree experiments show that the product is the same with the dryas with the moist gas, namely, nothing but chlorine:11.Arsenite oxidised. Chloride produced.1. 3'4 3 -52. 1 -9 1'953. 6 -55 6'65The slight excess of silver nitrate used may have been due to alittle hydrochloric acid carried over, although the actual amountsare not beyond the limits of accuracy of the method.When carbon dioxide is passed through a solution of bleachingpowder in water, the action is very rapid, but the result is exactlythe same, as the following experiments show :111.Arsenite oxidised.Chloride produced.1. 5 *6 5 -582. 3 -1 3'1The issuing gas again had a strong odour of chlorine, and noneat all of hypochlorous acid2544 TAYLOR : RESEARCHES ON BLEACHING POWDER.Action of Carbon Dioxide on a Mixture of Sodgum Chloride andHypochlorite, and on a Mixture of Bromide and Hypobromite.The mixture of sodium chloride and hypochlorite was preparedby passing chlorine into a moderately concentrated cold solution ofsodium hydroxide. The following experiments show that the act*ionis exactly the same as with a solution of bleaching powder :1.2.IV.Arsenite oxidised. Chloride produced.4 -1 4-126'1 6-13When carbon dioxide is passed through a solution containing amixture of a bromide and a hypobromite there is, as one wouldnaturally expect, an immediate and copious liberation of bromine.It is well known that carbon dioxide acts in a similar way on amixture of iodide and hypoiodite.It appears, from the foregoing experiments, that the action ofcarbonic acid on bleaching powder and similar substances is exactlylike that of any other acid.There has been much discussion asto the actual constitution of dry bleaching powder, but, whateverthat constitution may be, it may be taken that, in solution or inpresence of water, it is, to all intents and purposes, a mixture ofchloride and hypochlorite. The usual explanation of the actionof, say, sulphuric acid (when used in considerable quantity) onbleaching powder is t.hat the chloride and hypochlorite are bothdecomposed, with the simultaneous production of hydrochloric andhypochlorous acids, and that these decompose each other, with theliberation of chlorine.The question is whether or not we mustseek for some other explanation of the action of carbonic acid.There would seem to be no doubt that the action of carbonic acidis exactly like that of other acids. Of course, this involves theconclusion that calcium chloride (or sodium chloride, for example)is decomposed, when in solution, by carbonic acid, with the liberationof hydrochloric acid, and that, therefore, the action of hydrochloricacid on carbonates is a reversible one:CaCOs + ZHClI have tried to obtain some experimental evidence that this isthe case, and not altogether without success.Many years ago Miiller (Journ.Chem. SOC., 1870, 23, 36) statedthat a solution of lead chloride is decomposed when carbon dioxideis passed through it, with liberation of hydrochloric acid and pre-cipitation of it chlorocarbonate, and that some of the liberated acidcould actually be distilled off. He also stated that carbon dioxide,CaCI, + H,CO,TAYLOR : RESEARCHES ON BLEACHING POWDER. 2545under considerable pressure, would decompose sodium and calciumchlorides, when in solution in water, with liberation of hydro-chloric acid. He used ultramarine as an indicator, and stated that,whilst carbon dioxide alone does not decompose ultramarinesuspended in water, even under considerable pressure, if the watercontains common salt dissolved in it the colour of the ultramarineis destroyed.Carbon dioxide,when bubbled through water in which a little ultramarine issuspended, has no effect on it, whether the water contains salt ornot.Under a pressure of a few atmospheres, however, ultramarineis decomposed and decolorised by carbonic acid alone, and I havebeen unable to observe any difference in the action when the watercontained salt as well. The experiments I made were performedin an ordinary sparklet apparatus, in which the pressure attainsfive or six atmospheres. If distilled water with a little ultramarinesuspended in it is placed in such an apparatus, and then the liquidcharged with carbon dioxide in the usual way, there is no immediateeffect, but, in the course of a day or two, the colour of the u l t mmarine gradually disappears.As stated above, the presence ofsalt (or of calcium chloride) in the water makes no apparentdifference in the result.Methyl-orange is, however, a much more delicate indicator foracids than ultramarine. It is usually assumed that the former isnot affected by carbonic acid, but this is not quite correct. I f wellwashed carbon dioxide is bubbled through distilled water containinga little methyl-orange, there is a distinct alteration of the colour,although it does not turn pink. I f , however, the water containsalso a little pure salt, or calcium chloride, or potassium chloride,the colour becomes distinctly pink when the carbon dioxide isbubbled through.The change of colour is most striking in the caseof the common salt, but it is quite evident with the other chlorides.This may be taken as evidence that carbonic acid liberates a sensibleamount of hydrochloric acid in solutions of chlorides, that is to say,the action of hydrochloric acid on carbonates is a reversible one.Of course, the amount of hydrochloric acid thus liberated must beextremely small, but it will be quite sufficient t o explain the actionof carbonic acid on bleaching powder and similar substances. Thesmall amount of hydrochloric acid liberated will be at once decom-posed by the hypochlorous acid liberated simultaneously from thehypochlorite; this will enable the action of the carbonic acid toproceed it5 before, and so there will be a continuous evolution ofchlorine, and, if this is carried away as fast as it is formed, thebleaching powder will be almost completely decomposed.I am unable to confirm the latter observation2546 TAYLOR : RESEARCHES ON BLEACHING POWDER.It may here be noted that if carbon dioxide is bubbled throughwater containing potassium bromide or ammonium chloride andcoloured with methyl-orange, the change of colour is not so strikingas in the case of the three chlorides mentioned above.Pure watercoloured with methyl-orange becomes quite pink when charged withcarbon dioxide in a sparklet apparatus.Von Tiesenholt (Zoc. cit.) explains the production of chlorine whencarbon dioxide acts on bleaching powder by supposing that hypo-chlorous acid is first formed by the action of the carbon dioxide onthe hypochlorite present, and that this acts on the calcium chloride,liberating chlorine :CaCl, + 2HOC1= Cs(OH), + 2C1,.He finds, in confirmation of this view, that chlorine is liberatedwhen a solution of hypochlorous acid is added to calcium chlorideor to common salt.The experiments here described, however, showthat nothing but chlorine is produced by the action of carbonicacid on bleaching powder, so that all the hypochlorous acid whichis liberated must be decomposed. Apparently, if von T’iesenholt’sview is right, hypochlorous acid cannot exist in the presence of asufficient amount of a chloride, so that it would be impossible toexpel any hypochlorous acid from a solution which containschlorides. As will be seen later, however, mixtures of chlorineand hypochlorous acid containing a considerable proportion ofthe latter can be expelled from solutions of bleaching powder.Consequently, whilst it is possible that the action of hypochlorousacid on chlorides may account for some of the chlorine which isproduced in the case of concentrated solutions or the merely moistbleaching powder for example, von Tiesenholt’s explanation wouldnot appear t o be preferable to the one offered above.The Action of Air on Bleaching Powder.Although this was not the order in which the experiments wereactually tried, it will be best to describe first the effect of air fromwhich all the carbon dioxide has been removed.This was done bypassing the air through washing cylinders containing coke wet witha concentrated solution of sodium hydroxide.It was then bubbledthrough a milky solution of bleaching powder (about 5 to 10 percent.), and afterwards through the solution of sodium arsenite.In a31 the experiments with air, it was passed through at a rateof about 10 to 15 litres per hour.Air free from carbon dioxide is practically inert so far as bleach-ing powder is concerned, and naturally all that it can do is tosweep out any chlorine or hypochlorous acid which may happen tobe present. Consequently, the action is very slow, and the experiTAYLOR : RESEARCHES ON BLEACHING POWDER. 2547ments had to be carried on for a long time (from seventy-two toninety-six hours) in order to obtain sufficient oxidising action inthe solution of arsenite to be able to judge what was being carriedover.The following results were obtained in three separateexperiments :V.H ypochlorousArsenite Chloride acid, Chlorine,oxidised. produced. per cent. per cent.1. 1 -08 0.57 90 102. 1 *o 0-45 100 03. 0.8 0'38 100 0The amount of oxidation in these experiments was very little,but they appear to show that a small amount of free hypochlorousacid exists in slr solution of bleaching powder, which is simplyswept out by the passage of air free from carbon dioxide throughit. Probably the free hypochlorous acid is due to the calciumhypochlorite in it dilute solution being slightly hydrolysed, thus :Ca(OC1)2 + 2H,OThis possibly accounts for the fact that solutions of bleachingpowder have an odour of hypochlorous acid.One similar experiment t o the above was made in which thesolution of bleaching powder was kept at a temperature of about40° the whole of the time.In this experiment, also, practicallynothing but hypochlorous acid was swept out, the only differencebeing that, as one would expect, the time required was rather less.Ca(OH)? + 2HOC1.A ction of Ordinary Air on Bleaching Powder.A considerable number of experiments were made with ordinaryair, passing it through a tube containing dry bleaching powder, andthen through the solution of arsenite. A t first the action is ratherslow, but, as the bleaching powder gradually becomes wet, theaction proceeds more and more rapidly.In some of the experi-ments the moisture of the air was purposely increased by passing itthrough's tube containing wet glass wool. In each of the twofollowing series of experiments the same tube of bleaching powderwas used throughout. The time occupied by each experiment variedfrom about twenty-four hours at the beginning to six hours whenthe bleaching powder had become wet25481.2.3.4.5.1.2.3.TAYLOR : RESEARCHES ON BLEACHING POWDER.Arseniteoxidised.4 '452'92-557.07.92 -13.356.0VI.Series 1.H ypochlorousChloride acid,produced. per cent.4-0 112 -56 132.3 106.95 07.95 0Series 2.1 *83 153.1 86-05 0Chlorine,per cent.8987901001008692100The above experiments are selected from a considerable number,and they all tend to show that, at the outset, ordinary air sweepsout from bleaching powder a mixture containing from 80 t o 90 percent.of chlorine, and from 10 t o 20 per cent. of hypochlorous acid,but that, as the action proceeds, the amount of hypochlorous acidgradually diminishes, and at last nothing but chlorine appears.The gradation of the experiments is not the same in the two series,but that is partly due to the fact that some intermediate experimentsin both series were spoiled by going on too long.When ordinary air is passed through a solution of bleachingpowder (not filtered, and containing about 5 to 10 per cent. of thepowder), the proportion of hypochlorous acid swept out is consider-ably greater, as indeed one would expect if we accept the suggestionthat the hypochlorous acid is due t o hydrolysis of the calciumhypochlorite. As in the case of the dry powder, however, theamount of hypochlorous acid gradually diminishes as the experimentproceeds, although it does not disappear altogether.The followingexperiments were made with the same solution of bleaching powder,in the order in which they are given. In experiment No. 4, theproportion of hypochlorous acid appears to have risen slightly, butthe method of determining it is not accurate enough to enable oneto say that the amounts in experiments 3 and 4 were not sub-stantially the same. The action was very slow in the first experi-ment, but much more rapid afterwards:1.2.3.4.VII.Hy pochlorousArsenite Chloride acid, Clil orine,oxidised. produced.per cent. per cent.4'48 2'94 52 489-15 7-27 25 753 *18 2 '88 10 903.34 2.87 16 8TAYLOR : RESEAHCHES ON BLEACHING POWDER. 2549It must be pointed out that the above numbers, showing therelative amounts of hypochlorous acid and chlorine swept out ofthe liquid by the air, do not necessarily represent the actual pro-portions present at any moment in the liquid itself. There is nodoubt that chlorine, being less soluble in water than hypochlorousacid, will be swept out more readily, so that the proportion ofhypochlorous acid actually present in the liquid is certainly greaterthan the above numbers indicate.These results, showing the action of ordinary air on bleachingpowder, are very remarkable.The difference between the actionof ordinary air and air from which the carbon dioxide has beenremoved is, a t first sight, almost incredible. Whereas the lattersimply sweeps out from a solution of bleaching powder (althoughvery slowly) practically pure hypochlorous acid, the presence of thereally very small amount of carbon dioxide which usually existsin ordinary air causes the action to proceed much more rapidly(although not with anything like the rapidity with which purecarbon dioxide acts), and, after a time, has almost the same effect,so far as the product is concerned, as passing pure carbon dioxidethrough it.I have already expressed the opinion that the action of puremoist carbon dioxide on bleaching powder is the same as that ofother acids-it is a mass action, and the carbonic acid decomposesboth the chloride and the hypochlorite.Whilst one may acceptthis explanation in the case of pure carbon dioxide used in com-paratively large quantities, and always locally in large excess, it isimpossible to believe that the small amount of carbon dioxide presentin ordinary air can act in the same way. We must therefore lookfor some other explanat$ion.The Action of Chlorine 0% Alkalis a Reversible Action.I n former papers (Mem. Nanchester Phil. SOC., 1897, 41, No.VIII; Trans., 1900, 77, 725) I have pointed out that the actionof iodine on alkalis is a reversible one. If an alkali is added to asolution of iodine in water or in potassium iodide until the colourjust disappears, the addition of potassium iodide to the solutioncauses the liberation of some of the iodine:2KOH +I, K I + KO1 + H20.The addition of the extra amount of potassium iodide causes thereaction to proceed from right to left in the above expression.Also, and this, too, follows from the fact that the action is reversible,the amount of alkali needed to complete the reaction from left toright and to remove the colour of the iodine is considerably moreVOL.XCVII. 8 2550 TAYLOR : RESEARCHES ON BLEACHING POWDER.than is required by the equation, so that the almost colourlesssolution of iodide and hypoiodite always contains some free alkali.When these experiments were made, similar ones were alsoperformed with bromine and alkalis, but analogous results werenot obtained.The reason of this must have been that too stronga solution of bromine was used, because I find that the reversibilityof the action of bromine on alkalis is quite as striking as that ofiodine if a very dilute solution of bromine is employed. The actionis not nearly so easy to see with ordinary bromine water, but ifthis is diluted with ten to twenty times its bulk of water, and thensodium or potassium hydroxide added drop by drop until the colourof the bromine has disappeared, the addition of a little concentratedsolution of potassium bromide causes a manifest liberation ofbromine. The liberation of bromine is seen still more plainly if,instead of the solution of potassium bromide, a considerable amountof the powdered salt is added.The addition of the extra potassiumbromide causes the action to proceed from right to left:KBr + KOBr + H,O. 2 KOH + Br2It is perfectly reasonable to suppose, then, that the action ofchlorine on alkalis is also a reversible action. This has alreadybeen suggested by von Tiesenholt (Eoc. cit.), who describes it numberof experiments which point to this conclusion. I have been ableto demonstrate, by experiments which are described later, that thisconclusion is correct, and it will be seen that it supplies a perfectlysatisfactory explanation of the action of ordinary air on bleachingpowder, and that it also explains some well-known facts with regardto some bleaching solutions which have been hitherto apparentlyinexplicable.I f we represent the action of chlorine on sodium hydroxide andon slaked lime thus:2NaOH + C1, = NaCl + NaOCl + H,Oand2Ca(OH), + 2C1, CaCl, + Ca(OCl), + 2H,O.*it is plain that the chlorides produced by the action are continuallytending to reverse the reaction, so that, to carry it to a finish fromleft to right, there must always be a considerable amount of freesodium hydroxide or lime present.It is a well-known fact thatbleaching powder always contains a, considerable amount of freelime, and that it is impossible to prepare it otherwise. I f this freelime, or a portion of it, is removed, then the reaction will proceed* It is not suggested that this equation represents what actually occurs in themanufacture of bleaching powder, but simply the condition of equilibrium in whichit exists when wet or in solutionTAYLOR : RESEARCHES ON BLEACHING POWDER.2551in the opposite direction to a greater or less extent, and chlorinewill be liberated.A filteredsolution of bleaching powder was employed, having a specificgravity of 1-03 t'o 1-06 in different experiments. In order to removesome of the free lime, the solution was exposed to air for somehours in a shallow dish, with occasional shaking. The amount offree lime present in such a solution is considerable, and the latterbecomes very milky on exposure to air. The liquid was filteredfrom the precipitated calcium carbonate, and air free from carbondioxide wils passed through it and into the arsenite solution inthe usual way.A number of experiments were made to test this point.The following are some of the results obtained:VIII.Arseniteoxidised.1.4.02. 3.83. 2-64. 5'925. 4-356. 2-27Chlorideproduced.3.163-01.85.154 *041 '52Hypochlorousacid)per cent.26274415850Chlorine,per cent.747356a59250Most of the separate experiments were made with differentportions of the solution, which had been exposed to air for differentlengths of time, so that the extent to which the free lime wasremoved varied. Doubtless this accounts for the irregularity inthe results. In all the above experiments the action was muchmore rapid than was the case with the solution from which noneof the free lime had been removed, the rapidity evidently dependingon the extent to which this removal had been carried.It will benoted that besides the large quantities of free chlorine produced,in most of the experiments the amount of hypochlorous acid sweptout from the liquid was very much greater than was the case withthe solution from which no free lime had been removed. This isquite what one would expect to occur. The hypochlorous acid, asbefore stated, is probably due t o hydrolysis of the calcium hypo-chlorite in the solution. This also is a reversible action, and asone of the products of the hydrolysis is free lime, the removal ofthe lime naturally stimulates this action as well.These experimpla demonstrate quite sufficiently the reversibilityof the reaction betwen chlorine and calcium hydroxide. As thefree lime is more or less removed, the reaction proceeds in theopposite direction, and chlorine is liberated.I n these experimentsthe free chlorine is swept out of the solution, but it is continuallybeing reproduced, the steady removal of the chlorine allowing the8 D 2552 TAYLOR : RESEARCHES ON BLEACHINQ POWDER.reverse action to take place continuously. I f the free chlorine wereremoved from the solution in any other way, by bleaching, forexample, it would in the same way be continually reproduced aslong as any of the bleaching substance remained. It follows fromthis, of course, that the bleaching action of a solution of bleachingpowder will be stiaulated by the removal of free lime from thesolution.The action of ordinary moist air on bleaching powder, both solidand in solution, described on p.2548, is now perfectly intelligible.The carbon dioxide in the air combines with the free lime, and, asthis gradually diminishes and finally practically disappears, thereverse action proceeds freely, and, of course, chlorine is produced.*It is usually understood, and has been frequently stated, that apure solution of hypochlorous acid bleaches more energetically andmore rapidly than free chlorine. It may be doubted whether thisis really the case. I have prepared practically pure solutions ofhypochlorous acid, and compared its action with that of a solutionof chlorine on various colouring matters, and I have failed to findany evidence of the greater activity of hypochlorous acid.Ratherthe contrary. With a solution of indigwarmine, for example, thebleaching action of chlorine is much more rapid than that ofhypochlorous acid-in the case of the latter the action is to bedescribed as sluggish, rather than rapid. This is an importantpoint, because I am strongly of opinion that in the use of solutionsof bleaching powder and similar substances for bleaching purposes,most of the actions generally attributed to hypochlorous acid arereally due to chlorine, and that, in practice, hypochlorous acid playsonly a minor part in bleaching.It is remarkable how the bleaching action of a solution ofbleaching powder is stimulated by the mere removal of the freelime in it. I f a strip of Turkey-red calico is placed in a clearsolution of bleaching powder so that it is completely immersed inthe liquid, and if the liquid is kept in a, closed vessel so that airhas no access to it, there is scarcely any bleaching action at all,even after several days.I f , however, the solution is placed in a* It may be asked i f the removal of free lime by carbon dioxide is a satisfactoryexpIanation of the fact that ordinary air expels chlorine from bleaching powder,would not this also explain the action of pure carbon dioxide on bleaching powder,so that there would be no need to assume, as is done in the first part of this paper,that carbonic acid decomposes clilorides with the liberation of hydrochloric acid ?The author adheres to the latter explanation simply because the action of carbondioxide is so much more rapid than that of air.A stream of carbon dioxide througha solution of bleaching powder liberates chlorine from ten to twenty times morerapidly than air a t its quickest, and the action altogether suggests a rapid andcomplete decomposition, such as is effected by other acids, rather than the meresweeping out of chlorine produced by the reversed action.This will be referred to again laterTAYLOR : RESEARCHES ON BLEACHIXG POWDER. 2553basin or a shallow dish, so that air has free access, and if a smallportion of the red calico is left outside the liquid, so that it isreached by the solution and the air at the same time, the portionoutside is bleached quite rapidly.Further, if the coloured calicois completely immersed in a little of the solution contained, say,in a deep test-tube, and the test-tube is breathed into about halfa dmen times, shaking after each time, the calico is very soonbleached. Also, whilst, as stated above, a fresh solution of bleach-ing powder has very little, if any, bleaching action on a piece ofred calico completely immersed in it, if the solution has been exposedto air in a shallow dish for a few hours, with occasional shaking,then a piece of red calico completely immersed in it is bleachedrapidly.A simple but very striking experiment which illustrates the samepoint is to immerse a strip of ordinary red litmus paper in a freshsolution of bleaching powder. The paper is turned blue, and in ashort time it is bleached.I f , however, immediately after it hasbeen dipped in the solution, it is breathed upon, it is bleachedalmost instantly. A solution of bleaching powder which has beenwell exposed to air, as described above, bleaches litmus paper atonce.I n all these cases the more rapid bleaching action is simply dueto the removal of free lime, and I think it is plain, also, if referenceis made to the experiments, series VIII, on p. 2551, that theprincipal bleaching agent is chlorine, and not hypochlorous acid.Certainly those experiments show that in some cases a considerableproportion of hypochlorous acid is swept out, but in all cases therapidity of the bleaching action is roughly proportional to theextent to which the free lime is removed, and the more com-pletely that is done the greater is the proportion of chlorineliberated.It is a fact, well known in bleach-works, that an old vat is moreactive than a new one.Exposureto air, especially if the liquid is frequently st,irred, gradually causesthe removal of the free lime.If the action of chlorine on lime is, as I think the above experi-ments sufficiently demonstrate, a reversible action, then the reverseaction must be stimulated by the addition of calcium chloride to thesolution. Afterexperiment No. 2 (series VIII) on p. 2551 was finished, a consider-able amount of crystallised calcium chloride was added to the samesolution of bleaching powder and air free from carbon dioxidepassed through it again.The action became considerably morerapid, and the effect of the calcium chloride is seen by a comparisonThe reason for this is obvious.Experiments were made to see if this is the case2554 TAYLOR : RESEARCHES ON 3LEACHING POWDER.of the two experiments.of the calcium chloride.No. 1 was before, No. 2 after, the additionIX.HypochlorousArsenit e Chloride acid, Chlorine,oxidised. produced. per cent. per cent.1. 3.8 3.0 27 732. 4'75 4'73 0 100The solution used in experiment No. 6 (series VIII) was treatedin the same way with the following result:H ypochlorousArseiii tc Chloride acid, Chlorine,oxidised. produced. per cent. per cent.1. 2'27 1.52 50 502. 5.4 4.92 10 90These experiments show plainly that, as anticipated, the reverseaction is greatly increased by the addition of more calcium chloride.Other chlorides, of course, ought to have a similar effect.Thefollowing experiments show the effect of adding common salt to thesolution. As before, the greater part of the free lime in thesolution was removed by exposing it to air. Experiments 1 and 2were successive experiments before the addition of the salt, andNo. 3 shows the effect of the salt.siderable quantity-almost sufficientX.Arsenite Chlorideoxidised. produced.1. 1.75 1 -42. 5-25 4-13. 6.53 6.55The addition of the salt in theThe salt was added in con-to saturate the solution.Hypochlorousacid, Chlorine,per cent. per cent.25 7528 720 100"above experiment caused theaction to proceed much more rapidly.Thus, whilst in experimentNo. 2 it took twenty hours to oxidise 5-25 C.C. of the arsenitesolution, in experiment No. 3, 6.53 C.C. were oxidised in four hours,the carbon dioxide-free air passing through at approximately thesame rate in both experiments.* The apparently complete disappearance of hypochlorous acid indicated inexperiments 2 (IX) and 3 (X) is very remarkable, and seems difficult to explain.It is not claimed, however, that the method used for determining the relativeamounts of chlorine and hypochlorous acid is perfectly accurate. I t is doubtfulwhether it would be possible to detrrmine very small proportions of hypochloronsacid by it. It must also bc borne in mind (see p. 2549) that chlorine is moreeasily swept out from the solution than hypochlorous acid, so that it is possible that2he latter does not altogether disappear.Possibly, also, von Tiesenholt's explanation(see p. 2546) may apply here, and the hypochlorous acid may be all decomposed bythe large quantities of chlorides prescnt in the solutionTAYLOR : RESEARCHES ON BLEACEING POWDER. 2555Experiments were also made to see the effect of the addition ofcalcium chloride and salt to an ordinary solution of bleachingpowder, without removing any of the free lime. The free lime inthe solution, of course, tends to stop the reverse action, so that theeffect of addkg calcium chloride or salt to the solution is not nearlyso great as when the free lime is first removed. The following isthe result of the two experiments tried.To No.1 calcium chloride was added, and to No. 2 common salt- -both in large quantity. The action proceeded very slowly indeedin both experiments, but the results are sufficient to show that,even in ordinary bleaching powder solution, the addition of chlorideshas a sensible effect in reversing the action :H ypochlorousAraeiiite Chloride acid, Chlorine,oxidised. produced. per cent. per cent.1. 2-24 2 '22 0 1002. 2-66 2 '4 10 90It follows from the above experiments that the addition ofcalcium chloride or salt to a solution of bleaching powder mustexercise a stimulating effect on the bleaching action of the solution.This is actually the case. I f some of the free lime has been removedfrom the solution, the effect of the addition of considerable amountsof calcium chloride or salt on the bleaching action is very striking.With bleaching powder solution in its ordinary state, containing theusual amount of free lime, the effect on its bleaching action ofadding calcium chloride or salt is, for the reason pointed out above,not nearly so great, although it is quite sufficiently marked.I understand that it has been found, in actual bleaching, thatthe addition of either calcium chloride or salt stimulates the action,but I am not aware that any satisfactory explanation of thisstimulating effect has hitherto been given.It may perhaps be worththe while of practical bleachers to note that. the addition of calciumchloride or salt has a much greater effect when some of the freelime has been removed-by exposing the solution to air, forexample.Bleaching solutions made by the electrolysis of a solution of salthave latterly come into considerable use, and I understand thatthe fact has been frequently noted that a solution of sodiumhypochlorite thus prepared is more active than a solution of sodiumhypochlorite, containing the same proportion of available chlorine,prepared by the addition of sodium carbonate to a solution ofbleaching powder and allowing the precipitated calcium carbonateto settle.The explanation of this is obvious when it is understoodthat, in preparing the electrolytic bleaching solution, only a smallfraction of the salt in the solution is usually decomposed. Th2556 TAYLOR : RESEARCHES ON BLEACHING POWDER.solution thus differs from that made by the other method by con-taining a large amount of salt, and the effect of this is to increasethe reverse action and so to liberate chlorine in the solution.Also,in the electrolysis of the salt, chlorine and sodium hydroxide areproduced in exactly equivalent proportions, so that there cannotbe a sufficient amount of the latter to absorb the whole of thechlorine. Under these conditions, the reversing action of the excessof salt will naturally be very considerable. The greater bleachingactivity of such a solution is therefore perfectly natural, and exactlywhat one would expect.In addition to the experiments described in this paper, I haveused my method for distinguishing betwen hypochlorous acid andfree chlorine for investigating the action of various acids on bleach-ing powder and similar substances. This investigation is stillproceeding.Summ,ary.1. The action of carbon dioxide on bleaching powder and similarsubstances results in the liberation of chlorine only-no hypochlorousacid. The conclusion is drawn that the action is like that of anyother acid, and that carbonic acid decomposes both the chloride andthe hypochlorite in the bleaching powder. It follows from thisthat the action of hydrochloric acid on carbonates is a reversibleone.2. Ordinary moist air acts on solid bleaching powder, liberating atfirst both chlorine and hypochlorous acid, the former in muchthe larger amount. After a time nothing but chlorine is produced.When ordinary air is passed through a solution of bleaching powder,a mixture of hypochlorous acid and chlorine is swept out, at firstin about equal amounts; but, as the experiment proceeds, the formerdiminishes, and the latter increases to about 90 per cent.3. The action of chlorine on alkalis, like that of iodine andbromine, is a reversible one, as stated by von Tiesenholt. If the freelime in bleaching powder is removed, this causes the reverse actionto proceed, and thus chlorine is liberated. This explains the actionof ordinary air on bleaching powder. The reversibility of the actionalso explains the stimulating effect on bleaching which the additionof calcium chloride or of salt causes in a solution of bleachingpowder.4. I n the ordinary processes of bleaching the active bleachingagent is probably free chlorine, hypochlorous acid playing only aminor part.MUNICIPAL SCHOOL OF TECHNOLOGY,MANCHESTER