247
D E T E R M I N A T I O N O F T A N N I N G MATERIALS.
Our results on this sample were 0.048 by the acetate method and 0.055 by the volumetric. By the combination method described above, which seems to exclude many of the chances of error which occur in either the acetate or molybdate method, we found 0.0538 per cent. phosphorus. I t will be noticed that the results obtained by the acetate method are lower than either of the others. This has been our experience repeatedly, when we have compared the three methods, the acetate giving invariably the lowest results. W e will not enter into a discussion in regard to the causes for this as they are many, and this is not the purpose of our paper, but in our opinioii the proposed Committee on Standard Methods can do no greater benefit to the iron industry of our country than by a thorough investigation of the methods used for the determination of phosphorus in ores and finished products. From our personal observation we know that frequently injustice is done, and able chemists brought into disrepute, simply because their results have not agreed with those obtained by analytical chemists of wide reputation, whose methods have not told the whole truth.
ON THE QUALITATIVE DETERMINATION OF TANNING MATERIALS.' BY H . R .
PROCTER, F.
I. C., F. C. S.. LECTURERON LEATHER INDUSTRIES AT S H I R E COLLEGE, L E E D S ,
THE YORK-
ENGLAND.
IT
can hardly be said that any systematic attempt has been made to formulate a scheme for distinguishing the various tanning materials by their qualitative reactions, except perhaps a very slight table published by the writer, although, in view of the constantly increasing number of new tanning materials, and especially of new extracts which are offered to tanners, the matter has become one of considerable practical importance. T h e commercial value of an extract is dependent, not only.on the percentage of tanning matter as determined by analysis, but on the nature of the taiinin present, which influences the character of the leather produced. It is, of course obvious, that, failing a clear knowledge of the. 1 Kead before the World's Congress of Chetnists, August cle was kindly read by Professor Henry Trimble.
25,
1893. Proof of this arti-
248
H. K . PKOCTOK
tannins themse!ves any definite sclieiiie of separatioii is for the present impossible : since. altliougli ive hn1-e good reason to believe that the taiiiiiiis are :I class;, ~ v eare quite igiiorant oi their number, 1vhich riin!. ensill- ?le a coiiipa;-ntively small 0 1 1 ~ . as many members of the group ivliich at preieiit recci1.e i i i t l i vidual names are prol)a!)ly iiiere mixtures. Ti'hat can 11c (lolie is to separate the taiiiiiiig riiaterials iiito groups or iiidividuals b y reactions n-hicli 1iai.e L w i i ciiipirically drteriiiiiietl ; ol ivliich in many cases ive (lo riot kiic~\v the clieiiiical sigiiificarice. arid which may possihl!- i j e ofteii tluc t o t~tliercoii>titueiits of tlie materials than tlie taiiiiiiis theiiiseli-es. The questioli is still further complicatetl 1 ) ~ ' the fact tlint ivoods. barks, aiitl fruit of the same tree ofteii yield tariniiis Of distinctly differeiit character. In view of these facts, tlic jiroup tables giver1 must he only regarded as pointing out the geiiei-a1 chnracter of the material examined, ant1 their iritlicatioris iiiust a!iva~.s1)e colifirmed by direct comparison of ( 7 N the reactions obsenwl ivith those given by the material suspected. I t iiiust also be reiiieiiiliered that in some t x s e s the group reactioiis :ire uiicert:iiii, or ma?. 1)e siiiiulated by iiiixtiirc.~,\\.liicli 111:iy ofteii be tletermiiietl 11). comparison of tlie rebults oi tlie iiiore sl)eci:il tcsts. Iii this conriectioii it 1ii:iy 1 ) iiotccl ~ that iiwst o! thc reactioiis under the interrnetliatc groups might 1)c siiiiulattxl !)!. suitable mixtures of X aiid E' ; u i d tlint the iiicoiistaiic). oi the iroii reaction in closely allied trees, poiiits either to tlic-ir containing mixtures of tannins, or to the existence of a very large iiuniber of closely allied taniiiris of ver). similar constitution. T h e point is one very difficult of decision, since \ve have no really reliable iiiethod for separating different iiieiiilxrs of the faiiiil>-. I t was thought that some i1ifonii:itioIi a s to the ineariiiig of the reactions emplo!~ed iiiight ! ) e o1)taiiietl 1)). exaiiiiiiiiiji the behavior of the simpler deriv:tti1.es uiitler the sxiiie circuiiistances, aiid the results are gi\wi i i i taMe S . I t !vi11 be at once obvious that the hlue-l)lack reactioii \vith iron is cliaracteristic of the pyrogallol group : aiitl tlic ;;reen-i)lack equnily so of the catechol derivatives, while pliloroglucol. coiitrar)- to tlie statements of the test-lmoks. x i i w 110 iiiarkctl reaction \\.it11 iron. but is alone cliaracterizetl h y tlie v u ) ' delicate reaction
D E T E R I I I N A T I O N O F T A N N I N G 31ATERI.ILS.
249
with the vanillin present in deal. On the remaining reactions little light is thrown. I n qualitative work with tanning materials, a good deal depends on the strength of the solutions, and the way in which the tests are performed, and as the information in the Tables is necessarily very brief, a few preliniinary words are desirable. IilfZ~sionsof tanizing mafcrials are understood to be of about the strength used by the writer in determinations by hide-powder, viz., about 0.6 gram of dry soluble matter in IOO cc., but moderate divergencies from this do not influence results. Of coiirse where rea’ctions are feeble in any case, with very dilute solutions they may be imperceptible, and on the other hand. strong solutions will sonietiines give precipitates where only colorations are noted with weak ones. Broiiiiize water reaction.-It is best to add the broniine water drop by drop to 2-3 cc. of the infusion in a test tube, until the solution smells strongly. I n some cases the precipitate is slight, or forms slowly, and occasionally it is crystalline and on this account less conspicuous, but it is usually a distinct yellow or brown flocculent one. . I n most cases bromine precipitates such tannins as give a green-black with iron, and in general ternis it may be said to be a reagent for the catechol tannins. Nitrotis acid reaction.-This reaction, which is believed to be a new one, is obtained by adding to a few cc. of the infusion in a porcelain basin a distinct excess of solution of sodium or potassium nitrite, and then 3-5 drops of decinoriiial sulpliuricor hydrochloric acid, In typical cases the solution instaliti!. turns pink or crimson, and slowly changes through purple to a deep indigo-blue, but in others, as suniach where the renction is feeble, and niasked by other changes, the final color is gree:i or even brownish. I n a large nuiiiber of cases, nitrous acid produces a yellow or brown coloration or precipitate. but “reaction” in the Tables invariably means a series of colorchanges as above described. Nothing can as yet be said on the nature of the chernical changes produced. Tlie reaction i.; given by all tanning materials which yield ellagic ficid or “bloom,” but not by ellagic acid itself, nor by pure gnllotannic acid. It is possibly a reaction of ellagitannic acid.
250
H. R . P R O C T O R .
Fewic alum, A?izmo?zio-fe~rir siilfihnf~v.--rb one per cent. solution of this salt has been chosen as casilj- obtainable, neutral, and in a state of approxiiiiate pui-it).. Ferric chloride, which is generally used, is almost iiivarial)l!- strongly acid, xncl of n 1m-y persistent yellow. Acid ferric chloritic in excess \vi11 give a green-black reaction with gallotaniiic acid itself, niitl its results as a test are often very misleading. Ferric acetate. which has also been largely used gives heavy precipitates xvitii iiiost materials and is very strongly colored, so that it was fouiid less convenient than the iron alum. As the colorations arc geiierally very powerful, they are best seeii bj. diluting the taliiiin infusion freely, and adding the iron solution very cautiously. I n many cases the coloration first produced, and which is the cliaracteristic one, rapidly fades to a dull olive or hrowii in presence of excess of iron, probably b y oxidatioii atid destruction of the t a n n i n C u p ~ i csu@kaie a i d anzmo?ziu.--When solution of cupric sulphate is added to a tanning material infusion, it soinetiiiies gives a precipitate, and sonietinies not. This is a useful indication in sonie cases, but must not be regarded as of first rate importance as the presence of an)- weak organic salt would i n all cases cause a precipitate, by iieutraliziiig the sulphuric acid of the sulphate. Cupric acetate produces a precipitate with all tannins so far as I have observed, and it is interesting to note that in this case, as in many others of a like character, tlie tanriin combines with, and precipitates the salt as a whole. On the addition of ammonium carbonate, effervescence takes place, and the precipitate darkens, atid beconies a true tannate. When ammonia is added to the iiiisture of a tannin and cupric sulphate, a precipitate is invariably foriiied in tlie first instance, consisting of a tannate of copper with variable quaiitities of blue cupric hydroxide. %‘ith excess of ariinionia, the whole of this precipitate is i n iiiaiiy cases dissolvctl, ivhile in others, an irisoluble tannate reinailis. ’Phe Tatter is the case with all tannins derived from gallotannic acid and with many containing protocatechuic acid and foriiis a useful means of classification, though as yet the difference of structure to which it corresponds is unkno~vn. Tlie fact that hemlock arid other
D E T E R B I I N A T I O N OF T A N N I N G M A T E R I A L S .
2.51
pine barks, as well as cutch and gambier, yield cupric tannates soluble in ammonia is an instructive commentary on processes which have been proposed for quantitative estimation by precipitations by amnionio-cupric solutions. Where the precipitate redissolves it generally does so with the production of either a greenish brown, or a purplish brown coloration, which in presence of a small amount of the blue animonio-cupric sulphate, appears either as a green or a reddish violet, and is again a useful means of identification. I t need hardly be pointed out that to see this coloration well, the copper should be in the least possible excess. Hence it is desirable to use a very weak copper solution, say at most one per cent. Stannous chZonZe a n d hydrochZon*c acid.-This reagent, the application of which is here first published, consists in a strong solution of stannous chloride in concentrated hydrochloric acid. If about ten cc. of this are added to one cc. of the tanning material infusion, in a porcelain basin, and allowed to stand for ten minutes, coniferous tans, mimosas, and some others give a very marked pink coloration. This is specially distinct in the case of larch bark. If a small piece of larch-tanned leather be steeped in the reagent, the coloration appears very strongly. Deal shaving a n d N~~d~ochZoric acid.--A shaving or slip of any pine wood is moistened with the infusion, and then, either before or after drying, is again moistened n-ith concentrated hydrochloric acid. I n the case of cutch and gambier, and a fen other materials, the spot becomes at once a bright red or violet, but in many cases the reaction is faint, and only appears after some hours. It probably always indicates the presence of phloroglucol. Sodium SuQhite-This was mentioned in my previous manuscript on the subject, but the printer unfortunately substituted sodium sulphide, which does not answer in the laboratory. I n some cases these reactions will s e n e to determine the constituents of mixtures but in many cases this will continue impossible till further and more distinctive reactions are discovered. They can of course be applied to the recognition of tannates. when excess is present in leather which can be dissolved out
252
H . R. P R O C T O R .
with water; but the effect of dilute alkalies and other solverits on the reactions is worth further experinleiit. .I few drops of the solutioii are placed in contact \\ith a crl-stal of sulphite 011 a tile or in a basin. With valonia, a bright pur1)lisli pink is rapidly developed, apparently by oxidatioii. Many other tanniiig iiiaterials produce red or pink colorations, but in no case so marked a reactioii as valonia. Coizcenfrafed S Z @ I / L ~ I Y ~ C acid.-.I test tube i b rinsed out with t h e infusion, and drained so that only about a drop remains, aiid concentrated acid is cautiousl!- poured into the sloping tube, so as to foriii a layer uiiderneath the taiiriiii solution. T h e ring of color at the junction of the tiyo liquids is noted, mid then they are mixed hy shaking, am1 diluted with water. *2l,out half the iiiaterials give a deep purplish cririisoii, called ,siiiipl). s'criiiisoii" in the Tables, and in niaiiy cases this is decidedly pink 011 dilution. rvhile in other cases it is obscured b y 1)rowi products formed by the heat of mixture. A large nuiiiber of other iiiaterials give browns or yellows only, which when intense appear red. but dilute to yellows or yellow-browis. Lime water is a iiiost useful reagent, being wr!. varied in the color of the precipitates it produces. arid the changes they uiitlergo by oxidation. T h e reaction is best seeii i n a shallow porcelaiii basin.
I
I
1-
!iu;
i
Precipitate is insoluble in excess of E monia-Group A y. Table IV.
am.
LRM
.
LJ
y.-
.g
Precipitate redissolves with reen or olive brown coloration-Group A 13. %able 111. Precipitate redissolves with reddish violet Q. Table 11.
{z 8 colorahon-Group A
U
. .-
c
