X x x v I r r . - CONTRIBUTIONS FROM THE LABORATORY O F GONVILLE AND CAIUS COLLEGE CAMBRIDGE. NO. XII.-The action of j n e l y divided metals on solutions of Fer& Salts and a rapid method for the titration of the latter. By DOUGLAS J. CARNEGIE B.A. Demonstrator in Chemistry Gonville and Caius College. 1. I T is universally admitted in text-books that of existing methods for converting ferric salts into ferrous salts prior to titration with potassium permanganate the safest and best though by far the slowest method is to boil the acidified ferric solution with zinc in an inert atmosphere. The more rapid methods (excluding the stannous chloride method, which is not applicable if permanganate titration be employed) are hampered by the facts that in their employment it is difficult to deter-mine the point of exact reduction and excess of the reducing agent is as fatal as defect.It seemed to me that the safest method might be so modified as a t the same time to make it a rapid one by increasing the effective surface of the zinc through the employment of zinc-dust in the place of the usual granulated zinc. While experimenting in this direction I found that zinc-dust instantly reduces ferric to ferrous salt and this even in 7ieutraZ solufions. At the same time if the solction is neutral iron is precipitated partly as ferrous partly as ferric hydroxide. I n acid solutions no iron is precipitated but the reduction is less rapid the more free acid there is present. In every case zinc goes into soh-tion. 2. Very rapid and accurate estimations of ferric solutions are realised by the following method :-The bottom of a dry and narrow beaker is covered with zinc-dust, which has been sifted through fine muslin.A known volume of ferric solution previously nearly neu tralised by ammonia is now delivered into the beaker and shaken briskly with the zinc-dust. Finally a known volume of dilute sulphuric acid is added and the contents of the beaker are once more shaken. It is essential for rapid reduction that the above order be observed ; the nearly neutral ferric solution must Jirst be added to the zinc then the acid. I n order to withdraw for titration a definite volume of the ferrous solution free from particles of undissolved zinc I make use of the " reversed filter," figured in which n is a glass tube ; c miislin ; d filter-paper, held in position by an india-rubber ring b.When this filter i CARNEGIE THE ACTION OF FINELY DIVIDED METALS. 469 25 C.C. of an acidulated ferric chloride solution reduced by iron-free magne-Fium required 15.3 C.C. of a decinormal permanganate solution. immersed in the beaker the clear ferrous solution rises in it to the same level as the liquidin the beaker and may then be withdrawn by 25 C.C. of same solution reduced as above required 15.2 C.C. of same perman-ganate solution 470 CARNEGIE THS ASTPON OF FINELY DIVIDED METALS tried the action of the volatilised zinc crystals on ferric chloride. Their reducing power was unimpaired ; hence I had to seek for a new explanation. It is well known that ferric chloride in aqueous solution is in a state of partial dissociation as is roughly represented in the equation-Fe2C1 + (n + 3)H20 Z (Fe,,O,nH,O) + FeaCl + 6HC1.It might be urged that on adding zinc-dust to such a system the hydrochloric acid would be removed from the sphere of action with formation of zinc chloride and hydrogen and that the nascent hydrogen would reduce the ferric chloride existing as such while the soluble hydrated iron oxide might in virtue of this upsetting of the mobile equilibrium be simultaneously transformed into an insoluble hydrated form. According to this explanation it would follow that more iron hydroxide would be precipitated the higher the temperature at which reduc t'ion takes place €or the dissociation of ferric chloride increases with the temperature.And indeed experiment proved that about twice as much iron is precipitated as insoluble hydroxide when the reduction is effected a t 100" as when it takes place a t ordi-nary temperatures. But according to this explanation one would predict the improbability of reduction i f absolute alcohol were substi-tuted for water as the medium of the change ; whereas experiment shows that even under these conditions reduction readily takes place with great rise of temperature. 4. I am thus driven to the conclusion that the zinc acts merely as a &chlorinating agent much as stannous chloride acts -Fe2C!6 + Zn = ZnC1 + Fe,Cl,, and that the precipitate of iron hydroxides which occurs in neutral solutions is partly due to .the zinc oxide which is always present in the dust to the extent of about 50 per cent.partly to the zinc hydroxide formed during the reduction by the action of water on the finely divided zinc. Zinc-dust merely effects instantaneously the dechlorinat,ion which I found zinc-foil required several hours to effect. The change represented above is an exothermic one; the heat of formation in aqueous solution of ZnC1 [112,840] is greater than the negative thermal change in the passage from the system Fe2Cl,,Aq to the system FezS14,Aq [55,540]. 5. If this explanation of direct dechlorination be valid it seemed probable that all those metals whose chlorides have a heat of forma-tion in aqueous solution greater than 55,540 gram-units would in the finely-divided state reduce ferric solutions.I have made many experiments in this direction and I find that the following metals i ON SOLUTION3 OF FERRIC SALTS. 47 1 a finely-divided state reduce ferric solutions with varying degrees of rapidity :-iron mercury silver aluminium and copper as well as zinc. Sometimes the metals were employed in the shape of foil [aluminium copper silver] sometimes in the state of fine division in which they are precipitated from boiling alkaline solutions of their formates or from hot solutions of any of their salts by means of zinc-dust followed by repeated digestion with dilute acids suited to the occasion. I n the cases of aluminium and silver it was definitely proved that no precipitation of a salt of iron occurred. This without doubt would be the case with all metals M” where M” + H,O = M”O + H represents an endothermic change.Platinum and gold do not reduce ferric soliitions. Now with the exception of the last two named the heats of formation of the chlorides of all the fore-going metals are greater than 55,540. Nevertheless the rapidity of reduction by a metal- does not appear to be a function of the energy which runs down i n the formation of its chloride thus [AP Cl‘ Aq] = 475,650 whilst [Zn CP Aql = only 112,840; yet zinc reduces instantaneously whilst aluminium reduces the most slowly of all the metals experimented with. But experiment showed it to be undoubtedly the case that those metals reduce the quickest which are the most readily attacked a t ordinary temperatures by dilute chlorine-water. It is of interest to note that galena in a finely-divided state also reduces ferric chloride solution whereas antimony sulphide has not this power.6. From the whole of my experiments I conclude that zinc-dust is practically the best reducing agent for the purpose in hand. True it is that zinc-dust may sometimes contain a little iron and that titra-tion with permanganate cannot be conducted in an acid solution containing zinc but that the latter must be first removed. But in the first place zinc-dust contains so little iron and so slight a solution of zinc takes place before titration by my method that any error arising from this source is negligible. However in the attempt to elaborate a method which would preclude any uncertainty on this point I prepared zinc-dust free from zinc oxide as recom-mended by Sabatier by means of repeated digestion with dilute acid and also by what I found to be a more rapid method viz.by digestion with solutions of ammonium chloride and ammonia in both cases finally drying the product on porous tiles in a vacuum. This purified zinc was shaken up with a standard ferric chloride solution without the addition of any acid; the ferrous solution was filtered off, acidified and titrated; but the iron was not fully accounted for in the filtrate so rapidly does water attack the finely-divided zinc with formation of hydrogen and zinc hydroxide ; the latter precipitating solutions of iron salts in contact with it. I n fact i n solutions o 472 CARNr(:GIE THE ACTION OF FINELY DIVIDED METALS several metallic fialts MgClZ AL(S04)3 Co(NO,), MnS04 &c , finely-divided zinc very soon causes a precipitate either of the hydroxides or of basic salts of the metals present.I n the second place even when the reduction is effected by metals which can easily be obtained absolutely free from iron which are not oxidised by water and which do not evolve hydrogen with dilute acids separation from the finely-divided metal musf always precede titrntion; for even silver and aluminium are attacked i n feebly acid solutions bv permanganate. 7. Mitscherlich ( Z e d . anal. Chem. 2 72) has stated that in the reduction of ferric solutions it is absolutely necessary that the whole of the zinc shmld be dissolved before titration ; the reason adduced being that iron is precipitated on the surface of the zinc and does not dissolve until the last traces of the zinc themselves disappear.If this statement be accurate objection may be taken to my method detailed above ; but I much doubt its accuracy. Experiment showed that the titre of an acidulated iron solution was independent of the time it had remained i n contact with the zinc-dnst. This might be explained iu this special case by supposing the finely divided zinc to be practically enveloped in a protecting layer of hydrogen but other experiments would lead me to believe that such a supposition is unnecessary. Examination of pieces of granulated zinc free from iron removed either before or after complete reduction of both hot and cold ferric solntions always failed to give evidence of iron. Beebe’s method of reducing ferric solutions (Chem.News 53 269) would also be untrust-worthy were Mitscherlich’s statement correct. Wit’hout doubt the zinc in all cases becomes coated with a black deposit which as Rodwell Vogel and others have shown contains in addition to the iron present originally in the impure zinc iself zinc combined with lead sulphur aiid carbon. 8. After the work of which this paper is a short account was finished I casually came across a reference in Fremp’s EmyclopBdie to a paper by Brown on the reduction of ferric compounds b; zinc. I Jiare procured the paper referred to ( I r o n 1878 361) and find that E ~ o w ~ ’ s method consists in reducing iron ores directly by fusion with ~ Z V P T ~ S ~ ~ zinc (Hobson and Sylvester had shown that a t a temperature of 205” zinc becomes so brittle that i t may be powdered in a mortar).Brown has also used this pulverised zinc to reduce ferric salts in acid solutions ; but that his method is founded as is the current one on the reducing action of nascent hydrogen and not on the direct reducing. powers of the zinc is obvious from the following quotation :-“ There should be but a very small excess of sulpburic acid present so that at the end of an hour or fwo only about half the zinc will be dissolved.” Brown is also of opinion that the whole of the zinc must be dis ON SOLUTIONS OF FERRIC SALTS. 4 i 3 solved before titration but he does not state his grounds for that opinion. I prepared some pulverised zinc by Hobson and Sylvester's met.hod, but with three different specimens of zinc. I uniformly found that they did not become brittle at 205" but at higher temperatures and also that it was impossible thus to obtain anything approaching the fice division of zinc-dust. The pulverised zinc obtained reduced neutral ferric solutions but slowly. In conclusion I would express my thanks to Mr. Pattison Muir for the kindly suggestive interest he has taken in the work detailed, VOL. Lcrr