ANALYTICAL EDITION
June 15, 1942
519
Cooke, Proc. Chem. SOC.,19, 243 (1903). Durrant, J . Chem. Soc., 115, 134 (1919). Engel, Compt. rend., 96, 497 (1883). Engel and Bernard, Ibid., 122, 390 (1896). Evans, Analust, 52, 565 (1927). Ibid., 54, 523 (1929). Ibid., 57, 492 (1932). Fainberg and Gintrburg, Zavodskaya Lab., 7, 23 (1938). Fenimore and Wagner, J . Am. Chem. SOC.,53, 2472 (1931). Grippa, Ann. chim. applicata, 20, 249 (1930). Ibbotson and Aitchison, “.4nalysis of Non-Ferrous Alloys”, p. 124, London, Longmans, Green and Co., 1915. King and Rutterford, J . Chem. SOC.,1930, 2138. Knecht and Hibbert, “New Reduction Methods in Volurnetric Analysis”, p. 6 , 2nd ed., London, Longmans, Green and Co.,
(35) (36) (37) (38)
1925. Kolthoff and Amdur, IND.ENG.CHEM.,-4s.~~. ED., 12, 177 (1940). Kolthoff and Furman, “Volumetric iinalysis”, Vol. I, p. 235, New York, John Wiley & Sons, 1929. Ibid.,Vol. 11,pp. 356, 408.
THISpaper represents the second portion of the oompleted dissertation of Henry A. Sloviter in partial satisfaction of the requirements for the Ph.D. degree a t t h e University of Pennsylvania, 1942. The first portion was published in an earlier issue [IND. ENQ.CHEM, ANAL.ED.,13, 890 (1941)].
(26) (27) (28) (29) (30) (31) (32) (33) (34)
Koppeschaar, 2. anal. Chem., 15, 233 (1876). Lombardo, Met. ital., 29, 1 (1937). Maquenne, Bull. soc. chim., (3) 3, 401 (1890). Matthes, Pharm. Ztg., 71, 1508 (1926). Masretti and Agostini, Gam. chim. ital., 53, 257 (1923). Miloslavski, Lyubimova, and Belogorskaya, Zavodskaya Lab., 6, 1184 (1937). Oliverio, Ann. chim. applicata, 21, 211 (1931). Shat’ko, Zavodskaya Lab., 7 , 4 1 2 (1935). Sloviter, McNabb, and Wagner, ISD. ENG.CHEN.,ANAL.ED., 13, 890 (1941). Thiele, A n n . , 265, 55 (1891). Topf, 2. anal. Chem., 26, 184 (1887). Tsyvina and Dobkina, Zavodskaya Lab., 7, 1116 (1938). Washburn, J . Am. Chem. SOC.,3 0 , 4 4 (1903).
A New Procedure for Detecting Acidity’ FRITZ FEIGL AND PAUL0 E. BARBOSA Laboratorio Central da ProduGZo Mineral, Ministerio da Agricultura, Rio de Janeiro, Brazil
A
RECENT paper ( I ) from this laboratory described in detail a sensitive procedure for detecting “basicit.y” in both soluble and slightly soluble materials, such as hydroxides, oxides, phosphates, fluorides, organic bases, and the salts of weak organic acids. The reagents for this delicate test are prepared by treating neutral solutions of copper or nickel sulfat.e, chloride, or nitrate with a deficit of dimethylglyoxime, thionalide (P-aminonaphthalide), or cupron (benzoinosime) and then filtering. A typical instance, nickel solution treated with dimethylglyoxime (= DH,), car1 be represented : Ni++ 2DHz = Si(DH)a 2H’
+
+
The filtrates from these incomplete precipitations are slightly acid equilibrium solutions; each has a characteristic pH value. If a solution of this type is brought into contact with a material that decreases the effective hydrogen-ion concentration of the reagent, a highly visible precipitate of the inner-complex salt is produced a t once: nickel dirnethylglyoxime (red), nickel thionalide (black-brown), or copper benzoinoxime (green). This procedure has proved useful in a variety of practical cases, and has also found microchemical application in connection with spot tests. The successful analytical use of a procedure based on disturbing an established equilibrium in solutions of complex salts naturally led to attempts to extend this principle to other systems of this nature. Such solutions always present an equilibrium between the complex compound (or its ions) and the materials that have united t,o form the complex. The stability of the complex (or its ions) determines the position of the equilibrium. Equilibrium solutions of silver-ammine chromate were studied with a view to applying them in this new type of test,. I t was found that they can be used to detect materials that consume acid, ammonia, or ethylenediamine. Silver chromate dissolves readily in excess ammonia water. The yelloa solution (Cr04-- ion) contains the complex salt [-lg(SH8)2]2Cr04. Therefore, if silver chromate is digested with a quantity of ammonia w t e r insufficient to bring about
’ Translated b y
Ralph E. Oesper, University of Cincinnati.
complete solution, there will be obtained, on filtration, a yellow solution that smells of ammonia. I n this there have been established the equilibria Ag,Cr04
+ 4SH3@ [Ag(NH3)2]2Cr04Ft 2Ag(NH&+ + Cr04--
and any removal of ammonia will lead to the precipitation of the highly colored silver chromate. Consequently, this type of ammoniacal solution of silver chromate can serve to reveal the presence of materials that consume ammonia by uniting with it to form an ammonium salt, react with it to form an ammoniate, or consume ammonia in the precipitation of a hydroxide. The ammoniacal solution of silver chromate has the practical defect that it loses ammonia unless the reagent is stored in a tightly stoppered vessel. This loss results in the formation of a precipitate, even in the absence of nmmonia-consuming materials. If a drop of the solution is placed on filter paper a brown spot of silver chromate appears in a few minutes. On a spot plate the solution decomposes more slowly, and tiny crystals of black-brown silver chromate separate on the surface of the drop. Sonetheless, the ammoniacal solution can be used for the present purpose because the silver chromate produced by the slow breakdown of the solution differs in form and color from that arising immediately as the result of chemical action. A stable silver chromate equilibrium solution can be prepared by dissolving the salt in aqueous ethylenediamine ( = e n ) . Since this base boils a t 118”C., its vapor pressure in a silver-en chromate solution is so slight that the reagent can even be kept a t mater-bath temperature for as long as 5 minutes without danger of decomposition. The silver-en chromate is far more stable than the analogous silver ammonia chromate because coordination of the en-molecule on the silver ion produces a five-membered ring
INDUSTRIAL AND ENGINEERING CHEMISTRY
520
It is well established that such auxiliary valence rings contribute markedly to the stability of the respective complex ions. I n line with this is the fact that silver-en chromate can be isolated by crystallization, quite in contrast with the unstable ammonia compound. Solutions of the en-salt present the equilibrium 2[Ag-en]+
+ CrOa--
G 2en
+ Ag2Cr04
The equilibria in these ammoniacal and ethylenediaminesilver chromate solutions will be shifted above all by materials that furnish hydrogen ions which combine with ammonia or ethylenediamine SHI
+ H+
-P
KH4+ or en
+ H+
+
enH'
and so withdraw these basic materials from the complex. This causes a precipitation of silver chromate. All watersoluble acids, acid salts, and salts whose solutions react, acid because of hydrolysis, affect these reagents for this reason. It is worthy of special note that anhydrides which are not soluble or only slightly soluble in water react positively with silver chromate equilibrium solutions. Examples are tungstic anhydride, molybdic anhydride, and silica. In such cases it is difficult to decide whether this result is due to withdrawal of ammonia by the acid anhydride to form an ammonium salt, for instance MOOS
+ 2NH3 + HzO
+
(?I;H4)2M004
or whether the ammonia is adsorbed on the surface of a n acid adsorbent and thus immobilized. The latter explanation appears the more likely in the case of silica. A positive reaction with these reagents is also shown by solutions of salts that form ammoniates: cobalt, nickel, and cadmium sulfates, for instance. The shifting of equilibria in these solutions has provided a sensitive means of solving certain analytical problems. Special emphasis needs to be placed on the necessity of excluding halides in these tests, because they will disturb the equilibrium by forming silver halide. Experimental REAGENTS. Silver-Ammonia Chromate. Silver chromate, produced by the interaction of silver nitrate and potassium chromate, is well washed, and small portions of the moist salt are vigorously swirled with 6 N ammonia until considerable quantities of the solid remain undissolved. The suspension is allowed to stand one hour, with occasional swirling, and then filtered into a bottle that can be tightly stoppered. If used repeatedly, small amounts of silver chromate are deposited because of the loss of ammonia, but the reagent can be restored to usefulness by merely filtering. Silver-Ethylenediamine Chromate. Freshly precipitated, moist silver chromate is shaken continuously with an aqueous solution of ethylenediamine kept at 60" C. Additional portions of the salt are introduced until considerable quantities remain undissolved. The suspension is filtered, and the filtrate allowed to stand 12 hours at room temperature. Large crystals of [Ag-enIzCrOI are deposited; these are collected on a filter, washed once with ice-cold water, and then dissolved in warm water. The solution is qtable for weeks.
Procedure One drop of the unknown solution or a few milligrams of the solid to be tested are placed on a white spot plate and treated with 1 t o 2 drops of one of the reagents. If an acid or a baseconsuming compound is present in the sample an immediate precipitation occurs; the quantity of red-brown silver chromate indicates roughly the proportion of active material in the sample, If the ammoniacal reagent is employed, fine black crystals will always appear on the surface of the liquid after a few minutes because of the loss of ammonia. This should not lead to unwarranted conclusions.
Vol. 14, No. 6
-4pplications The silver chromate equilibrium solutions react positirely with all soluble and almost all slightly soluble compounds of acid character, with salts whose solutions are acid, and with those metal ions that can form the more stable ammine ions, The present discussion is limited to applications that are new and interesting to the analytical chemist. BEHAVIORTOWARD SILICA. Hydrous silica was prepared from solutions of alkali silicates by repeated evaporations with hydrochloric acid on the water bath. The residue was washed with hot water until all traces of acid were removed. A deep red-brown appeared at once when this hydrated silica was tested on a spot plate with 1 to 2 drops of reagent. Prompt reaction was shown also by samples of this product that had been exposed to the air for several days, or that were partially dehydrated (aged) at 140" C. Another portion of the hydrous silica was ignited to constant weight in platinum; during the ignition samples were removed from time t o time and tested. The reaction was positive in all cases. A sample that had been brought to constant weight was ignited for 4 hours longer. It produced a color that was less intense than that brought forth by the unignited material, or by the portions that had not been blasted so long. Finely ground sand and quartz-that is, naturally occurring crystalline silica-showed no reaction, even after long contact with the stable silver-en chromate equilibrium solution. Some natural silicates (certain zeolites, kaolin, opal, garnierite) gave definite positive results, while others (amazonite, talc, muscovite) showed no reaction whatsoever. The dissimilar behavior of different forms of silica and natural silicates is noteworthy and requires comment. Precipitated and ignited silica is resistant to alkalies at ordinary temperatures; partial solution occurs only on warming bith concentrated lyes. Therefore the immediate precipitation of silver chromate from the equilibrium solutions by silica is certainly not due to the formation of soluble ammonium, or ethylenediamine silicate. Since silica cannot be credited with being a source of hydrogen ions, the assumption is compelled that some forms of silica have the ability to adsorb ammonia or ethylenediamine. This surface adsorption of the base has the same effect as reaction with hydrogen ionsnamely, inactivation-the equilibrium is disturbed, and silver chromate comes out of solution. It appears that only amorphous silica has the ability to affect silver chromate equilibrium solutions; the crystalline varieties are without this power. Not enough cases have been studied to warrant the general statement that all forms of amorphous silica are reactive in this sense. This property may possibly be possessed only by those varieties that are characterized by exceptionally great surface development The diminution of activity with prolonged ignition indicates that the free surface plays a decisive role. The varying behavior of silica toward these reagents may be useful in petrography with reference to characterizing certain silicate rocks. I t seems likely that the gangue of ores consists, in part at least, of amorphous silica. The acid-free washed residues of magnesites, limestones, and pyrolusites from various sources gave distinct positive reactions. The silica obtained by dissolving a number of commercial irons in acid also responded to this test. This method of detecting silicic acid can be employed in organic qualitative analysis to test "acid-insoluble residues" for silica. BEHAVIORTOWr\RD METAL-ACIDANHYDRIDES.Tungstic and molybdic anhydrides are not soluble in water, but they react with silver chromate equilibrium solutions. A few grains of the powdered oxide immediately turn red-broan if spot-tested with 1 to 2 drops of the reagent.
June 15, 1942
521
ANALYTICAL EDITION
If an alkali tungstate is evaporated with hydrochloric acid, and the resulting yellow tungstic acid is thoroughly washed to remove any adherent hydrochloric acid, the product gives a strikingly strong positive reaction. Minute quantities of tungstic acid are sharply revealed if moistened on a filter paper with silver-en chromate reagent. If the tungstic acid is ignited for several hours in platinum, the reactivity toward the equilibrium solution gradually diminishes but never disappears entirely. The anhydride of vanadic acid gives an immediate reaction, but this is not seen so distinctly as in the case of molybdic and tungstic anhydride, because the brown vanadium pentoxide masks the result to some extent. The behavior of niobic and tantalic anhydride is remarkable. Two milligrams of the commercial oxides were tested on a spot plate with 2 drops of silver-en chromate solution. A distinct reaction was observed a t once with niobic pentoxide. I n contrast, tantalic anhydride gave no color; even after standing in contact with the reagent for a n hour, nor when the test mixture was warmed briefly by a current of hot air. Since the positive reaction with the niobic anhydride might have been due to silica in the specimen, 2 grams of the oxide were heated in a platinum crucible with hydrofluoric and sulfuric acid and fumed to dryness, and the residue was ignited for 2 minutes. This silica-free niobic anhydride reacted about as definitely as freshly precipitated silicic acid. Tantalic anhydride subjected to this same treatment gave a weak but detectable reaction. These purified oxides were then ignited for an hour, pulverized, and retested. I n both cases the result was now negative. These experiments were repeated several times; the findings were always the same. Consequently niobic anhydride can be definitely distinguished from tantalic anhydride by the silver-en chromate test, provided the samples have not been ignited too long. A spectrographic examination of the niobic oxide before and after ignition might reveal traces of some other more volatile, more reactive oxide that is removed by the ignition. The activity of the water-insoluble metal acid anhydrides toward the equilibrium solutions is, in all likelihood, to be ascribed not to the formation of an ammonium salt, but rather to adsorption of ammonia or ethylenediamine. This hypothesis is particularly applicable in the cases of tungstic and niobic anhydrides, whose activity falls off distinctly or even vanishes on prolonged, intense heating. BEHAVIORTOWARD HYDROLYZABLE SALTS. Salts derived from weak bases and strong acids hydrolyze in aqueous solution. These solutions exhibit an acid reaction toward the usual indicators and cause an immediate precipitation of silver chromate from the reagents. Since the hydrolysis of the salt comes to equilibrium these hydrolyzed solutions are likewise equilibrium solutions, and consequently their reartion with the silver-en chromate reagent presents the interesting case of the mutual interaction of two equilibrium solutions. Aqueous solutions of ferric, chromic, titanic, bismuth, and zirconium sulfate react a t once with silver-en chromate solution, Likewise, basic salts, such as the evaporation and ignition residue of alum, give an immediate color when spottested with this reagent. The high sensitivity of this test for aluminum is noteworthy. On a white spot plate as little as 1 microgram of aluminum can be plainly detected in 1 drop (0.15 ml.) of aluminum sulfate solution. The test is still more sensitive if cne drop of the aluininum solution is evaporated to dryness on a porcelain crucible lid and the residue is then spot-tested. I n this way it is possible to detect as little as 0.4 microgram of aluminum. A striking demonstration is provided by the following comparative procedure. Two 3-ml. portions of a 1 to 250,000
aluminum sulfate solution are put into test tubes. One specimen is treated with sprnral drops of ammonia water, the other with about 0.1 ml. of silver-en chromate solution. No precipitate mill appear in the first portion even after warming, while a distinct turbidity of silver chromate immediately will be seen in the portion that has been treated with the equilibrium solution. BEHAVIORTOWARD AMMINESALTS.Those metal ions whose complexes with ammonia and ethylenediamine, respectively, are more stable than the corresponding silver complexes will react positively with the silver chromate equilibrium solutions. Solutions of the sulfates and nitrates of copper, cadmium, zinc, cobalt, and nickel produce immediate precipitation of silver chromate. The detection limits (in micrograms) when 1 drop (0.05 ml.) ot a sulfate solution was brought into reaction with 1 drop of silver-en reagent were found to be: copper, 2.5; cadmium, 7; cobalt, 2; nickel, 2.5; zinc, 2.5. The difference in these figures shows that the reactivity of the ammine saltformers with silver chromate equilibrium solution is dependent on the degree of tenacity with which the ethylenediamine is held in the respective complex ions. The findings discussed above are summarized in Table I.
T.4BLE I. REACTION TOWARD SILVER CHROMATE EQUILIBRIUM SOLUTIONS Material Tested Silica Freshly precipitated Partially dehydrated a t room temperature or a t 140' C . Ignited t o constant weight Quartz sand Zeolite;, kaAin, opal, garnierite Amasonite, talc, muscovite MOO3
WO8
After ignition
VZOS NbnOa After treatment with H2Fz and HzSOn Treated a s before and then ignited Tan05 rlfter treatment with H2Fn and HzSOa Treated a s before and then ignited Ferric, chromic, titanic, zirconium, a n d bismuth sulfate Basic aluminum sulfate Copper, zinc, cadmium, cobalt, a n d nickel sulfate
Reaction Strongly positive Prompt Slow decrease in intensity b u t always positive Xegative Positive Xegative Strongly positive Strongly ositive Decrease$ intensity, b u t always positve Immediate, somewhat masked b y color,of V Z O ~ Immediate Immediate Negative Negative Weakly positive Negative Strongly positive Strongly positive Strongly positive
A useful preliminary test to establish the possible presence of metal ions that are capable of forming ammine complexes
is easily carried out. The solution of the sample must be freed of halide by fuming with sulfuric or nitric acid. After proper dilution, the solution is treated with excess ammonia water, warmed, and filtered. A few drops of the clear a t r a t e are evaporated to dryness in a small porcelain crucible, and the residue is heated to drive off the ammonium salts. After cooling, the contents of the crucible are spot-tested with 1 t o 2 drops of the silver-en reagent. If silver chromate appears, the presence of a metal ion capable of forming an ammine complex is established. This procedure can be used in the ordinary qualitative scheme to furnish a reliable indication of the presence of zinc, which will be found in the ammoniacal filtrate obtained from the precipitation of the hydrous aluminum oxide.
Literature Cited (1) Feigl, F., and Silva, C. da, IND. ENG.CHEM.,ANAL.ED.,14, 316
(1942).
