Spot Reaction Experiments Part II:
Reactions on Filter Paper FRITZ FEIGL
Laboratoria Central da Produ~EoMineral, Ministerio d o Agricultura, Rio d e Janeiro, Brmil
(Translated by Ralfih E. Oesfier, University of Cincinnati)
I
N THE first paper of this series' it was pointed out that spot analysis and semimicro spot reactions in many divisions of chemistry are valuable not only as lecture demonstrations, but especially as instructive experiments for the student himself to carty out and to repeat a t will, with small expenditure of material and in limited working space. Some of the following spot reactions are of particular interest because they illustrate effects, hitherto unknown, that can be utilized for analytical purposes. If filter paper impregnated with a salt solution is treated with a suitable precipitant, the insoluble compound will present a far greater area of free surface than that exposed by the precipitate when formed in a test tube. This great development of surface is due primarily to the fact that the solid, in so far as i t does not form a more or less coherent film on the surface of the paper, is dispersed throughout the innumerable capillaries of the paper. Consequently, aggregation into compact coarser fragments does not occur. The free surface of solids is extremely significant with respect to their chemical changes, because i t is an important factor in determining the reaction velocity. This statement is to be taken in the following sense. The particular form of the solid product has no effect on the reaction velocity of the dissolved material, which is in equilibrium with the solid phase. Invariably, a certain portion of the solid must pass into solution before a reaction occurs there. However the rate a t which a solid is transferred to the dissolved phase-that is, the physical rate of solution-is a function of the free surface and varies directly with it. On the other hand, there are reactions (particularly of very slightly soluble materials) which can occur directly on
the free surfaces of solids, either with or without the intervention of the dissolved portions. Obviously such reactions will proceed more quickly the greater the reactive surface. In addition, i t should be noted that the appearance or disappearance of small quantities of colored solid materials can be seen far better on the white background of porous papers than in a water suspension, for instance. It is clear then that phenomena may be encountered in spot reactions, involving solids highly dispersed in the capillaries of paper, which will entirely escape observation when the same reaction is carried out with compact pieces in test tubes, or which can be detected there only by far more tedious methods. The following experiments illustrate these points. 4. Union of Sulfur or Selenium with Thallium SulIt was not known hitherto that solid thallium sulfide reacts with dissolved sulfur or selenium to form thallium polysulfides, T12S :S, and TlzSe:Se,. Probably this fact escaped observation because compact thallium sulfide unites only slowly with sulfur or selenium dissolved in an organic liquid. However, if filter paper impregpated with TleS is spotted with a solution of sulfur or selenium, combination is rapid and extensive because of the increase of the TlzS surface. The products, red-brown with sulfur and black-brown with selenium, are insoluble and become quite visible if the spotted paper is bathed in dilute acid; whereas the unchanged T1,S is dissolved almost instantly. Consequently the black-brown thallium sulfide paper loses its color except where characteristically colored spots of the addition compounds of TLS with sulfur or * selenium remain. Solutions of sulfur in alkali sulfides-that is, poly-
' Experiments with N
p
I
T n ~ JOURNAL, s 20,137-41 (1043).
Rraile
sulfides-react with finely divided TlzSin the same way as do solutions of sulfur in carbon disulfide, pyridime, etc. This result cannot be seen in a test tube reaction because solutions of thallium salts treated with alkali polysulfide produce a precipitate of black T1,S. The following experiments therefore give visible evidence of the importance of the free active surface of a solid in starting a reaction and influencing its rate. Procedure: Filter paper is soaked in 0.1 N thallium carbonate solution. While still moist, the paper is held over a beaker of warm yellow ammonium sulfide. Black thallous sulfide is thus precipitated on the paper and in its capillaries. The paper is washed quickly and dried in a current of heated air. It will keep for several days in a tightly closed container, but on longer standinp, especially in moist air, the sulfide is oxidized to sulfate. A drop of diluted ammonium polysulfide solution is placed on a strip of TbS-paper, dried, and then immersed in 1 N nitric acid. The DaDer will turn white as the TlsS dissolves, but a b r o w k i d spot will be left. The ammonium polysulfide can be replaced by an alkali polysulfide; other dilute mineral acids can be substituted for the nitric acid. The same experiment made with colorless ammonium sulfidewillleave no colored spot. As another experiment, thallium sulfide paper is treated with a drop of a one per cent solution of sulfur, or of a saturated solution of selenium, in carbon disulfide. As soon as the solvent has evaporated, the redbrown addition compound of sulfur will be plainly visible, even on the dark brown thallium sulfide paper. In the case of selenium, the color of the addition compound is so similar to that of the reagent paper that the dark brown spot can be seen plainly only after the paper has been bathed in dilute acid. Very minute quantities of sulfur or selenium suffice to demonstrate the addition compounds with thallium sulfide on filter paper; the identification limit is three y sulfur or one y selenium in one drop (one gamma, y, = 1 X 10-6 g.). Consequently, the procedure described here can be used to detect the presence of these elements dissolved in carbon disulfide or in other organic liquids. This new test for sulfur can be used to reveal the presence of free sulfur in fungicides, mineral products, medicinals, etc. For instance, free sulfur can be detected in vulcanized rubber as follows: The sample is shaken for several minutes with carbon disulfide in a stoppered bottle. A drop of the extract is placed on thallium sulfide paper and allowed to evaporate. When the paper is put into dilute nitric acid, a brown spot will be left. When considerable quantities of sulfur are placed on the reagent paper, red-brown spots are left, whereas brown spots result if only small quantities of sulfur are involved. The explanation is that acid-stable polysulfide is formed on the surface and protects the underlvinc , -TlsS - from the solvent action of the acid. This "protectivelayer effect" will he discussed in more detail in the next paper of this series.
5. Reaction of jinely d i d e d manganese dioxide Manganese dioxide is dissolved easily by concentrated hydrochloric acid, particularly when warmed: MnOz
+ 4HC1 = MnClz + 2Hs0 + CIS
The process of solution can be followed easily by noting the disappearance of the solid, as well as the liberation of chlorine (bluing of starch-iodide paper). Dilute hydrochloric acid a t room temperature reacts with manganese dioxide quite slowly. The same is true of dilute solutions of other reducing agents, such as oxalic acid, hydrogen peroxide, sulfurous acid, etc. Accordingly, if precipitated MnOzis shaken in a test tube with dilute sulfuric and oxalic acids, consumption of the solid by the reaction MnOp
+ H2C204+ H2S0, = MUSO, + 2C01 + 2H3O
cannot be observed directly but can be established only by a quantitative analysis. The conditions are quite different, however, if MnOz is highly dispersed in the capillaries of the paper. Under such circumstances, the reaction is almost immediate when a drop of a dilute solution of a reducing agent is placed on the paper. A white spot on a light brown or yellow field appears where the manganese dioxide is reduced. This paper can be used for a spot reaction to reveal the presence of various inorganic and organic reducing agents The reactivity of highly dispersed MnOzis so great that even a drop of dilute sulfuric acid, or even moderately strong acetic acid, produces a white spot on paper impregnated with a very little MnOz. In this way, reactions can be observed that are so slight that they are completely undetectable when carried out in test tubes. Procedure: Manganese dioxide papers of varying MnOz content can be prepared by soaking strips of quantitative filter paper in potassium permanganate solutions of different concentrations from one per cent down. A little sodium hydroxide solution must he added to the permanganate solution. The permanganate, according to its concentration and the length of exposure, oxidizes the cellulose to a greater or lesser degree. Finely divided manganese dioxide is precipitated in the capillaries of the paper and cannot he washed out. Deep brown to scarcely yellow papers can he prepared. If they are washed thoroughly in flowing water, and dried, they keep indefinitely. Mmute quantities (> 0.5 y) of reducing materials obviously require the use of weakly impregnated papers, and several minutes are needed to give a clearly evident reduction. Action on MnOz can be made visible on paper so weakly impregnated that i t appears almost white, if the spotting and dlying of the paper is followed by immersion in an alcohol solution of benzidine. The latter is a sensitive reagent for traces of MnOz, which turns benzidine blue. Accordingly, if weakly impregnated MnOz paper is spotted with a drop of the solution of a reducing agent that consumes MnOz, the whole strip of paper will turn intensely blue with
,Se FEIGL, e ,,Sgecifir and
reactions,,, Tmns OESPER, Nordeman Publishing Co., New York, 1940, p. 106.
the exception of the spotted area which remains colorless. 6. Reaction of Mercuric Oxide with Hydroxyhmine IIydrochlo~ide.~If a water solution of hydroxylamine hydrochloride is warmed gently with an excess of mercuric oxide, black metallic mercury is precipitated:
an effect that is usually taken for granted only for the more active halogens, chlorine and bromine. Procedure: One drop of a one per cent solution of c.p. sulfur in carbon disulfide is placed on neutral litmus paper. The solvent is allowed to evaporate and the spot is treated with a drop of 0.5 N iodine in potassium iodide solution. For comparison, a drop of pure car2NHsOH. HCl + 2HgO = Hg + HgCl? + NI + 4Hz0 bon disulfide is placed on the same strip. of indicator This is the net equation for a reaction that actually paper and, after evaporation of the liqtlid, treated with a drop of the iodine solution. The litmrrs paper takes place in two stages: is then placed in water, which dissolves the iodine off 2NHnOH.HCI HgO = HgCI: + H20 + 2NH10H (1) the paper. A red spot will be left on the litmus paper HgO ~NHIOH= Hg + N2 + 3H10 (2) a t the place where free sulfur remained after evaporation of the carbon disulfide. The sulfur came in conThe spot reaction described here permits the simple tact there with iodine, producing sulfuric acid. This separation of these partial reactions, which can be seen proves that an oxidation of sulfur as shown by (2) occurs after even a short exposure to iodine. plainly, in succession, on the surface of filter paper. 8. Conoersion of Metal Suljides into iodide^.^ HithProcedure: Filter paper is impregnated with 0.2 N solution of mercuric chloride in alcohol, dried, and erto it has not been known that numerous heavy metal then dipped momentarily in 1 N sodium hydroxide sulfides can be converted directly by iodine into the solution. The paper is washed thoroughly and then corresponding iodides: dried in a current of warm air. It must be stored in MS+L=MI>+S the dark. A drop of a dilute solution of hydroxylamine hydrochloride is placed on the yellow paper and the The proof of this conversion was made first by means capillary picture that develops is observed. Proceed- of spot reactions. They offer the best possibility of ing from the place of application, a white circle forms demonstrating this new method of forming iodides. first (zone of partial reaction (I)),and adjoining this is a Filter paper impregnated with the heavy metal sulfide black ring, due to the deposition of mercury (zone of is spotted with a solution of iodine in carbon disulfide partial reaction (2)). The whole is surrounded by un- or ether. The solvent and the excess iodine are volatilaltered yellow mercuric oxide. To show that mercuric ized, and spots, whose colors depend upon the particuchloride is formed in the central circle the paper is lar iodides, are left on the paper. The conversion ocfumed with hydrogen sulfide, or spotted with dilute curs rapidly because the sulfide is highly dispersed in potassium iodide solution. Black mercuric sulfide or the paper and thus exposes an extraordinarily large red mercuric iodide is produced. reaction surface to the iodine. The experiment may be carried out with hydrazine As might be anticipated from these experiments, the sulfate in place of hydroxylamine hydrochloride. conversion into iodide can be accomplished also with Analogous partial reactions occur. compact fragments of sulfides, provided an excess of 7. Action of Iodine on Free Sulfur. Acid-soluble iodine is allowed to react long enough. In such cases sulfides are commonly determined titrimetrically by the precipitated or powdered sulfide is shaken with adding the samples to an excess of an acidified stand- solutions of iodine in carbon disulfide or ether. Alterard solution of iodine. The reaction natively, the sulfide is mixed with solid .iodine and heated in a closed glass vessel on a water bath, thus bringing iodine vapor into action. Under these condioccurs and the unconsumed iodine is back-titrated with tions, amorphous sulfides almost always produce crysstandard sodium thiosulfate solution. This titration talline iodides. The following sulfides can be converted into iodides involves a slight error because the excess iodine may by spot reactions on sulfide paper, or by direct converoxidize the free sulfur to sulfate: sion of the precipitated and powdered materials: AgzS, HgS, PbS, Bits3, CuS, CdS, TltS, SnSz. Sb&, The extent of this side reaction, (2), which leads to a AS&. The conversion of sulfides, finely dispersed in the consumption of iodine in excess of that demanded by (I), is ordinarily very small if the determination is car- capillaries of paper, is practically finished within one or ried out judiciously, that is, if the excess iodine is back- two minutes. Considerable differences in the rate of titrated immediately. Under such circumstances the conversion are shown by sulfide ores and by compact error can be neglected for all practical purposes. Never- products that have been precipitated and dried. For theless, the proof of the oxidation of sulfur to sulfate by instance, precipitated HgS and PbS react relatively means of the following spot reaction has an additional quickly, that is, the beginning of the iodine formation interest because it demonstrates, in a simple fashion, can be seen after about three hours. In contrast, Ag2S,
+
+
' FEIGL,op. cit.. p. 165.
"tudies
with L. I. Miranda.
and particularly CuS, require several days. The energetic reaction of TlnS is noteworthy; the precipitated and dried sulfide swells and glows when i t is shaken with crystals of iodine. The fact that solid metal sulfides react promptly with iodine dissolved in carbon disulfide or ether proves that the formation of iodide does not take place through reaction of dissolved single molecules of the reactants, because metal sulfides are absolutely insoluble in these organic liquids. The reactions begin with an attack on the molecules of the free surface of the solid sulfide. For this reason only sulfides that are finely dispersed in the capillaries of paper show uniform rapid reaction. With compact specimens, on the other hand, a superficial coating of the sulfide by iodide occurs first. Continuance of the reaction depends upon the production of new reactive surfaces by detachment of the iodide. The reaction of solid sulfides with dissolved iodine or with iodine vapor probably takes place in two stages. The iodine molecules add to the sulfide molecules of the surface, (I), and the addition compound thus formed undergoes an inner molecular change, as shown in (2). The summation of (1) and (2) gives the net equation for the formation of the iodide:
of free iodine. However, in reality i t consists of dark brown thallic iodide (TlIs), or thallous polyiodide (TlI.L), which on warming or on treatment with a suitable solvent gives up iodine and goes over to T1I. The formation of the brown central fleck can be explained as follows: The concentration of iodine is highest a t the point of application. Consequently, the thallium compound richest in iodine forms the* as a circle. The iodine which migrates toward the periphery suffices only to produce the compound containing less iodine, namely, yellow T1I. To demonstrate that the formation of thallium iodide is a strongly exothermic process, a little powdered iodine (tip of knife blade) is placed in a dry test tube. An equal bulk of dry thallous sulfide is added and the mixture shaken. As soon as some of the iodine volatilizes, the formation of iodide begins, the material glows, and the test tube becomes warm. In this case also, TlIa is the initial product. This can be proved by heating, whereupon iodine is evolved and yellow TI1 remains. The thallous sulfide required for this demonstration is prepared by the action of (NH4)S on 0.1 N thauous carbonate solution. The precipitate is dried by washing with alcohol. (b) Formation of Mercuric Iodide (or Lead Iodide). Mercuric sulfide paper is prepared like thallous sulfide paper but using HgC12. A drop of three per cent solution of iodine in carbon disulfide is placed on the This hypothesis of the addition of iodine to sulfide, with reagent paper, and after the solvent has evaporated the activation of the iodine, is supported by the fact the excess iodine is removed by a blast of hot air. A that even insoluble metal sulfides catalytically ac- yellow-red spot is left on the black paper. It can be celerate the reaction between iodine and sodium azide.& proved that this consists of HgIa, and not of the red modification of HgS (cinnabar), by treating the spot with one or two drops of one per cent potassium iodide TOPOCHEMICAL REACTIONS Chemical changes in which the reaction products solution. The HgIe dissolves, forming &HgL, and a remain or are fixed a t their place of origin are known as colorless circle then remains on the black paper. The following experiment demonstrates the slow topochemical reactions. The following experiments conversion of compact HgS on treatment with solid illustrate the topoehemical synthesis of iodides, as well as the effect of the free reactive surface on the reaction iodine: Some powdered iodine is placed in a porcelain velocity of solids. They furthermore permit the ob- crucible and covered with a thin layer of black HgS servation, during the formation of thallous and mer- (obtained by precipitation and drying). The crucible curic iodides, of additional interesting details that will is covered with a watch glass and allowed to stand a t about 30°C. The iodine gradually volatilizes and rebe pointed out in the discussion of the experiments. Procedure: (a) Formation of Thallium Iodide. The acts with the sulfide. After about 10 hours, the conpreparation of thallium sulfide paper has been described version is quite evident; the color has become brown. in Experiment 4. A drop of three per cent solution of Almost all the black amorphous HgS is converted to iodine in carhon disulfide is placed on a strip of the red crystalline Hg12in about two to three days. The conversion of lead sulfide into lead iodide can be reagent paper and the solvent allowed to evaporate. demonstrated by spotting PbS-paper with a drop of the The spotted area leaves a dark brown circle surrounded by a yellow border of TlI. If the paper is allowed to iodine solution. A yellow spot of PbIl remains on the stand for some time, the yellow ring spreads. When black paper after the iodine has volatilized. Lead the spot is exposed to a blast of heated air, the dark iodide is formed also if powdered iodine is placed in a brown inner circle disappears rapidly and a round yel- crucible and covered with a layer of dried, precipitated low spot of T1I is formed. The same conversion can be PbS. (Pulverized galena may also be used.) The accomplished also by placing the spotted paper in five iodine is volatilized by gentle warming. When no per cent potassium iodide solution, or in pure carbon more violet fumes appear, the contents of the crucible disulfide. The appearance of the brown central circle are transferred to a test tube. Water is added and the might lead to the conclusion that it consists of TlzS or suspension heated to boiling and filtered. On cooling, quantities of yellow, crystalline lead iodide separate from the filtrate. THLSJOURNAL. 20,13741 (1943).
9. Reaction of Mercuric Sulfide with Alkali Poly- the HgS into Hg4 (3). This in turn reacts with KI icdide. Mercuric sulfide dissolves in potassium poly- and forms RsHgI. (4). The correct reaction mechaiodide (solution of iodme in potassium iodide) with lih- nism, accordingly, is represented as follows: eration of sulfur. This reaction is the basis of an iodometric method of determining mercuric sulfide. The essential equation is HgS
+ 2KI + I, = KlHgI, + S
It has always been believed that the foregoing reaction does not occur directly but that potassium mercuric iodide and potassium sulfide are formed in the initial stage, (I), followed by the action of iodine with the potassium sulfide, (2). The summation of the two partial equations gives the net equation HgS K,S
K8 ++ 114KI == K?HgI,+ 2KI + S
(1)
The assumption of this reaction sequence is erroneous, however. The experiments descrihed in Experiment 8, showing the direct action of iodine on sulfides, make it probable that the iodine of the polyiodide converts
++ 2KI L = HgI* + S = KnHgIa HgS + 2KI + 4 = KzHgIl + S HgS HgIl
(3) (4) (3)
+ (4)
It is easy to demonstrate this succession of rsactions by means of spot reactions on mercuric sulfide paper. Procedure: Mercuric sulfide paper is prepared as descrihed in Experiment 8. A drop of 0.5 per cent solution of iodine in carhon disulfide is placed next to a drop of 0.2 N iodme solution (in KI) on a strip of HgS paper. After about two minutes, the paper is put into water and the following will he seen: The area spotted with the aqueous iodine solution turns perfectly white in accordance with the net reaction equation. The portion spotted with the carhon disulfide solution of iodine turns yellow, showing the formation of mercuric iodide as indicated by (3). Only after the paper is placed in potassium iodide solution is a white spot produced here also in accordance with (4).
