(34C) Polakovic, J. and Polakovicova, J., Sb. Prac. Chem. Fak. SVST 1966, 87. (333) Qureshi, M. and Qureshi, S. Z., Anal. Chem. 38, 1936 (1966). (36C) Rosenburg, J. P., Rtw. B r a d . Quim. 60, 291 (1965). (37C) Shimizu, T., Anal. Chim. Acta 37, 75 (1967). (38C) Simek, M., Chem. Listy 60, 817 (1966). (39C) Strelow, F. W. E., Anal. Chem. 38.
Organic Analysis
(1D) Akimov, V. K., Smirnov, 0. K. and Emel'yanova, I. A., Zh. Analit Khim. 21,610 (1966). (2D) Kreshkov, A. P. and Tumovskii, L. A., J . Anal. Chem. USSR 21, 541 ( 1966). (3L)) Lewandowski, A. and Wojcicka, E., Poznan. Towarz. Przyjaciol Nauk,
Wydzial Mat. Przyrod. Prace Komisji
New Ion Exchange Materials
Mat. Przyrod. 12, 67 (1967). (4D) Webster, P. V., Wilson, J. N., and Franks. M. C.. Anal. Chim. Acta 38. 193 (1967). ' (5D). West, P. W., Qureshi, M., and Qureshi, S. Z., Zbid. 37,97 (1966).
( I F ) Baetsle, L. H., HLIYS,D., and van Deyck, D., J . Znorg. Nucl. Chem. 28, 2835 (1966). (2F) Gustrow, J . Prakt. Chem. 31, 320 (1966). \ - - - - ,
(3F) Hering, R. and Haupt, D., Z . Chem. 6, 192 (1966). (4F) Hering, Ii., Trenne, K., and Neske, P., J . Prakt. Chem. 32, 291 (1966). (5F) Hering, R., Ibid., 34, 69 (1966). (6F) Koster. G. and Schmuckler., G.. - , ' Anal. Chim. Acta 38, 179 (1967). (7F) Kun, K. A., J . Polymer Sci. AS, 1833 (1965). (8F) Kun, K. A., Ibid., A4. 847; 859 (1966). (9F) Leikin, Y. A., Davankov, A. B., and Korshak. V. Y.. Vusokomolekul. Soedin., A9, 619 (1967): " (10F) Manecke, G. and Bourwieg, G., Makromol. Chem. 99, 175 (1966). (11F) hlarhol, J. and Chmelicek, J., Collection Czech. Chem. Commim. 31, 3881 (1966). (12F) Petrow, H. G. and Levine, H., Anal. Chem. 39, 360 (1967). (13F) S'asil'eva, Ye. 11. and Gavurina, It. K., Polymer Sci. ( U S S R ) (English Transl.) 8, 781 (1966). (14F) Veruovic, B., Chem. Prumysl 17, 21 (1967).
Pharmaceutical and Biological Analysis
Anal. Chem. 37,692 (1963). ' (4E) Frizel, D. E., Malleson, A. G., and Marks, V., Clin. Chim. Acta 16, 45 (1967). (5E) Klotz, L. and Rehfeld, S., Pharm. Zentralhalle 106, 3 (1967). (6E) Manikowski, W. and Niezgodzki, L., Farm. Polska 21,657 (1965). (7E) Montgomery, K. 0. and Weinswig, bl.H., J . Pharm. Sci.55, 1141 (1966). (8E) Reid, R. H. P., Craft, M. C Roberts, E. G. G., and Wise, L., Technl: con Symp., 2nd iV. Y . London 1965, 671 (1966).
Inorganic Microchemical and Trace Analysis Philip W . West, Coafes Chemical laboratories, Louisiana State University, Baton Rouge, l a . Foymae K . West, Gulf South Research Institute, Baton Rouge, l a .
N
and trends have taken place during the past two years which hold promise for significant advances in the field of inorganic microchemical and trace analysis. Membrane electrodes have been developed which show conclusively that electroanalytical methods are becoming available that are selective and even specific and have sufficient sensitivity and simplicity to make them competitive with other established methods. A%tomicabsorption spectroscopy which is inherently specific is now being improved by chemical adjuncts which improve the accuracy and provide means for very significant increases in sensitivity. The dramatic increase in interest in studying minute amounts of material and analyzing trace systems is leading to increasing interest in catalyzed and induced reactions. The electron microprobe is proving to be a n invaluable tool in studying individual species of atoms concentrated on minute areas. The elegantly' simple ring oven method is proving useful for a wide number of diverse st'udies and is now established as a .valuable quantitative technique for microanalysis. Gas chromatography is E W DEVELOPMENTS
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now established as a tool in studying metals. This review, covering January 1, 1966 to December 31, 1967, is contiguous to the 1966 review of Inorganic -Microchemistry (226) although the scope and emphasis are slightly changed. Because of the voluminous literature which has appeared dealing with various aspects of microchemical and trace studies, it is no longer possible to attempt the recording of specific and detailed facts as was done in the past. I t is unfortunate that masking agents, isolated separations procedures, new organic reagents, and new gadgets for microcheniical manipulations can no longer be recorded for the convenience of the reader. Likewise, topics reviewed elsewhere will be nientioiied only when necessary to show applicatioiis in the development of significant new trends. ,4number of valuable and interesting reviews have appeared in the literature which serve to supplement and augment the present review. T. S. West (232) has reviewed inorganic trace analysis, hlinczewski (149) has reviewed methods of separation together with methods for determining trace impurities, and
Helbig has reviewed the status of ultramicro analytical techniques with special emphasis on electrochen~ical methods (94). In a review of developments in aiialytical chemistry, 13elcher (10) has discussed other techniques of interest in microchemical and trace studies. Various methods for concentrating elements for microchemical and trace studies h a s been reviewed (135)and individual techniques such as solvent extraction (84, the ring oven method (179), thin-layer chromatography (191), and the use of ion exchange beads as separation and reaction media (75) have all been discussed. Rechnitz has discussed the status of cation-sensitive glass electrodes (176, 177) as well as membrane electrodes applicable to anion studies (175). l'ungor, Havas, and Toth also reviewed the preparation and applications of membrane electrodes (167). Hanna and Siggia have reviewed kinetic methods of analysis (91), Walsh (217) has discussed recent major advances in atomic absorption spectroscopy, the use of laser-excited spectra for qualitative studies in geology and metallurgy has been summarized (150), and the principles and techniques of scanning
clrctroii probe microanalysis have been discussrd (235). Geiicral sources of information of iiiterest to the inorganic microanalyst include the translation of Yatsimirskii’s and Vasil’ev’s compilation of instability constants of complex compounds (237), the treatise on solvent extraction chemistry of metals (144), and the bibliography on flame spectroscopy (141). The proceediiigs of various congresses and symposia are important sources of iiiformation. For example, a n extensive survey of various aspects of the electron niicroprobe has appeared in the proceedings of the Symposium of the Electrothermics and Metallurgy Division of the Electrochemical Society (145). Likewise, collected papers from a colloquium on metallurgical analysis emphasizing the electron beam microanalysis have been published (48). Proceedings of a symposium sponsored by the Electrocheinical Society on the electron niicroprobe have been published (208). Various papers (101) and the plenary lectures (100) of the 1965 Iiiternatioiial Symposium on Microchemical Techniques have been published. Other proceedings of interest include those of the Graz Symposium on hnalytical Chemistry (166) and the combined meeting of the German Chemical Society, the Austrian Society for Microchemistry and h a lytical Chemistry, and the Swiss Society for Analytical and General Chemistry (147) have also appeared. Papers presented a t the X X t h International Congress of IUP=1C held in Moscow in 1965 (214) include a number of contributions of interest to the microanalyst. OPTICAL METHODS
I n general, optical methods provide great variety in applicatioas and scope. The selectivity and even specificity which characterizes a number of the methods make them especially attractive. They are often very sensitive or can be made so by combination with suitable chemical enhancement techniques. One of the methods of greatest current interest is atomic absorption spectroscopy. I n general, it provides excellent senstitivity and is free from interferences. About the only distortion of results arises from those physical or chemical phenomena that reduce the atomic population in the flame. Bec a u v measurements are made on characteriitic spectra of the respective metah, miqidentification or high results are no problem. Low results that have sometimes been encountered in the past are non- becoming less of a problem because it is usually possible to employ chemical techniques such as the use of releasing agents, complexing ligands, aiid organic solvents so as to establish
working environments that provide efficient atomization. The extraction of metal complexes into organic solvents is becoming a widely used technique which often provides the ultimate in reliability and sensitivity. Calcium and magnesium have been determined (171) with impressive reliability. I t is of interest that 27, isopropanol medium was effective in overcoming several interferences in the determination of magnesium. The determination of magnesium has also been studied by Suzuki who recommends the prior extraction of the 8-quinolinol comples into isobutylmethyl ketone (200). The method has been applied to the analysis of natural water, brines, sodium bicarbonate, and to aluminum and its alloys. Chakrabarti (40) found that extraction of tellurium diethyldithiocarbamate extracted into methyl isobutyl ketone gives a two-fold enhancement in sensitivity compared with aqueous solutions. The determination of silver in the parts per billion range was found to be possible when the metal-dithizonate is extracted into ethyl propionate and final nieasurement made by atomic absorption spectroscopy (223). The complexing ligand and the solvent used for the extraction both prove to be critically important in establishing the ultimate sensitivity. Various investigators (5, 186, 202) have considered solvent effects in atomic absorption spectrometry. Feldman and his coworkers evaluated four solvents for the determination of trace amounts of manganese (68). They found as little as 2 ppb of manganese may be determined in a n acetone medium. Sachdev, Robinson, and West determined vanadium with excellent results (187) by employing a nitrous oxide-acetylene flame. The sensitivity was improved by use of methyl isobutyl ketone together with the addition of aluminum ions and diethylene glycol diethyl ether. Methyl isobutyl ketone was also used by Feldman, Knoblock, and Purdy (69) in the determination of chromium in biological materials by atomic absorption spectroscopy. The combination of chelation together with extraction and subsequent atomic absorption spectroscopy has proved very effective for the study of such difficult media as sea water or saline waters where excellent results have been obtained in the determination of copper (33, 132). Further evidence of the advantages of employing organic solvents is demoiistrated by the work of Butler and Matthews (37) who determined molybdenum down to the parts per billion range by extracting metal chelates into n-amyl methyl ketone. They employed this technique to insure greater reliability as well as improved sensitivity. Likewise, copper, cadmium, and zinc have been determined in parts per billion levels by atomic absorption spectroscopy (17 2 ) .
Improved reliability was obtained by employing E D T A as a releasing agent to minimize any interfering effects. The use of various sources for atomic absorption spectroscopy has been coiisidered by Ginzburg aiid Satarina (82), Fassel and his associates (63) have evaluated spectrocoiitinua as primary sources for atomic absorption spectroscopy, and a comparison has been made by de Galan, McGee, aiid Wiiiefordner of line and continuous sources in atomic absorption studies. They concluded (79) that with a n instrument having a good monochromator, a continuous source provides detection limits comparable with those obtained with a hollow cathode discharge tube and offers several distinct advantages. Their conclusions are in general agreement with those reached by Fassel and his group. In connection with the use of coiitinua, the study of the xenon-mercury arc as a primary source for atomic absorption spectrometry is of interest ( 7 2 ) . A number of quite widely different studies of atomic absorption y)ectroscopy have been reported that are of interest. The possibility that this method can be applied to continuous monitoring has been shown by Thilliez (210) who describes the continuous determination of traces of lead in the eiivironment. AL;little as 10+ gram per cubic meter of air can be determined. The method is also applicable to the determination of mercury iii the atmosphere. Resonance nioiiochromators are being developed for use in specific applications such as the determination of lithium in blood serum (29). =in integrating analogue computer for atomic absorption spectrometry has been described which permits rapid and accurate measurements where instrumeiit noise might otherwise lead to unsatisfactory results (28). Spectrographers will benefit from the work of Margoshes who has presented a method for the prediction of the relative seiisitivities of two or more absorption lines of a giveii element. Line selection is made on the basis of published data obtained with both hollow cathode lamps and continuum sources (137). The influence of flame temperature on atomic emission and atomic absorption flame spectrometry has been considered (18) and Wendt and Fassel have studied atomic absorption spectroscopy with plasmas. They conclude (222) that induction-coupled plasma seems superior to combustion flames. Other optical methods of general interest include the use of low n a t t age microwave induced argon plasmas for the excitation of metals which makes possible a detection limit of 10-12 gram of individual metal (184). The use of reflectance spectroscopy (239) for the identification of cations separated by thin-layer chromatography is of interest VOL. 40, NO. 5 , APRIL 1968
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as is the use of X-ray spectrography in the evaluatioii of ion exchange resinloaded discs which has been applied to micro aiid trace analysis (39). Frei and Ryan (73) also used reflectance spectroscopy for trace metal analysis. They employed chromatoplates to separate copper, cobalt, and nickel; and after treatment with rubeaiiic acid, determilied the amounts present by diffuse reflectance spectroscopy. As little as 0.05 p g of the elements can be determined iii this manner with a n error of only 2 4 % . Fluorometric methods have special appeal for the study of traces. The methods are characterized by great sensitivity and are usually quite reliable. Atomic fluorescence is particularly exciting because of its amazing sensitivity. Dagnall, \Vest, and Young, for example, compared determinations of cadmium by atomic absorption and atomic fluorescence (50) mid found t h a t the fluorescent procedure is much inore sensitive than the absorption iiietliod aiid is equally free from inter-element interferences. Dagnall, Thompson, and R e s t have investigated some of the experinieiital parameters in atomic fluorescence spectrophotometry (56). The same investigators have studied atomic fluorescence methods for the determiiiatioii of seleniuni and tellurium (54) employing microwave-excited electrodeless discharge tubes as spectral sources. They made parallel atomic absorption studies and reported advantages for the atomic fluorescence procedures. Xrnieiit,rout (4) has studied the determination of nickel by atomic fluorescence flame spectrophotometry. X-ray fluorescence spectrography is also of interest. Microgram quantities of tin have been det,ermiiied by a coinbiiiatioii of ion exchange aiid X-ray fluorescence (41), and tin, antimony, selenium, arsenic, gerniaiiiuni, aiid copper (254) have been determined in the microgram range. The nietal sulfides were precipitated using copper as a carrier and the precipitate collected on a membrane filter for subsequent exainiiiation by X-ray fluorescence. ilIolecular fluorescence, although i t does not exhibit, the specificity of atomic fluorescence, still is a valuable tool for the study of trace materials. Dagnall, Smith, and West’, for example, have demonstrated excellent sensitivity for the method in the determiliation of maaiiesiuiii (49)and aluminum (53). Interferences can generally be minimized by judicious use of masking agents. For example, a highly selective iiiethod for determining copper in the sub parts per billion range has been proposed (7)baaed on a preliminary extraction of bis(2,9,-diniethyl-l,10phenanthroliiie) copper(1) nitrate from a n E D T A medium. -1ternary complex of copper is filially extracted into chloro140 R
ANALYTICAL CHEMISTRY
form for the actual measurement of its fluorescence. Although restricted by the cost of the equipment and the skill required of the analyst, both mass spectrometry and the electron microprobe are becoming invaluable tools for the study of microchemical problems. The methods have fantastic sensitivities and reliabilities. McHugh and Sheffield have used mass spectrometry to determine subnanogram amounts of iodine (141) and beryllium (149). I n the case of beryllium, for example, less than lo-’* gram can be detected based on the generation of positive 9Be ions. Spark-source mass spectrometry has been employed for determining trace elements in titanium(IV) oxide pigments (103). By using Kb in low concentrations as internal standards, quantitative results are possible with a coefficient of variation in the order of 15%. The spark-source approach has also been applied (211) to the st,udy of boronated graphite crystals. It appears possible to use the method to detect variations in boron concentration between various regions on the surface of a single crystal. The electron beam microprobe technique, like the mass spectrometry procedures discussed above, provides exceptional sensitivity and reliability. I n reviewing electron beam microanalysis, Xoack (156) points out that as little as 10-14 gram of copper can be detected and surface distributions can be studied. Techniques for the application of the microprobe are being rapidly developed, and great versatility is already evident. Bock and Zimmer (82) have isolated material (e.g., copper) from solution by electrolytic deposition on the end of a platinum mire sealed in a capillary tube. The microprobe determination of the deposited material can then be carried out with an error of approximately 10%. The preparation of samples for laser microprobe studies has been reported by Rosaii (181) who obtained a homogeneous dispersion of sample by applying droplets of sample solution to a gel of unfised spectrographic emulsion. Microprobe analyses have been programmed (‘70) with the aid of a mult,ichannel analyzer and a comprehensive computer prograni for electron probe microanalysis has been described (54). Of fundamental importance t,o those interested in the electron microprobe are the general assessment given by Kelly (112) of mass absorption coefficients, and the discussion by Ziebold of precision and sensitivit,y in electron probe microanalysis (240). ELECTROANALYTICAL METHODS
Essentially for the first time, electroanalytical methods are now becoming available that hold promise for selective
or even specific measurements. Membrane electrodes may show a high degree of specificity and reliability. They are generally very sensitive and are readily adaptable to moiiitoring systems which make them specially attractive. Although the response to concentration changes is logarithmic, this is not necessarily a serious handicap in the coiicentration ranges for applications will generally be made. The familiar glass electrode for p H measurements has undergone development for inally years and it can be anticipated that membrane electrodes for other measurements will likewise require a considerable period of time for ultimate perfection. Severtheless, there are a number of quite satisfactory membrane electrodes now available; and these give promise for important future developments in the field. Electrodes for fluoride, calcium, silver, sulfide to mention only a few, are now available that seem very satisfactory. K i t h the interest now apparent in these electrodes, it is certain that future developments will be rapid. The reviews dealing with meiiibrane electrodes referred to earlier should be consulted. Some indication of the applications may be obtained from the current literature. A modified fluoride electrode (5g) has been applied to the study of 50 pl samples. Potentiometric measurements of chloride and of bromide have been made with membrane electrodes (1’74), and an iodide-sensitive electrode has been evaluated (1?3). .kstudy has been made of paraffin nienibranes, polyvinyl chloride membranes, and liquid ion exchange electrodes for the measurement of Ca2+activity (192). A membrane electrode for potassium (81) has been used as the indicator electrode for the potentiometric titration using tetraphenylborate solution as the titrant. Ijsseling and van Daleii (99) have considered theoretical aspects of potentiometric titrations with ion eschange electrodes. They considered the various parameters such as diffuaion coefficients of the ions, the conceiitratioiis of the solutions, and the caiiacity of the niembranes employed. The discussion of menibraiie electrodes emphasized the desirability of specificit,y. I t is not to be implied, however, that iionselective methods are necessarily of less interest. Anodic and cathodic stripping techniques, for esample, are of great value; and the very fact that they are general in application to the study of many metals and noiimetals is often a n asset. Stroiiiberg (19;) has discussed the possibility of determining substances in coiicentrations as low as 1 O - l 1 S by depositionstripping polarography. Anodic stripping polarography has been used for the determination of as little as 1 iig of manganese by XIonnier, Martin, and Haerdi ( l j l ) , and the same authors
have studied the factors influencing the determination of traces of cadmium (139) by anodic stripping a t a hanging mercury drop electrode. Anodic stripping voltammetry has been used to determine zinc in sea water (130), thallium in high purity indium ( I & ) , and germanium (196). Stripping voltammetry has been proposed for the determination of manganese (98),and t'he determination of subniicro amounts of a iiumber of metals through the use of anodic strippiiig voltammetry has been discussed (138). Duyckaerts aiid Coseinans have discussed various methods of analysis by aiiodic stripping and have considered theoretical aspects and experimental parameters of importance (60). Perone and Stapelfeldt,have proposed the use of solid micro electrodes for derivative striiiping voltainnictry (162). Cathodic st~ippiiigmethods may also be useful. Sulfate in concentration ranges of 0.005 to 1 ~g per milliliter have been determined (180) by reduction to H2S using titanium(II1) in H3PO4 medium. The sulfide is then deposited 011 a mercury drop produced by electrodeposition on a platinum electrode aiid the film thus produced is dissolved cathodically and the peak height obtained nieasured to determine the original sulfate concentration. Internal electrolysis is often valuable for t,he separatioii of ions. The conditions for iiiternal electrolysis such as adjustmeiit of pH aiid addition of complexing agent$ often make it possible t o separate metals from each other and from diverse other ions (153). Hlanchard (20) has denionstrat,ed the efficieilcy of internal electrolysis by em1)loying the spontaiicous deposition of Po and Ui on nickel from HC1 solutioiis. Coulometric determinations are valuable in niicrochemical and trace studies and are particularly attractive when adapted to continuous monitoring instrumelits. method for the ultramicro coulometric titration of chromium (45)is of interest as are the coulometric methods for determining selenium (2) and sulfide (38). In other electroanalytical studies, it is interesting to note a modification of the so called Kest-Gaeke method for the determination of sulfur dioxide. Ciaccio and Cotsis have used a polarogralihic finish to determine SO2 which has been iqolated and stabilized as the [HgC12S02]2-complex. The complex is broken down by means of hydrazine in a basic medium ( 4 7 ) ; after removal of nitrcury, the released SO2 is determined I)olarogral)hically. Alkalimetric titrat,ioiis for ultramicro aiialyses using a gold electrode have been discussed by l'etrikova and illiiiiarin ( I C s ) , and inverw pulse currents have been used for thc microdetermination of cadmium, lead, and inanganese (194). The use of various electrochemical methods for tjhe
determination of copper (95) provides a n interesting perspective of different possibilities. APPARATUS
Optical equipment and associated attachments are of general interest. F o r atomic absorption spectroscopy the enhancement of intensity of hollow cathodes is important in estending the sensitivity of the method. Sullivan and Walsh (198) have used a discharge to produce atomic vapors by cathodic sputtering. The resulting vapor was excited b y a second discharge electrically isolated from the first, and the resultant resonance lines prove to be two orders of magnitude more intense than those obtained from conventional hollow cathode discharge lamps. Kirsten and Bertilsson have described (116) a n ultrasonic nebulizer for flame photometry aiid flame absorption spectrophotometry, and Zacha arid Winefordner (238) have described a simple instrumental system for measurement of flame emission. T h e instrument used a n argon-hydrogenentrained air flame with a total consumption burner. The complete unit' proves suitable for t h e detection of 14 metals in parts per billion concentrations. l : demountable hot hollow cathode lamp suitable for obtaining the fluorescence response of 14 elements has been described by Diiiniii (58): a i d Dagiiall, Thompson, aiid West have given details for the production and operation of microwave-escited and electrodelcss discharge tubes for use (65) in atomic fluorescence and atomic absorption spectroscopy. A simple atomic absorption photometer has been described (228) which proves to be quite sat,isfactory for the analysis of waters for t h e corninon metal inipurit~ies. Atomic resonance units are finding applications now in the design of simple si~ectro1,hotometers. Sullivan and Walsh (199) point out that the isolation of resonance lines in atomic absorption spectroscopy can be accomplished using a n appropriate metal resonance monochromator. Such a n approach has been used by Ling (126) who has described a simple mercury photometer. This general approach has also been employed by Kuznetsov aiid Chabovskii who have described aii apparatus (128) for the automatic rapid determination of mercury in powdered Samples. A helium-glow photometer suitable for the determination of picomole amounts of alkali metals has been described (216), and a combination electron microscope/electron microprobe instrument has been designed (139). As has been anticipated, lasers are finding applications iii analyt,ical studies. A simple micro apparatus based on a laser beam has been described (108)
for use in spectrographic analysis, aiid a He-Ke gas laser has been propoqed as the excitation source in a commercial R a m a n spectrophotometer (43). I t is claimed t h a t the unit is less likely to cauSe photodecomposition of samples and t h a t it may be applied to colored samples. Review of electronic equipment for analytical applications is not possible, but attention is called to the precise coulometer proposed by Quaylc aiid Cooper (170). The instrument measure, coulombs with a probable error of + 25 ppm. -411automatic microcoulometer employing a n integrating motor whose rotations are counted optically is also of interest (193). 13ecauie of the interest in the hanging mercury drop electrode and its application to trace studies, the electrolytic cell described by Kemula (113) is of interest. SEPARATIONS
Separations of various types are essential for most methods that are used in microchemical or tracc studies. Particularly with conceiitratioris in the parts per billion range preconce~itration or separation or both may be necessary. Farquhar, Hill, and English have determined 39 metallic impurities in potassium chloride by a precoiicentration step employing carrier precipitatioii with 8-quinolinol thioiialide and tannic acid followed by spectrographic esamination (62). Entrainment' has long been a standard method of separation, and the hydrous metal oxides or metal sulfides are probably most often used as collectors. -1 study has been made recently of the manganese dioside collection of lead (178). LIarczenko (134) has proposed a riuniber of new separation methods among which was the use of t,he nickel diniethyl glyoximate precipitation to carry down traces of palladium in separations from platiiiuin, gold, silver, alumiiium, and iron. Other separations include calcium and magnesium collected with lanthaiiuni phosphate silica coprecipitated with niobic acid and chloride precipitated as AgC1 with barium sulfate as carrier. Solvent extraction is one of the fundamental techniques for concentrating trace metals. Equally important is the fact t h a t estractiori methods are often uniquely effective in accomplishing difficult separations. Piischel has summarized some estraction methods for use in concentrating tracc metals (169) prior to X-ray fluorescence measurements. The use of alkyl phosphoric acid for the coiicentrat,ion and separation of indium, gallium, thallium, antimony, and bismuth has been described (124); and Luke (I2?g) has described the applications of hesone extraction of metal ions in t h e study of metals aud alloys. The extraction VOL. 40, NO. 5, APRIL 1968
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procedures are especially attractive because each of the trace metals of concern can be isolated using a standard procedure applicable to any and all of the matrix metals and alloys being considered. Such general separations of groups of metals are particularly important when specificity can be obtained by final measurement such as atomic absorption spectroscopy. There are always needs, however, for more selective separations and for methods that can accomplish difficult separations. Germanium has been separated from arsenic (83) by extraction with dioctyl methylene diphosphonic acid. Lithium has been selectively separated from beryllium (3) by extraction from a caustic solution into a diethylether solution of dipivaloylmethane and submicrogram amounts of cesium have been extracted (119) with calcium dipicrylaminate into nitrobenzene. The value of extraction methods in trace work is exemplified in the studies of boron in which the boron-curcumin complex is extracted into ethylmethylketone-chloroform which contains phenol (209). The extraction of the colored product permits the determination of boron down to 8 mpg. An interesting study has been made of the mechanism of the extraction of selenium(1V) with saturated aliphatic monoketones. Jordanov and Futekov (106) showed that at a particular acidity, chlorocomplexes of selenium are present which react with the ketones to form organic selenium compounds that are soluble in chloroform or carbon tetrachloride. On the basis of the mechanism studies, specific extraction separation of selenium from all other elements becomes a possibility. A somewhat similar fundamental study has been made of the extraction of niobium. Jurriaarise and Moore (107) have shown that it is possible to induce the selective extraction of K b by tatnoyltrifluoroacetone by the addition of n-butanol through the aqueous phase. I n some instances extraction procedures are used as a n adjunct to other methods of separation and concentration. Antimony has been collected from seawater with hydrous manganese dioxide and then separated from accompanying interferring elements by extraction (164) of the iodide into methyl isobutyl ketone. Thin-layer chromatography, like solvent extraction discussed above, is reviewed in detail elsewhere. However, because of its general importance to microchemical and trace studies, at least brief mention of its applications to the study of inorganic systems seems necessary. A system of identification of 40 cations and 19 anions has been proposed (93) by Hashmi and his coworkers. The scheme is simple and permits complete analysis of a n unknown mixture in less than 3 hours.
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Thin-layer chromatography has been proposed for the separation of alkaline earths (76) and the halides and pseudohalides ('77). Silica gel suspended in dithizone has been used as the medium for separating mercury, copper, cadmium, nickel, and zinc (57),and a number of the first row transition metals have been separated on thin layers of cellulose and silica gel impregnated with tributyl phosphate (9). Urinkman and deVries have proposed a technique employing liquid ion exchangers for the separation of ternary and quaternary mixtures of metals (SO). Resinous reagents are finding special applications in microchemical studies. Pyridine-2,6-dicarboxylicacid eschangers have proved useful as chelate-forniiiig resins capable of separatiiig calciiim from strontium @ I ) , and a HSCN-type resinous reagent has been described (90) which was used to detect copper, iron, cobalt, mercury, and nickel. Other resinous reagents were prepared and their applications described. Ion exchange reagent beads have been prepared which are claimed to be selective in the detection of molybdenum (154), inagnesium (110), cesium (155), beryllium ( f o g ) , and chromium (201). Somewhat similar in principle and application are some of the reagent papers that have been described. An anion exchange paper useful in the determination of microgram amounts of boron (131) has been described. Ai reagent impregnated paper has been developed for the determination of microgram amounts of nickel ( 1 1 4 ) , and a scheme for the systematic analysis for anions has been proposed (205) based on a 5-group separation and the use of suitably iinpregnated test papers for detecting the individual constituents of the separated groups. Among the special techniques used for microchemical separations is the use of ion exchange membranes. Blaedel and Christensen (19) have studied the selectivity of anion exchange membranes and Blaedel and Haupert (28) have discussed and illustrated the potentialities of ion eschange membranes for the separation, extraction, and concentration of ionic species. They have provided a theoretical study of the cation exchange equilibrium involved aiid have shown experimental verification. MASKING
Starting with the first revierv of inorganic microchemistry published 20 years ago, references have been made to the use of masking technique? for the minimizing or elimination of interferring effects caused by diverse substances. The technique of masking is one of the most elegant means available for conditioning systems in the enhancement of selectivity. N o s t optical methods have
incorporated the use of masking agents and the same can be said of the majority of electroaiialytical procedures. I n a sense, it is surprising that something as important as masking remains essentially as an art. Even when a critical interference is eliminated by the addition of an appropriate masking agent, the fact is seldoni indexed in the abstract' systems. The importance of the problem can be illustrated by reference to a study of methods for the determination of nitrate (229). The determination of nitrate in air and especially in water is of vital importance because 20 ppm or more nheii ingested by infants may cause death. Still the method for determining nitrates remained essentially unchanged for almost 80 years and was subject to serious error ill the presence of chloride. True, chlorides can be removed by precipitation; but this tedious process was all too often omitted iii the routine laboratory schedules. S o w a coiivenient aiid effective means has been provided to elimiiiate the chloride interference based 011 the addition of antimony sulfate as a niaskiiig agent. It is to be hoped that theoretical considerations will soon provide a means for calculating the effectiveness of coniplesing ligands in the stabilizing of systems aiid the eliniiiiatioiis of interferences. Up to now, the technique is primarily empirical anti probably will remain so except for isolated ideal situations. I t is iiiteresting that Kelly and Suttoii (111) have presented a theoretical consideratioii of precipitation reactions taking place in the presence of chelating agents. They have considered the precipitation of a nuniber of metals i i i the preseiice of EDTh through such comnion precipitants as hydroxide, sulfate, sulfide, oxalate, and 8-quinolinol. The importance of niaskiiig is sometimes recognized even in the title of papers. For csaml)le, l'riliil, who is certainly one of the recognized authorities in the use of such techniques, has recently 1)ublished a paper entitled, "Contributions to the Basic Problems of Coniplesometry. XIX Determination of Zinc and Cadmium: 3-Mereaptopropionic Acid as a 11 for Cadmium." The phasizrs (f 65) the necessity of s u c c e 4 v e masking for the complesinietric iiivestigation of a mixture of ziiic, cadmium, and copper. Hopefully other authors will join in calling attentiou to significant applications of masking techniques. Two masking agents that should be added to the list of coniplesing ligands are dithiocarbaininoacetic acid aiid 110tassium trithiocarbonate. Ijudevsky, Russeva, and Mesrob (36) have found t h a t dithiocarbaminoacetic acid forms very stable water-soluble conil)lcses with bismuth (II I) , indium (II I ) , t hallium( 111), cadniium(II), lead (11), nier-
cury(II), and copper(I1). Johri and Singh have reported (105) that iron(I1) and iron(II1) are made stable in solution thus permitting the precipitation of titanium, beryllium. and zirconium hydroxides in their presence. Stable complexes are reported to be formed with iron, cobalt, nickel, copper, molybdenum, vanadium, and palladium. RING OVEN METHODS
T h e ring oven technique introduced has continued to attract interest and is being applied in many ways and in many fields. F o r example. it is proving to be a very valuable tool in air pollution studies where it is used for t,he identification and determination of air borne particulates. The ring oven method for determining selenium in atmospheres (227) is better than even such sophisticated methods as neutron activation. The ring oven combined with the w e of reagent crayons has been proposed for the microdetermination of zinc (230) and copper (231). The methods apply in the microgram to nanogram range and errors can be kept under lo%, which is comparable with the accuracies obtained by most other methods that are applied a t such levels of determination. 111this connection we disagree wit,h the modest' designation of our good friend Professor R'eisz who introduced the ring oven a few years ago and who refers to such methods as being semiquantitative. If the ring oven is semiquantitative at these levels then so are such methods as polarography, emission spectroscopy, and colorimetry. In fact, few methods applied in the determination of microgram or nanogram quantities can justify reporting results to more than two significant figures. On this basis, then, the ring oven is certainly comparable to other more generally used techniques. '1 somewhat similar study to the ones cited above was that of Loley and Malissa (127) who used the ring oven for the qualitative and quantitative separation of long lived radionuclides from uranium fission. Dust samples originating from nuclear studies in 1961-62 were analyzed for radioactive Cs, Sr, and Ru. The sensitivity of the method is shown by the quantitative evaluation of strontium-90 a t the IO-'* Ci activity level. Weisz and Klockow have used the ring oven in the identification and semiquantitative determination of radio nuclide3 (219, 220), and radiochemical separations by the ring oven method have been applied to molybdenum and technetium (9?!117) and cesium (96). Ring oven methods have been used in combination with thin-layer chromatography ( f 5 8 , 1 8 8 ) . The method has been used for the determination of traces of selenium in water (14) and the quantitative analysis of some alloys (15, 16). Qualitative studies based on the ring
oven have been applied to gold plating (31); platinum plating (32); some gold and silver alloys (159); and various copper, aluminum, and zinc alloys (216). The detection and semiquantitative determination of thallium (218) has been described. X method has been proposed for the separation of calcium, strontium, and barium ( 4 2 ) ; and ring colorimetry has been suggested (104) for the determination of micro amounts of easily reducible elements. Tests for phosphate and silicate have been investigated by the ring oven method (80) and a general discussion has been published ( I ) , covering the ring oven separation of mivtures of ion
