A
Selective Spot Reaction for Cadmium’
FRITZ FEIGL AND L. I. MIRANDA, Minirterio da Agriculture, Laboratorio de Produfao Mineral, Rio de Jeneiro, Brazil
I
S 1898 Blau (1) showed that a,a’-dipyridyl and a,a’-phenan-
throline form intensely red, water-soluble, acid-resistant ferrous salts.
D
Q-Q
C N ,
a ,a’-dipyridyl
01,
\ N D
a’-phenanthroline
They owe their color to the complex ions Fe( a,a’-dipyridyl)3+ and Fe(a,a’-phenanthr~line)~++.These cations belong to the hexammine type, since six coordination positions of the iron atom are occupied by three molecules of base, each with two groups. I n concoordinating nitrogen atoms or rather -N= trast to most ferrous salts, which readily undergo oxidation, these complex ions are remarkably stable, and are extraordinarily resistant against attack by even alkalies and alkali sulfides. The analytical application of these complexes came many years after their discovery, +
In 1930, Hill (6), and then independently Feigl and his colaborators (9) in 1931, described the use of the dipyridyl compound for the detection and colorimetric determtnation of iron. The phenanthroline complex was used in the same way somewhat later. The numerous publications (6, 8, 9, 10,li?,19, 16, 17, 18) testify to the value of these complexes for these purposes and also as redox indicators. Komarowski and Poluektoff (7) used the a,a’dipyridyl complex t o detect molybdenum (after reduction with stannous chloride). Poluektoff and Nazarenko (11) described the microchemical detection of various anions through characteristic crystalline precipitates containing these complexes. Ferrari (4) utilized the formation of stable Fe(a,a dipyridyl),++ ions to obviate the interference of iron in certain precipitation reactions of magnesium, beryllium, aluminum, and titanium. The a,a’-dipyridyl acts as a masking agent in such cases, Some years ago, the senior author (2)showed that red precipitates result when considerable quantities of alkali iodides or dilute solutions of potassium-nickel cyanide are added to the red solution of ferrous dipyridyl salts. The precipitates consist of Fe(a,(~’dipyridyl)a.I~ and Fe(a,af-dipyridyl)a.Ni(CN)2,respectively. The ferrous phenanthroline salts show a like behavior. These observations suggested that solutions of these complex. salts might function as general precipitants for voluminous anions. Exploratory trials in this laboratory have shown that such precipitates are formed by molybdic, tungstic, H3PO4.12MoOs.aq., H1P04.{2WO,.aq., periodic, picric, picrolonic, and aurintricarboxylic acids, and by the following complex anions: HgL--, BiL-, CdI,--, Ni(CN)r--, CO(CN)~---, Zn(ChT)a--, Cd(CK)a--, and Hg(CNS)r--. All the preci itates produced by the union of Fe(a,a’-dipyridyl)s++or FeL,a’-phenanthroline)3++ with the foregoing compounds or ions are crystalline. Their color which ranges from red, red violet, to blackish red, indicates that the red complex iron ions constitute the chromotropic constituent of the respective, slightly soluble salts.
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The analytical utility of precipitating these complex anions with Fe(a,a’-dipyridyl)3++ and Fe( a,a’-phenanthroline)3++ has been studied in this laboratory. New methods of detection based on these precipitates have been developed, and furthermore new procedures for the quantitative determination of mercury, bismuth, nickel, zinc, cadmium, and phosphate have been worked out. However, satisfactory methods for detecting and determining these materials are already available, and in view of the relatively high price of a,a’-dipyridyl and a,a’-phenanthroline, it seems best a t this time to limit the discussion to a qingle new application of these reagents, since it presents an nctunl advance and advantage.
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+ Fe(a,a’-dipyridyl)3++ + 41- Fe(a.a ’-diovridvl) C‘dL Cd(NH3)4++ + Fe(a,01’-dipyridyl),++ f - 4 1 Fe(a,a’dipyridyl)3.Cd14 + 4NHs Cd++
?.
A “
” ’-
a,&-Phenanthroline may be used in place of the qd-dipyridyl. DETECTION OF CADMIUM IN THE PRESENCE OF OTHER METALS
PRECIPITANT. a,a’-Dipyridyl (0.25 gram) and ferrous sulfate
heptahydrate (0.146 gram) are dissolved in 50 ml. of water. Ten grams of potassium iodide are added, the solution is shaken vigorously for 30 minutes, and the precipitate is filtered off. The filtrate is a saturated solution of Fe(a,a‘-dipyridyl)& containing an excess of potassium iodide. The rea ent is stable. If it becomes cloudy after long standing, it can%e restored to usefulness by filtering. PROCEDURE. Thick filter paper (S. and S. 601 or Whatman 120) is used. A strip is laid across a small beaker or crucible. A dro of the weakly acidified, neutral, or ammoniacal test solution is pfaced on the paper. Before the drop is absorbed, the spot is treated with a drop of the precipitant. Reaction takes place a t once and after the liquid has soaked in, the cadmium-bearing prec:pitate is left as a red fleck or ring. The color is intense enough so that the precipitate is easily seen against the red stain left by the spreading of the reagent. A blank or comparison test is necessary only when very small amounts of cadmium are suspected. The test solution and precipitant may also be brought together in a microcentrifuge tube. It is best to introduce the drop of the precipitant first, then a drop of the test solution, and to centrifuge immediately. For reasons that are not apparent, the sensitivity of the test is appreciably less if the drops are mixed before starting to centrifuge. When cadmium is present, a red precipitate collects in the constricted end of the tube. Identification limit: 0.05 microgram of cadmium; limiting proportion: 1 to 1,000,000. DETECTION OF CADMIUM IN THE PRESENCE OF OTHER METALS
Metal ions that form slightly soluble or complex iodides interfere with the direct use of Fe(a,a’-dipyridyl)& as precipitant for cadmium. The iodide of the reagent is thrown down by silver, thallium, and lead salts, and the precipitates are made red by adsorption of Fe(a,~~’-dipyridyl)~++ ions. Cupric salts, in neutral or acidified solutions, liberate iodine with simultaneous precipitation of cuprous iodide, which forms colored addition camplexes with a,a’-dipyridyl and a,a’-phenanthroline (16). Bismuth, mercuric, stannous, and stannic salts form soluble complex iodides, which likewise are deposited as red complexes
1 Tianrlated from the G e r m a n manuscript ln Ralph E Oesper, Univ ~ i \ i t \( i f Cincinnati. !
The precipitation of red Fe(a,d-dipyridyl),.CdI, is the basis of a spot reaction that can reveal the presence of minute quantities of cadmium. If carried out in ammoniacal solution, this test succeeds even though copper and zinc are present. The simplicity of the procedure and the attainable identification limits and limiting proportions make it superior to all previous spot tests for this element. In addition, it appears probable that the precipitation of Fe( a,a’-dipyridyl)~.CdI~ from ammoniacal solution will provide a simple method of quantitatively separating cadmium from considerable quantities of copper and zinc. This separation is extremely difficult when the methods available up to now are used. The new method of detecting cadmium and separating it from other metals requires the maintenance of conditions a t which there is no coprecipitation of red Fe( a,af-dipyridyl)3.1~or other complex iodides. These conditions are easily secured by working in ammoniacal solution and by employing a saturated solution of Fe(a,a’-dipyridyl)Jg that contains an excess of iodide ions. This reagent then contains all the ionic species requisite to the formation of Fe( 01,a’-dipyridyl)~.CdI,in accord with the equations:
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142
INDUSTRIAL A N D ENGINEERING CHEMISTRY
containing a,a’-dipyridyl. (These latter precipitations are not nearly so complete as the corresponding cadmium reaction.) None the less, the test for cadmium becomes almost specific if the test solution is treated beforehand with ammonia water. The interfering lead, mercuric, bismuth, sbannous, and stannic ions are removed as hydrous oxides and the ammoniacal filtrate can then be tested with the reagent. The Cd(NHa)++ions in the filtrate react promptly, whereas any Cu(KHJ++ and Zn(NHs)4++ ions are inactive. The only other interfering ions in the ammoniacal filtrate are Ag(NH8)*+ and thallium. If, therefore, cadmium is to be detected in the presence of these and other ions that form slightly soluble iodides the following procedure can be used: Dilute hydrochloric acid is added to the test ortion; any precipitate (silver, lead, thallous chloride) is filtereaoff. The filtrate is made ammoniacal, filtered if need be, and the clear solution tested with the reagent. All these operations can be carried out with one or two drops of the test solution in a microcentrifuge tube. The various precipitates are easily segregated by centrifuging. The effectiveness of this test for cadmium was measured with the aid of ammoniacal solutions containing known quantities of Cd++ and Cu++, n n d Cd++ and Xn++. The findings were that 0.08 microgram in one drop it is possible to detect 0.1 microgram copper in the presence of 5000 times this quantity of zinc Far better limiting proportions are obtained if the detection of ahout, 0.2 microgram of cadmium is all that is required. OUANTITATIVE DETERMINATION OF CADMIUM
Although t,he precipitate conforms to the formula Fe( ( Y , c Y ’ dipyridyl)a.CdI4, the writers have not yet been able to use this material as the basis of a direct method for determining cadmium. When washed with water to remove the adhering reagent, the precipitate dissolves to some extent and also decomposes slightly. Consequently, low values are obtained. None the less, cadmium can be quantitatively separated from other metals, particularly copper and zinc, by precipitation from an ammoniacal solution by means of ferrous dipyridyl and iodide. After the Fe(ap’dipyridyl)a.CdI4 has been isolated and decomposed, the cadmium can be determined by any of the accepted procedures. The precipitation by means of mercaptobenzothiazole ( I d ) is recomnirrided particularly. The reagent solution used for the detection of cadmium is employed for its quantit>ativeprecipitation. The preci itate is collected on paper or in a filtering crucible and washer? with diluted precipitant, then dissolved in hot sulfuric acid (1t o 4). An excess of strong bromine water is added to the solution, which is then boiled for 10 minutes. The iron is thus oxidized and the iodide is converted to iodate. The cooled 6olution is made ammoniacal and then warmed. The hydrous iron oxide precipitate is filtered off. The cadmium is then precipitated by adding an ammoniacal solution of mercaptobenzothiazole to the clear filtrate. The precipitate is washed wit’h dilute ammonia and brought t o constant weight at 110” to 120’ C. The factor to cadmium is 0.2330. This procedure gave very satisfactory results with solutions t,hat contained 100 mg. of cadmium along with 200 times this weight of copper, or 100 times this quantity of zinc. Lack of the reagent prevented trials on mixtures containing other proportions or other materials. It is hoped therefore that this promising procedure mag be tried by other workers. LITERATURE CITED
Blau, Momtah., 19, 647 (1898). (2) Feigl, “Specific and Special Reactions”, translated by Oesper, p 1 If3 NPW Yo&. Nordeman Piihlishing C o . , 1940
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Vol. 16, No. 2
(3) Feigl and Hamburg, Z. anal. Chem., 86, 1 (1931); 90, 199 (1932). (4) Ferrari, Ann. chin&.appZicata, 27, 479 (1937). (5) Fortune and Mellon, IND. ENQ.CHEM.,ANAL.E n . , 10, BO (1938). (6) Hill, Proc. Roy. Soc. (London), A, 107, 205 (1930). (7) Komarowski and Poluektoff, Mikrochim. Acta, 1, 264 (1937). (8) Lang, Ibid., 3, 116 (1938). (9) Linder and Kirk, Mikrochemie, 22, 291 (1937). (10) McFarlan, IND.ENO.CHEM., ANAL.ED.,8, 124 (1936). (11) Poluektoff and Nazarenko, J . applied Chem. (U.S.8.h!.), 10, 2105 (1937). (12) Scharrer, 2. Pflanzerndhr, Dtingung, 33, 336 (1934). (13) Schulek and Floderer, Bm. ungar. pharm. Ges., 15, 210 (1939). (14) Spacu and Kuras, Z . a d . Chem., 102,24 (1935) ; 104,88 (19:3A), (15) Szebelledy and bjtai, hfilcrochim. Acta, 2, 299 (1937). (18) Tartarini, Gazz. chim. ital., 63, 597 (1933). (17) Thiel et al.. Ber.. 70. 2491 (1937): 71. 756 (1938). (18) Walden, Hammnett,’and Chapmb, Am: Chem. Soc., 53, 3908 (1931): 55 264 (1933).
i.
A
Simple Microtorch EDWARD N. DACUS
Beacon Research Laboratories, The Texas Company, Beacon,
N. Y.
A
HYPODERMIC needle (which can probably be obtained from a First Aid Room) makes an excellent nozzle for a microflame. Details of a hand torch that has found many U Y ~ S in this laboratory are shown in the accompanying diagram. The parts can be made with simple hand tools and solderctl together to form a single unit. No special skill is required.
Rubber H o s e
/ Cork Stoppers
Copper wire
Jokier
I &‘‘Copper Tubing
Tublnq Copper
Hypodrrrnic
Needle
The beveled point is ground off the hypodermic needle and the shield is made to estend about 4.7 to 6 mm. (0.188 to 0.25 inch) beyond its tip. The only function of the shield is to protect the flame from drafts, Air holes in the base of the shield are neither necessary nor desirable. A nonreducing flame is obtained without them, and they tend t o decrease the stability of the flame, which otherwise is extremely difficult to extinguish by drafts or sudden movements through the air. The convenient hand grip shown in the diagram can be provided, as an additional refinement, by boring several cork stoppers and slipping them over the barrel of the torch with their large and small ends alternately facing. They should be shellacked in place and have their edges sanded smooth. A Becton and Dickinson No. 25 needle, which has an inside bore of the order of 0.25 mm. (0.01 inch), gives a nonreducing “pin flame” that is hot enough for soldering electrical connections and small parts. A still hotter flame, for work wth quartz fibers and the like, can be made by using a slightly larger (No. 18) needle and feeding oxygen into the gas through a T placed well back in the rubber tubing. In such case, the gas should be turned on and lighted before admitting oxygen to the line, and the oxygen should be turned off before turning off the gas. A gas flame about 1 mm. in diameter and from 1 to about 30 mm. in length can be obtained with a No. 25 needle. With a No. 18 needle and a somewhat shorter overhang of the shield, a gas-oxygen flame about 3 mm. in length is obtained. While no further tests have been made, it is obvious that the rize of the flame can. within limits, be varied to suit by pmp7p1 r-lioice of needle - I Z P
