ANALYTICAL EDITION
I N D U ST RIAL andENGINEERING C H E MIS TRY Harrison E. Howe, Editor
Organic Reagents in Inorganic Analysis’ F. FEIGL University of Vienna, Vienna, Austria
A
LTHOUGH the methods of chemical analysis have been permanently influenced through the advances in all fields of chemistry, this influence is not always immediate, since in chemical analysis thorough testing and criticism are necessary and long periods of time frequently elapse before new methods and procedures become established. This is also true of the more recently popularized investigation and use of organic reagents. I n inorganic analysis, which is undoubtedly one of the oldest branches of chemistry, inorganic reagents or purely inorganic reactions have, with a few exceptions, long dominated, even during a time when synthetic organic chemistry was experiencing striking triumphs tnrough the evolution of classical syntheses and through the elucidation of constitution, etc., and an abundance of compounds was available. Up to the end of the preceding century only a few analysts devoted much consideration to organic reagents and then only where the value of such reagents was immediately obvious. As pioneers must be mentioned the French chemists, Denighs and Cazeneuve, with whose names are associated analytical reactions frequently used. The genuine beginning of a systematic research into the possibilities of organic reagents, however, occurred at a later date. It was distinguished by the founding and development of the chemistrv of the complex compounds by A. Werner and his school, further by the improvement of microchemical technic, and finally by the tendency toward rationalization and simplification of analytical methods. Today it is generally accepted that in the evaluation of analytical methods two criteria are determinative: the sensitivity and the specificity of the involved reactions. A comparison by both criteria of the effectiveness of inorganic and organic reagents shows that the latter exhibit many intrinsic advantages. Although organic substances had heen occasionally used earlier for precipitations and color reactions [the precipitation of metal oxalates, of hydroxides by organic baees, and the Griess reaction (60) for nitrous acid described in 1879, may be mentioned], it was only in 1905 that the significance of organic reagents in the analysis of inorganic eubstances was brought to attention in an obvious and emphatic manner. This was by the nickel reagent, diacetyldioxime (dimethylglyoxime), described by L. Tschugaeff, with which A. Rrunck carried out a series of valuable separations. The Tschugaeff nickel reaction may still be considered as ideal in a certain sense. Because of the intense color and insolubility of the nickel salt, the reaction furnishes a sensitive method for the detection
of nickel; its exceptional purity and definite composition, making possible direct weighing, afford a direct determination of small amounts of nickel; and above all-and this is its particular merit-it is highly specific in its action. The nickeldiacetyldioxime reaction indicates for the first time, in that it hints at the great reservoir of organic compounds, the possibility of a far-reaching simplification of difficult analytical separations through the use of specifically acting organic reagents. A survey of organic compounds for their use in solving analytical problems indicates that much empirical research is necessary, but that certain guideposts and directing principles are available for searching out new reagents or improving old reactions.
Salt-Forming Properties and Inner Complex Compounds First consideration must be given to the salt-forming properties of the organic compound, which are due to the presence or the formation of definite acidic atomic groupings; such salt-forming groups are the sulfonic (SOsH)-, sulfinic (A02H)-, carboxyl (CO0H)-, hydroxyl (OH)-, sulfhydryl (SH)-,oxime (NOH)=, and imine (NH)= groups, the hydrogen atoms of which are replaceable by metal atoms. At the same time conditions must be such in the organic m o l e cule that a complex compound can be formed through the saturation of the secondary valences of the metal atom contained as a neutral part in the molecule. In the analytical evaluation of the salt-forming ability of an organic compound, as high a sensitivity and specificity as possible are first to be desired, assuming the salt to possess the necessary solubility relations (insolubility in water, solubility in organic reagents), and to have a color different from that of the reagent. Although one of the above acid-forming groups is indispensable for salt formation, other atoms or atomic groupings occurring in the molecule frequently exert essential influences on the specificity of the salt formation as well as on the color and the solubility of the salt concerned. It is important to observe that in all cases the metal not only replaces the acidic hydrogen atom but also is bound to other atoms of the same molecule through the saturation of the secondary valences of the metal atom (inner complex salt formation). The conditions affecting the dependence of analytical effects on constitution are therefore important because, as is well known, condensation and substitution reactions frequently cause great changes in the structure of organic molecules, thus modifying the salt-forming properties of the compounds, and this may be made profitable from an
Translation from the German by Harvey C. Diehl, University of Michigan.
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analytical viewpoint. In a survey of the analytical evaluation of new reagents and the improvement of the activity of organic reagents already known, it is of great significance that certain atomic groupings in organic compounds exert a very definite influence on the solubility, color, etc., of the salts, so that it is safe to speak of the relation between certain atomic groupings and specific activity. It should be pointed out here, however, that the reaction medium, as well as the definite atomic groupings appearing in the molecule, is of great importance in the determination of the specificity of organic reagents. Thus a reagent may be highly specific in acid solution but in alkaline solution its uniqueness is lost or wholly changed in nature. Furthermore, it is often possible, through the addition of certain compounds, to decrease the concentration of disturbing ions so much that the trouble may be eliminated. As a starting point for these considerations we may select the reagent already mentioned, diacetyldioxime. As is well known, the red insoluble nickel salt of diacetyldioxime is interpreted as an inner complex compound according to the formula HsC-C-G-CHs
contrast to diacetyldioxime, is very soluble in water. The example of the dioximes indicates, therefore, that it is possible to change the molecule of the organic reagent so that its analytical value may be improved, if the specifically acting group is known and maintained unchanged throughout the variations in the rest of the molecule. This relation of analytically important properties to particular and characteristic groupings is also shown by two copper reagents-namely, benzoinoxime ($0) ,CsHS-CH(OH)-C(NOH)-C&, and salicylaldoxime (ZZ?), C,&CH(NOH)OH. The first reagent is specific for copper in ammoniacal solution, the latter furnishes a separation of copper in acetic acid solution; in both cases the organic copper compounds are suitable for direct weighing in either macro- or microanalytical procedures because of the high purity and definite composition of the precipitates. With both reagents there are produced inner complex copper salts corresponding to Formulas 1 and 2.
II
II
Not only diacetyldioxime but also numerous other syndioximes of the general formula R--C(NOH)-C(N0H)-R react in the same manner. As nickel reagents, benzildioxime, COHE-C (NOH)-C(NOH)-CCH6, was recommended by Atack (4) and the more soluble furildioxime, C4H30-C(N0H)-C(N0H)-C4H30, was recommended by Soule (111). It appears at first, therefore, to be merely the atomic group
H
It has been shown that the atomic groupings active in forming the complex compounds 1 and 2, -C(OH)C(N0H)- and A/OH
-c-c&OH
HOI!
which is responsible for the specificity for nickel; that this is true only to a limited extent, however, is shown by the behavior of the dioximes,
[lp" NOH
and
\
I n these dioximes the specific activity of the dioxime groups has been lost. The compounds behave as dibasic acids and yield yellow precipitates with almost all metal ions in neutral solutions. The aromatic ring, therefore, destroys the specific activity of the two oxime groups. The nickel specificity returns, however, when the double bonds of the aromatic rings have been removed as is shown by the behavior of cyclohexanediondioxime
are responsible for the formation of the green or bright yellowgreen Cu(l1) salts and that this function is quite independent of the other groups attached to the molecule (28). Comparing the activity of benaoinoxime with the other acyloinoximes of the general formula R-CH(OH)-C(NOH)-RI, it is found that for the formation of green insoluble copper salts the OH- and NOH- groups in a-positions are necessary. The green copper salts are stable towards ammonia, however, only if there are also present in the molecule atomic groupings capable of occupying the free coordination positions of the copper atoms (Formula 1). Accordingly, cyclohexanolonoxime, for example, yields a green water-insoluble copper salt of the formula CHI 0
CH2
@: E This compound, described by Wallach (I,%), yields a nickel salt which can be differentiated from nickel diacetyldioxime only with difficulty; it is produced under the same conditions and it can be used analytically in the same manner. The dioxime just referred to also shows that the insolubility of the nickel compound is in no way related to the insolubility of the reagent in water, for cyclohexanediondioxime, in
ci which is, however, soluble in ammonia. The use of benzoinoxime as a specific reagent for copper has been pointed out; this reagent also precipitates copper from an ammoniacal solution in the presence of tartrate or glycerol, making possible the separation of copper from aluminum, iron, and other metal ions preripitated by ammonia. In mineral acid solution the precipitation of copper is impossible. Copper benzoinoxime is also soluble in mineral acids. On the other hand, according to Knowles (76), benzoinoxime is able to separate quantitatively molybdenum,
ANALYTICAL EDITION
NOVEMBER 15, 1936
tungsten, and vanadium, the procedures being useful analytically. However, a true salt of molybdenum or tungsten with benzoinoxime is not produced, but probably only addition compounds between molybdic or tungstic acid and benzoinoxime of the nature of the so-called heteropoly acids. Salicylaldoxime, which precipitates copper quantitatively in an acetic acid solution, also, according to Holzer (70), separates palladium, but not platinum, quantitatively from acid solution, the precipitate also being suitable for weighing. Both 01the reagents mentioned therefore are not limited to one element in their activity. Such reagents are designated as specific reagents and are differentiated from the so-called special reagents-that is, those which are characteristic for a single substance. It should be noted that only a very few special reagents are available. However, in numerous cases through a choice of suitable experimental conditions a limitation of the activity of a specific reagent may result and the practical effects of a special reagent be achieved. I n a review of the specific activity of definite atomic groupings mention must be made of the cobalt reagent, a-nitroso/?-naphthol, described by Illinsky and von Knorre (71), which forms a reddish brown inner complex cobalt (111) salt o=N--c0/3
403
sensitive detection of zirconium. A recent investigation by Mayr (87) has shown that a-nitro-P-naphthol may likewise be used for the separation and determination of cobalt. Undoubtedly this again involves the formation of an inner complex trivalent cobalt salt. Insoluble inner complex salts, in which the organic component possesses the character of a dyestuff, as in the previously mentioned nitrosonaphtholates, are called color lakes. The color lakes of alizarin and other hydroxyanthraquinones possess considerable analytical significance. Atack (5) was the first to point out that aluminum could be detected with great sensitivity by means of the red color formed with alizarin8 (alizarin sulfonic acid). This is a question again of the formation of an inner complex salt corresponding to the coordination formula A1/3
Atack, Underhill and Petermann (119), and Yoe and Hill (124) applied the alizarin reaction to the colorimetric determination of aluminum. Quinalizarin O H 0 OH and which is one of the oldest organic precipitation reagents known. Also the isomeric /?-nitroso-or-naphtholate of cobalt possesses an inner complex character which in color and solubility is inappreciably different from the compound first mentioned. Both cobalt nitroso compounds have, until recently, been used merely as precipitants, since the cobalt precipitates formed in acetic a d d solutions do not have the theoretical composition. Recently Mayr and Feigl (88) have succeeded, by converting the cobalt into the trivalent state before precipitation, in obtaining a pure Co(II1)-nitrosonaphtholatewhich after drying is suitable for weighing, and furnishes, therefore, a direct determination of cobalt. That the inner complexforming groups are responsible for the cobalt affinity of both of the nitrosonaphthols is shown by nitroso R-salt, N-OH
recommended by van Klooster (73) as a cobalt reagent, as well as the behavior of 2-isonitroso-1-ketotetralin 0
/\/\C=NOH
investigated by Strauss and Ekkard (118). The latter compound behaves toward cobalt salts in the same manner as the nitrosonaphtholates. Iron and palladium are also precipitated as inner complex salts by nitrosonaphthol and by this means may be separated from the similar elements aluminum and platinurn. It is to be noted that in acid solution zirconium yields with the isomeric nitrosonaphthols insoluble zirconyl compounds, (C10H602N)2(Zr0),of which that with the a-form is colored green-yellow, that with the B-form, red. The latter is recommended by Bellucci and Savoia (7) for the
also forms lakes, and Fischer (50)has recommended the beryllium lake of quinalizarin for the detection and determinat.ion of beryllium. The magnesium lake of quinalizarin, according to Hahn (63), is suitable for the sensitive detection and for the colorimetric determination of magnesium. Finally, there should be mentioned aurintricarboxylic acid, the yellpw solution of which gives a red aluminum lake, probably corresponding to the coordination formula
Aurintricarboxylic acid was recommended as an aluminum reagent by Hammett and Sottery (65) and wag later used by Winter and co-workers (123) and by Roller (105) for the colorimetric determination of aluminum. There should also be emphasized the lake which aluminum forms with the dyestuff of fustic wood, the so-called morin,
K observed by Goppelsroeder (69). The reagent produces with aluminum salts in neutral or acetic acid solution an intense green fluorescence, which is due to a neutral aluminum salt of morin, A1(C1~H907)3,in colloidal solution, and makes possible the recognition of 0.005 y of aluminum (20, 108).
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Although the morin reaction is the only fluorescent reaction of metals with organic compounds used analytically up to the present time, it is expected that still other examples will be found. The ammonium salt of nitrosophenylhydroxylamine, C6H6N(NO)ONH4,recommended by Baudisch (6) in 1909 has been very wide!y used. This compound retains the name “cupferron,” because it was first shown to be very suitable for the separation of copper and iron from numerous other metals. Here also, an inner complex salt is formed according to the formula ~ N N
--I
I
o w 0
Me Later it was shown, especially by American investigators, that with control of the conditions of precipitation, a series of otherwise difficult separations of various metals can be effected and appreciably simplified. Thus the precipitation of thorium (116) and the separation of gallium (93) from Al, Cr, In, Ce, and U, of uranium (6Q)from Al, Zn, Ca, and Mg, of bismuth (96) from As and Sb, of tin (54,95) from Co, Ni, Zn, Mn, As, Pb, and Sb, and finally the separation of zirconium (15) from A1 have all been made possible by cupferron. I n all these reactions there are again formed inner complex salts with the metal ion precipitated. These compounds are suitable forms in which to precipitate the metal, but are not satisfactory for weighing because the metallic compound is always contaminated with some of the reagent which is insoluble in water. The solubility in water (or in ammonia, alkalies, or acids) plays an important role in the practical use of organic reagents, especially for quantitative purposes. When an alcoholic or acetone solution of an organic reagent is added to an aqueous solution the problem is always complicated by the danger that the reagent will be thrown out as such by the water, thus contaminating the precipitate, from which it can be removed only with difficulty. In such cases the conversion of the precipitate into a form suitable for weighing, through ignition or other treatment, is necessary; the essential advantage of a low percentage of the determined material in the weighed precipitate is, however, then lost. An example of an organic reagent forming inner complex compounds which have a satisfactory solubility and the theoretical composition is 8-hydroxyquinoline. This compound, which goes under the commercial name of “oxine,” was introduced into analytical practice by F. L. Hahn (1926) and by R. Berg (1927). Oxine is very soluble in alcohol and acetic acid and with numerous metals yields insoluble, for the most part brightly colored, inner complex salts corresponding to the coordination formula
VOL. 8, NO. 6
oxine, too, the formation of inner complex salts depends on the atomic groupings as set forth in the coordination rules already given; accordingly the halogen derivatives of hydroxyquinoline such as 5,7-dichloro- and 5,7-dibromohydroxyquinoline are suitable for analytical purposes and even present certain advantages over the parent compound. Although hydroxyquinoline reacts with numerous metals, the reagent can be utilized in the solution of numerous problems in gravimetric analysis if control is exercised over the conditions of precipitation (8). Two o-aminocarboxylic acids able to form inner complex compounds, which have been adapted recently to the purposes of gravimetric analysis, are anthranilic and quinaldinic acids. By means of anthranilic acid Funk (63) and coworkers have carried out a series of gravimetric determinations of divalent metals and have shown that through the bromometric determination of anthranilic acid a volumetric determination of the metal is possible. Quinaldinic acid was used by Rby and Bose (99) for the gravimetric determination of copper, cadmium, and zinc. I n the analytical use of anthranilic and quinaldinic acids inner complex salts are formed according to the formulas 0
Of organic reagents in which a sulfur atom is involved in the formation of inner complex compounds, there should first be mentioned dithizone, thionalide, and rubeanic acid. Dithizone is the commercial name of a compound easily obtained, diphenylthiocarbazone, H s8; : : ; : ; :C < S investigated by Fischer (51), which reacts with numerous metals partly in the keto form as written, partly in the enol form
Derived from either the keto or the enol form, inner complex salts result corresponding to the coordination formulas
b-ihe The metal salts of oxine as a rule may be dried a t 105” to 140” C. and weighed. The phenolic nature of the reagent, moreover, makes possible a volumetric and a colorimetric determination, the former through bromination of the oxine with the formation of 5,7-dibromo-8-hydroxyquinoline,the latter through the color reaction with phosphotungstomolybdic acid, according to Folin and Denis (529, or through the coupling of the oxine with diazo oompounds in alkaline solution. These volumetric and colorimetric determinations may be carried out either on the solution of the purified oxinate in acid or on the excess of reagent. With
The first, produced in acid or neutral solution, is completely insoluble in water, although easily soluble in organic solvents, yielding intensely colored solutions ; the latter form is preferred for univalent metals, and is equally insoluble in water but also insoluble in organic solvents. In spite of the lack of specificity in its action, by standardization of the
NOVEMBER 15, 1936
ANALYTICAL EDITION
reaction conditions, dithizone furnishes numerous sensitive and important tests and colorimetric quantitative procedures. Thionalide is the commercial name of thioglycollic acid P-aminonaphthalide, HSCH2C0.NHCloH,, recommended by Berg and Roebling (IO) as an analytical reagent. It is capable of forming inner complex salts corresponding to the coordination formula Me
,**'\s
C~~H,--NH,~' k d H 2
I n neutral solutions thionalide reacts as a derivative of hydrogen sulfide with all heavy metals which are precipitated by the sulfide ion. Noteworthy is the precipitation of metal ions from mineral acid solution: all metal ions up to Cd(I1) and Pb(I1) are separated in the form of yellow or white precipitates. Although the depth of color of the heavy metal sulfides is not carried over in spite of the sulfur linkage, the sensitivity of the reaction is far greater than that of the sulfide precipitation. For example, in 5 ml. of solution arsenic can be identified in a dilution of 1 to 100,000,000 by the formation of a white precipitate. With thionalide also much of the difficulty due to its lack of specificity may be overcome by control of the conditions of precipitation-for example, by precipitating from mineral acid solution, or from alkaline solutions containing tartrate or cyanide. Of particular value is the reaction with thallium in alkaline solutions containing tartrate or cyanide, which is very specific and sensitive to a dilution of 1 to 10,000,000. Thionalide also affords certain quantitative determinations. The copper and mercury salts may be isolated as such and after drying a t 105" are suitable for weighing; moreover, the organic component of the precipitate may be determined iodometrically after removal of the excess reagent and the metal content calculated accordingly. Thioglycollic acid, the parent compound of thionalide, has been recommended by Lyons (85) for the colorimetric determination of small amounts of iron; it also shows the property characteristic of thionalide of forming insoluble inner complex salts. The effect of the amino group introduced into the series thioglycollic acid-thioglycollic acid analide-thioglycollic acidP-aminonaphthalide is shown by the increasing insolubility of the metal salts (9).Rubeanic acid has recently been recommended by Rky and R&y (100, 101) as a qualitative reagent for copper, cobalt, and nickel. This compound, which is the diamide of dithioxalic acid, exists in solution in equilibrium with its tautomeric (aci), diimido, form, according to SG-NHZ HS-C=NH S-C=NH -S -NH2 HS--L=I\" * 2 H + + [ S- =NH
A
]
*
The acid form yields colored insoluble nickel, cobalt, and copper salts corresponding to inner complex compounds of the formula
c c
These complex salts are produced when the concentration of the aci form of the rubeanic acid is so great that the solubility product of the rubeanate formed is exceeded; as is seen from the above equation, this occurs when the hydrogen-ion concentration is diminished by the addition of sodium acetate, ammonium acetate, or alkalies. The copper, nickel, and cobalt compounds, precipitated quantitatively from strongly ammoniacal solution, when once formed, are insoluble in dilute mineral acids; this is in harmony with the idea that these compounds are inner complex salts. The stability of
405
these salts is exceeded only by the stability of the corresponding cyanide complexes, as is shown by the facts that the rubeanates are soluble in potassium cyanide solutions and are not precipitated from solutions containing cyanide. The sensitivity of the detection of copper, nickel, and cobalt by means of rubeanic acid is very high; in the form of a spot reaction as little as 0.006 y of copper, 0.012 y of nickel, and 0.03 y of cobalt can be detected. I n a mixture of these three metals, the identification is still possible by means of a capillary separation procedure and in this manner 0.05 y of copper can be recognized in the presence of 20,000 times as much nickel (43). A compound, capable of forming inner complex salts of high sulfur content with heavy metals, is 2,5-dimercaptothiodiazol, discovered by Dubsky and co-workers (20) This compound, functioning according to the tautomeric equilibrium N-N HN-N I
I,
HS-8
I,
8-SH
Y
_r
S=c!
4-SH
$'
reacts in the mercapto form with numerous metals, forming inner complex salts of the following form: HN-C=S
Of particular interest is the color of the bismuth salt, which is deep red in contrast to the colors of other metal salts which are white to yellow. Here again the specificity caused by the difference in color is based on the formation of an inner complex ring as pictmed above, and it is independent of the groups which can be easily introduced in place of the imino hydrogen atom.
Specificity in Normal Salts So far only those reagents have been mentioned which, because of the neighboring position of the salt-forming and coordinating groups, have offered the possibility of forming the so-called inner complex salts. As pointed out, such inner complex compounds are frequently characterized by an abnormal solubility and by a deep coloration. These two properties frequently contribute a high sensitivity to methods for the detection and determination of metals but, on the other hand, at times leave something to be desired with respect to specificity. This is readily understood, inasmuch as the ability of nitrogen, oxygen, and sulfur atoms to coordinate in the formation of inner complex salts is not specific in action but is exerted toward numerous metal atoms, as is shown by numerous examples in that great class of substances, the ammines and aquo compounds. I n attempting to arrive a t a state in which the greatest possible number of specific reagents is available, those organic compounds which are merely capable of forming normal heteropolar salts should also be investigated for their analytical applications. Actually such compounds are frequently highly specific in their activity. Thus it has long been known that arsenic acid precipitated white insoluble zirconium arsenate from zirconium salts in solutions strongly acid with nitric or hydrochloric acids. Rice, Fogg, and James (103) were the first to find that this property of arsenic acid was also characteristic of organic derivatives of arsenic acid-for example, phenylarsonic acid. This acid makes possible the determination of zirconium and its separation from titanium and calcium. Thorium, which is very similar to zirconium, is precipitated by phenylarsonic acid only in solutions
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406
buffered with acetate; this makes possible the determination of zirconium and thorium in the presence of each other by the same reagent. That the -AsO(OH)z group alone is responsible for the activity of the organic derivatives of arsenic acid has been proved by Arnold and Chandlee (3) who showed that n-propylarsonic acid was equally satisfactory as a reagent for the quantitative estimation of zirconium. It should be noted that p-n-butylphenylarsonic acid also precipitates iron quantitatively ( 1 7 ) . Phenylarsonic acid, in which substitutions have been made in various positions and in various manners, has been shown by Feigl and his co-workers to function just as the parent compound (27, 46). This made possible the introduction of a group which contributed color to the molecule with subsequent improvement in the sensitivity of the reaction. An example of this is p-dimethylbenzeneazophenylarsonic acid (27,46) by means of which 0.1 y of zirconium can he detected by a spot reaction. Further examples of the specific activity of more simply built organic compounds are indicated by the saltforming properties of sulfinic acid, RSOSH, and the analogous seleninic acid, RSe02H. It has been long known (116) that benzenesulfinic acid and ferric salts in mineral acid solutions yield a difficultly soluble, orange-yellow precipitate of the formula Fe(02SCJ35)3. Other aromatic sulfinic acids behave in II. similar manner, substitution in the aromatic nucleus having no effect. The analogous aromatic seleninic acids, according to Feigl and Anger (M), also form insoluble yellow ferric salts. This same work has also shown that tin (IV), ceric (IV), thorium (IV), and uranium (IV) salts are precipitated in strongly acid solutions by sulfinic and seleninic acids. The sulfinic and seleninic acids, therefore, exhibit a noteworthy specificity in action and this activity is again Iocalized in particular groups. In contrast to the arsonic acids mentioned previously, in which it was irrelevant whether the arsonic group was coupled with aromatic or aliphatic residues, a direct combination of the sulfinic or seleninic groups with an aromatic ring is necessary (38). Of the compounds C&k%" CsH,CH2S02H, and CJ3,S02H, only the first has the power of precipitating the above metal ions from acid solution. According to experiments to be reported later, it seems possible to use sulfinic acid for the quantitative macro- and microdetermination of the metals mentioned above (as well as for molybdenum and tungsten) and for their separation from other metals. A further example of the specific action of certain atomic groupings in which no complex formation is involved is the heterocyclic compound rhodanine HN CO
and its derivatives, which were investigated by Feigl (30). The imino hydrogen atom of rhodanin is replaceable by silver, mercury, gold, and palladium, producing a precipitate of yellow metal rhodanine compounds in acid solution. Because of the reactivity of the hydrogen atoms of the CH2 group in the rhodanine molecule, condensation with numerous aldehydes and ketones is possible and all the derivatives so obtained have the same properties as the parent compound with respect to its precipitating ability. Thus, by the introduction of a chromophore group, colored silver salts can be obtained. Such a reagent is p-dimethylaminobenzalrhodanine HN
CO
r7
VOL. 8 , NO. 6
the violet silver salt of which permits the identification of silver in a dilution of 1 to 5,000,000, the most sensitive method known for silver up to the present time (36). p-Dimethylaminobenzalrhodanine has the advantage of yielding an insoluble silver salt but the disadvantage that the reagent must be used in alcohol or acetone solution. Accordingly the reagent is excellent for qualitative purposes but is not usable for gravimetric determinations, because the reagent is thrown down by water and cannot be quantitatively removed from the precipitate of silver salt. A survey of the possibilities of improving the reagent shows, however, that rhodanine also condenses with sulfonated aldehydes-for example, with o-benzaldehydesulfonic acid yielding a sulfonated benaalrhodanine which is readily soluble in water, alkalies, and acids (3s)* Among the organic heavy metal compounds which do not belong to the group of inner complex compounds must be mentioned the cadmium salt of mercaptobenzothiazole
NHs According to Spacu and KuraF; (112) this compound is produced by precipitation from an ammoniacal cadmium solution with mercaptobenzothiazole in the form of a yellow precipitate which is sufficiently pure to permit direct weighing in gravimetric work. The reagent is of particular value for the separation of cadmium from copper, since copper is precipitated in acid solution, cadmium being determined quantitatively in the filtrate from the copper precipitate. Even among the alkali and alkaline earth metals the formation of normal salts may make possible a differentiation of similar metals, as shown by the following examples. By means of rhodizonic acid
barium and strontium may be detected in the presence of magnesium and the alkali metals according to Feigl (26), and by maintaining certain experimental conditions barium may also be identified in the presence of calcium and strontium, and strontium in the presence of barium and calcium. A satisfactory reagent for gravimetric analysis has been found in picrolonic acid
I
NO8
which forms insoluble salts of high molecular weight and of definite composition; the reagent was first recommended by Kisser (78) for the microchemical identification of potassium and has since been used for the gravimetric estimation of potassium (IN), calcium (8,21),lead (66),and thorium (67). According to Poluektoff (97) potassium can be identified by means of the water-soluble sodium salt of p-dipicrylamine, owing to the formation of an orange-yellow precipitate of the following formula:
NOVEMBER 15, 1936
ANALYTICAL EDITION
This reaction is specific for potassium in the presence of the elements of groups 4 and 5, with the exception of rubidium and cesium, and only large amounts of ammonia must be previously removed. The identification of the NHd group is now possible by direct salt formation. This is by means of the sensitive reaction proposed by Riegler (104) involving pnitrodiazobenzene .
407
intense red, water-soluble complex ion (111) which is stable toward dilute acids. This complex belongs to the hexammine type, since the six coordination positions of the iron atom are occupied by the nitrogen atoms of the three molecules of the organic base. Of especial interest is the great sensitivity and stability of the ferrous dipyridyl and ferrous phenanthroline reactions, by means of which it is pomible to detect even traces of ferrous salts (and therefore ferric by reduction). The reactions have been made the basis of a very sensitive colorimetric determination of iron by Feigl and Hamburg (42) but the procedure is difficult to carry out. I n both reagents the activity is due to the presence of the group
o~N-CI>-N~C~
and alkali yielding the red ammonium salt of p-nitrophenylnitrosamine O~N-~-N=N-ONH, Only slightly investigated from an analytical viewpoint up to the present time are the reactions of complex metallic acids with organic bases. A few examples will indicate that this field is capable of expansion: (Bib)- is precipitated by cinchonine (84) and oxine (II), [Cd(CNS)J -- is precipitated by pyridine (lor), and cadmium can be determined volumetrically by means of &naphthoquinoline (11). Recently Krumholz and Krumholz (80) discovered a specific and sensitive test for zinc in the reaction of a basic styryl dye with [Zn(CNS)d] Finally it must be pointed out that normal salt formation with organic compounds plays an important role in the identification and estimation of anions. Well known are the gravimetric and volumetric methods for the determination of sulfate and tungstate by means of the organic base benzidine (74, Qq) and of nitrate by the base nitron (16). Thelatter has recently been applied by Geilmann and Voigt (56) to the gravimetric determination of perrhenate. The salt formation with methylene blue is the basis of the best macro- and microdetermination of perchlorate ( l e ) , and Allen and Furman (1) have recently recommended triphenyl tin chloride, (CsH&SnC1, for the gravimetric determination of fluorine, the insoluble fluoride having the nature of a normal salt.
--.
Neutral Constituents in Complex Salts and Adsorption Compounds All the organic reagents mentioned previously, because of their acidic character or the additional presence of groups possessing residual valence, have had the power of forming normal salts or inner complex salts. Experience shows, however, there are many compounds of useful nature which do not possess hydrogen atoms replaceable by metals, but which, by means of secondary valence forces alone are able t o form addition products with inorganic compounds. This addition t o the metal of one or more molecules of the organic reagent as a “neutral constituent” may produce insoluble or characteristically colored complex compounds. Of this type of reaction we have all too few examples. Thus, a,a’-dipyridyl (I) and a,a’-phenanthroline (11) were found by Blau ( I S ) t o yield with ferrous salts the
0-0
