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The mechanism of interaction between molybdogermanic acid and the basic dye Crystal Violet
T&mu Vol. 27. pp. 10% to 1059 Pergamon Press Ltd 1980. Rinted
in Great Britain
THE MECHANISM OF INTERACTION BETWEEN MOLYBDOGERMANIC ACID AND THE BASIC DYE CRYSTAL VIOLET F. V. MIRZOYAN, V. M. TARAYAN, E. KH. HAIRIYAN and N. A. GRICGRIAN Institute of Inorganic and General Chemistry, Armenian Academy of Sciences, Yerevan, Yerevan State University, USSR (Received 23 April 1979. Revised 1 October 1979.* Accepted 20 June 1980)
Summary-On the basis of light-absorption studies on solutions of Crystal Violet (CV) molybdogermanate in acetone, the optimal pH conditions for quantitative formation of molybdogermanic acid (MGA) have been determined. Di. tri-, and tetra-salts of MGA have been formed and isolated. It has been shown that formation of the higher salts is favoured by lowering the acidity, but this increases also the amount of the solid co-product CV-isopolymolybdate. To overcome this inconvenience the surplus molybdate ions are masked by adding oxalate ions, thus allowing the separation of the corresponding solid tetra-salt up to pH = 6.5; the molar absorptivity of this compound in acetone solution is very high (4.2 x lo5 l.mole-‘.cm-I).
It is of great interest to study the composition and separation conditions of the compounds of basic dyes with various heteropoly acids, with a view to establishing their mechanism of formation and also elucidating the conditions favouring increase of the analytical sensitivity for the determination of the element
Preparation and separation of CV-MGA
compounds
forming the heteropoly acid. So far there are few examples of basic dyes being used as reagents for molybdogermanic acid (MGA).1-6 Among the triphenylmethane dyes the highest sensitivity is given by Brilliant Green (molar absorptivity 1.93 x lo5 l.mo1e-‘.cm-‘).2 The formation of CV-MGA compounds (CV = Crystal Violet) has been studied in fairly acidic solutions (06-0.7M nitric acid), and the di-salt of MGA obtained.* This paper presents a detailed investigation of the interaction of CV and MGA over a wide range of acidity and concentration of the reacting components, with the aim of establishing the optima1 conditions for the formation and separation of more highly substituted solid salts of MGA, and thus increasing the analytical sensitivity of the determination of germanium.
To a solution [containing a definite amount of Ge(lV)] in a conical centrifuge tube, a definite amount of molybdate was added followed by nitric acid until the optimum acidity (referred to as the initial acidity, pHi) required for the quantitative formation of MGA was reached, and the volume was made up to 5 ml with distilled water. The solution was stirred and left for 10-15 min for maximum formation of MGA. Then the optimum acidity (called the final acidity, pHr) for the separation of CV-MGA was established, and a certain amount of oxalate solution (if necessary) and the reagent dye were added and the volume was brought to 10 ml with distilled water. After mixing and formation of the considerable amount of precipitate. the mixture was’ centrifuged, the solution carefully decanted and the pH of the solution measured. The precipitate was washed in a test-tube with 2 ml of water, then separated by centrifuging and dissolved in 10 ml of acetone. The degree of combination of Ge(IV) in MGA and then in CV-MGA was estimated from the absorbance of the acetone solution. A blank test was performed to check the formation of CV-isopolymolybdate salts. The absorbance A of the solutions was measured at 595 nm. in l-mm cells. Solid compounds were separated by centrifuging for 1-2 min at 3000 rpm.
EXPERIMENTAL
RESULTS AND DISCUSSION
Reagenrs
The optimal
A 0.005M solution of Ge(lV) (pH 7.2) was prepared by dissolving the appropriate weight of (especially pure) GeOz in distilled water by adding small portions of sodium hydroxide solution, and was further diluted as required. Reagents used were sodium molybdate (purej, 0.024 and 0.012M solutions, CV (pure) 0.1% aqueous solution, sodium oxalate (pure) aqueous solution, nitric acid (especially pure. s.g. 1.41) acetone (pure). All the solutions were kept in polyethylene bottles.
* Lost in the post; copy received 17 June 1980.
conditionsjar the formation of MGA
In establishing the optimal acidity for the formation of MGA some authors’ have used the lightabsorption properties of MGA and tried to find the conditions under which the yellow colour of MGA is maximal. However, it is known that, depending on the acidity and the concentration of molybdate, various isomers of MGA (a- and @MGA) are formed, which differ in their stability and spectral characteristics.7-9 Therefore the maximum development of the yellow colour
1055
F. V. MIRZOYAN et al.
1056
1.0
2.0
30
40
w
Fig. I. Dependence of yield of MGA on acidity at various initial concentrations of molybdate ion. [Ge(IV)] = 1 x 10-‘M: [CV] = 1.2 x 10-4M; pH, = 0.8; [Moo:-]: I, 2.4 x 10-3M; 2, 1.2 x 10-3M; 3,0.7 x 10-3M.
of MGA does not unequivocally
indicate the quantitative formation of MGA. Consequently it is more convincing to investigate the formation of MGA not on the basis of its own light-absorption characteristics, but on that of the light-absorption of the salt of MGA with CV. It should also be taken into account that though its formation is very sensitive to the acidity of the medium, MGA (once formed) can exist in more acidic medium without being decomposed.*‘*” This allows the complete suppression of the formation of isopolymolybdates, by increasing the acidity to pH, = 0.8, at which decomposition of MGA does not take place, and accordingly a quantitative yield of CV-MGA is ensured. Figure 1 shows the dependence of absorbance on pHi for acetone solutions of CV-MGA at various initial concentrations of molybdate ions (pHi was measured in separate experiments). The acidity intervals given in Fig. 1 show the optimal conditions for MGA formation.* The validity of the considerations above is confirmed and the acidity interval for the formation of MGA is clearly indicated (pHi = 1.5-3.8 for 1.2 x lo-‘M molybdate and pH, = 1.2-3.8 for 2.4 x 10W3A4molybdate). Thus, the absorbance of the acetone solutions obtained under these optimal conditions is practically independent of the molybdate concentration and the acidity, in contrast to the behavior of MGA itself. Hence CV-MGA is of greater interest for analytical use. The oprimal conditions for separating pounds
CV-MGA
com-
Figure 2 shows the yield of CV-MGA compounds as a function of pHI at constant pHi. It shows that pHf = 0.7-0.9 ensures maximal and practically constant absorbance for the acetone solutions. * Estimated cm-‘.
on the basis of c = 4.2 x lo5 l.mole-’
.
The precipitates of CV-MGA are violet, characteristic of the singly-charged form of CV. If the pH is lowered the acetone solution decreases in absorbance, and the solid compound becomes green (acidity from pHI 0.4 to 4M nitric acid). From the colour of the precipitate it can be concluded that under these conditions MGA interacts with the green singly-protonated doubly-charged form of CV. An acetone solution of the product is violet, however, and presumably, when the precipitate is dissolved in acetone, the doubly-charged form of CV changes into the singlycharged form. Solutions of CV-MGA, at acidities from pH 0.4 to 2.OM nitric acid, have practically constant molar absorptivity (1.8 x 10’ l.mole- ‘.cm- I), which is indicative of the constancy of the composition of the CV-MGA product under these conditions. It therefore appears that the system stu$ed can give rise to compounds of various compositions. Figure 2 shows that the absorbance of an acetone solution of CV-MGA is a function of pHP but is independent of it over two ranges, in the vicinity of pHr = 0.1 and 0.8, the absorbance being greater at the higher pHI value. For both these pHI values the concentration ranges of the reactants that will give constant absorbance for the product have been determined. They also permit the pure CV-MGA compounds to be obtained. They are listed in Table 1. Use of higher concentrations of the components leads to appreciable separation of CV-isopolymolybdates and hence to decrease in the yield of CV-MGA compounds. The preparation of CV-MGA acidity conditions for MGA
compounds
at optimal
According to the results above the investigation of the interaction between MGA and CV at pH, > 1.0 is impossible because of the simultaneous precipitation of CV-isopolymolybdates (Fig. 2). which have similar spectral characteristics to those of the CV-MGA compounds. Hence it is necessary to mask the free
Molybdogermanic
1057
acid and Crystal Violet
Fig. 2. Dependence of yield of (1) CV-MGA and (2) CV-isopolymolybdate on pH,. [Ge(IV)] = = 1.2 x 10-3M. 1 x IO-5M; pH, = 2.5; [CV] = 1.2 x 10-4M; [Moo:-]
isopolymolybdate ions. Earlier this was done with oxalic acid at high acidity (0.60.7M nitric acid) which lowers the efficiency of the masking action. It was now found that the formation of the isopolymolybdates can be prevented by adding mineral acid to give pH < 1.0, without the need for the masking agent (Fig. 2). In establishing the optimal acidity for the formation of MGA. it was shown that the quantitative formation of MGA is possible at relatively low acidity (pHi 1.5-3.8). This makes it possible to form CV-MGA in this acidity range. with simultaneous masking of isopolymolybdates with oxalic acid. The effect of oxalate on the yield of CV-MGA as a function of pH is shown in Fig. 3. It can be seen that CV-MGA is formed from a 1.2 x 10m3M sodium molybdate/O.OlM oxalate solution in the acidity range pHr 0.5-7.0, quantitatively at pHr 1.5-2.5 or 4.0-6.0. The yield is lower at pHr 2.M.O because of the concurrent reactions of CV with both isopolymolybdates and MGA under these reaction conditions (Fig. 3. curve 1). According to Alexeyeva,” the HMo20; ion occurs in this acidity range, and this (or some similar species) appears to form a precipitate with CV. causing a significant rise in the absorbance of the blank solutions. An eightfold increase in the oxalate concentration (Fig. 3. curve 2) does not affect the quantitative separation of CV-MGA in the pH range 2.0-5.5 but completely eliminates the formation of CV-isopolymolybdates. Unfortunately MGA is stable for only 15 min under these conditions. Therefore it must be prepared and separated during that interval of time. The masking efficiency of the oxalate is mainly determined by the molybdate concentration. The yield Table 1. The optimal reagent concentrations
PH, 0.8
0.1
[Moo:-].
10-3M
0.8-1.8 0.848
of CV-MGA as a function of pH was studied at O.OSM oxalate concentration and fairly high concentrations of molybdate (Fig. 3. curve 3). Under these conditions the optimal range of acidity for formation of CV-MGA is expanded (pH 2.0-6.5). At pH 4.5 and constant concentration of molybdate (1.2 x 10e3M) and oxalate (1.0 x lo-‘M) the yield of CV-MGA was found to be quantitative with (0.9-5.0) x 10_4M cv. The correlation of these results with data obtained in the absence of oxalate indicates that the proposed variant of the method has markedly higher sensitivity. The acetone solutions of CV-MGA formed in presence of oxalate obey Beer’s law over the Ge(lV) concentration range 1 x lo-‘-l.6 x 10v5M. the molar absorptivity being 4.2 x 10’. This fact is unequivocally indicative of the change in composition of the CV-MGA compounds. The composition of CV-MGA cornpods The composition of the principal compounds formed between CV and MGA was determined by Job’s method (Fig. 4), at different final acidities: pHr 0.1 (curves 3 and 4) and pHr 0.8 (curves 1 and 2) and also at pHr 4.0 in the presence of oxalate as masking agent (curves 5 and 6). The maximum in the curves obtained at pH, 0.1 corresponds to the ratio CV : MGA = 2 : 1. Therefore. in more acidic solutions (acidity between pHr 0.4 and 2M nitric acid). according to the data in Fig. 2. the MGA salt precipitated contains the singly-protonated doubly-charged cationic form of the reagent dye (HCV”) (but see next paragraph). The formation of a compound with similar composition was reported for the preparation of CV-MGA compounds
[CV], 10-4M
[Ge(IV)], IOehM
Molar absorptivity. f.mole-‘.ctYL (595 nm)
1.1-2.2 1.1-3.0
0.2-16.0 0.3-I 8.0
3.2 x 10’ 1.8 x 10’
1058
F. V.
MIRZOYAN
er al.
Fig. 3. Dependence of yield of CV-MGA (1. 2, 3) and CV-isopolymolybdate (1’. 2’, 3’) on pHr, for various initial concentrations of oxalate ion. [Ge(IV)] = 1 x 10-5M; pH, = 2.2; [CV] = 1.2 x IO-“M; [Na2Mo0,]: curves I. 2-1.2 x 10-3M; curve 3.6.0 x 10-‘M. [Na2C204]: curve /.+.I.OlOM: curves 2, 3-0.080M.
earlier.’ in separation of CV-MGA compounds by flotation with organic solvent from 0.6-0.7N solutions of the acid. By the method proposed here, this compound is formed over appreciably wider ranges of concentration of the reactants. At pHr 0.8 the reacting ratio of MGA with CV is 1:3 (Fig. 4. curves 1 and 2). Therefore in the pH, range 0.8-1.0. according to the data in Fig. 2, the trisubstituted acid salt of MGA is formed, containing singly-charged CV cations. This raises the question of the nature of the 2:1 CV:GMA species referred to in the last paragraph. Arguments based on protonation of germanomolybdate would lead us to expect (CVf)r(H2GMAL-) rather than the (HCVZ+)r(GMA4-) which is postulated on the basis of the colour of the product. There seems to be no direct evidence either way. If the acidity is further decreased and oxalate is added, the MGA interacts with the dye in I:4 ratio (Fig. 4, curves 5 and 6) and the molar absorptivity of the acetone solution of the CV-MGA compound sharply increases (to 4.2 x lo5 I. mole-’ .cm- *). The reliability of the composition established for the CV-MGA compounds is confirmed by the fact that the molar absorptivity of the compound formed at pHr 0.1 is twice that of CV determined in acetone that of the (ecv = 1.05 x 10’ l.mole-l.cm-‘), pH,-O.8 compound is 3 x lcv and that for the oxalate system is 4 x cc,,. The absorbance of the CV-MGA compounds is evidently solely due to their CV content. The close correlation of the theoretically expected and experimental E values, and also the correlation of the practically observed and theoretically estimated* sensitivity of the determination of Ge clearly indicates that under these experimental conditions the CV-MGA compounds are quantitatively formed and separated. Therefore determination of the molybde* Note that if pH, = pH, = 0.8, MGA (and hence CV-MGA) will not be formed.
num content of the precipitates obtained at a definite Ge concentration should enable us to define clearly the inner co-ordination sphere of the compounds. The CV-MGA precipitate obtained under the conditions mentioned ([Moo:-] = 1.2 x 10e3M; [CV] = 1.2 x 10e4M; acidity given in Table 2) and containing 0.10 pmole of Ge(IV), after centrifuging and washing with water was dissolved in the same test-tube in 2-5 ml of concentrated sulphuric acid. The solution was diluted to volume with water in a 25ml standard flask, and its molybdenum content was determined by the thiocyanate method.13 To avoid spectral interference by the reagent dye, the molybdenum thiocyanate was extracted with 10 ml of butyl acetate and its absorbance measured at 460 nm. A blank determination was also done. A single extraction was separately shown to be quantitative. The calibration curve was linear. The mean recovery of molybdenum was identical (29; relative error) irrespective of pHr in the range 0.3-4.5 when pH, was 2.5, and irrespective of pHi (1.7-3.8) when pHr was 0.7,
0
40
cl4
06
0.2
I.6
20
Ceml
36
3.2
26
2.4
2.0
Ci,ml
Fig. 4. Continuous-variations series for the system CV-MGA. pH, = 2.5; [Moo:-] = 1.2 x l0-3M; pHf: curves 1, 2,-0.8; curves 3. h-0.1; curves 5. 64.0; [Na,C,O,] = 0.04M. Z[Ge(IV)] + [CV]: curves I, 4. 6-8.0 x lo-‘M; curves 2,3,54.0 x 10-5hf.
Molybdogermanic and corresponded to a ratio of Mo:Ge = 8: 1 in the CV-MGA compounds. It deserves attention that this is the stoichiometry of the MGA regardless of initial acidity in the range pHi = 1.7-3.8. Thus in addition to the well-known existence of the 12molybdogermanate and 1 l-molybdogermanate species in this acidity range,14,15 it can be stated that 8-molybdogermanate is also present. which appears to be stabilized as a result of the precipitation. Generalizing the results obtained, it can be conciuded that under the reaction conditions described, molybdogermanic acid (H4GeMos0aa) interacts with the dve reagent to form various ion-association compounds with different compositions, irrespective of the final acidity. The tetrasubstituted salt of MGA, which gives intensely coloured acetone solutions (E = 4.2 x 10’ I .mole” .cm-‘), is used in a highly sensitive and simple method for the photometric determination of germanium.
acid and Crystal
1059
Violet
The method has been used for determining Ge in various natural samples after its preliminary extraction and separ-
ation as GeC14.rh The results obtained were checked by the addition method. and the relative error was less than 3”,, at the
IO-‘M level.
REFERENCES
I. L. 1. Ganago and I. A. Prostak. Dokl.
Akad. Nauk SSR. 1964, 13;345. Idrm, Zh. Analit. Khim.. 1971. 26, 104. Gr. Popa and 1. Paralescu. Tdanra. 1964. IO, 315. Idem, Rrr. Chimir (Bucharer). 1970. 43, 21. V. P. Zhivopistsev and T. B. Tsheropanova. Zh. Andir. Khim., 1977. 32. 977. L. I. Ganago and I. A. Prostak. IX. Vhishikh Uchd-
Brlorousskoi
2. 3. 4. 5. 6.
0.5
nikh Zaledrnii. Khim. i Khim Teknol.. 1971, 14. 1165. 7. R. A. Chalmers and A. G. Sinclair. Anal. Chim. Acrcc. 1965, 33. 384. 8. F. Chauveau, P. Souchay and R. Schaal, Bull. Sot. Chim. France, 1959, 1190. 9. A. Hal&z and E. Pungor. Talantu. 1971. 18, 557. 565. 10. R. K. Motorkina. Zh. Ncorgan. Khim.. 1957. 2, 92. Il. Z. Ph. Shakhova and R. K. Motorkina, Merotli analix dkikh i rsrrmikh mural/or, p. 47. Izd. MGU. Moscow,
with nitric acid (the total volume must not exceed 9 ml at this point) and after mixing let the solution stand for 10
1956. 12. I. I. Alexeyeva.
min. If the volume is less than 9 ml. bring value with distilled water. then add 0.5 ml of oxalate. mix. then add 0.5 ml of O.l”,, CV shake the mixture until a precipitate appears. 3000 rpm. decant the liquid and dissolve the 10 ml of acetone. Measure the absorbance at termine the amount of Ge from a calibration
13. L. B. Zaychikova. Zarotlsk. Luh.. 1949. 15, 1025. 14. P. Souchay and A. Tchakirian. Ann. Chim.. 1946. 1, 232. 249. 15. M. Biquard and M. Lamache. Bd. Sot. Chin,. Fruncc,. 1971. 32. 16. V. A. Nazarenko. Analirichrska~u khimiu gwmaniu. p. 177. Izd Nauka. Moscow, 1973.
Procrtlurr
To the test solution containing 0.07-l 1.60 ldg of Ge, add ml of 0.024M sodium molybdate. adjust to pH 2.0-2.5
it up to this 0.2M sodium solution and Centrifuge at precipitate in 595 nm. Decurve,
Zh. Ntorgan.
Khim..
1967. 12, 1840.
in Great Britain
THE MECHANISM OF INTERACTION BETWEEN MOLYBDOGERMANIC ACID AND THE BASIC DYE CRYSTAL VIOLET F. V. MIRZOYAN, V. M. TARAYAN, E. KH. HAIRIYAN and N. A. GRICGRIAN Institute of Inorganic and General Chemistry, Armenian Academy of Sciences, Yerevan, Yerevan State University, USSR (Received 23 April 1979. Revised 1 October 1979.* Accepted 20 June 1980)
Summary-On the basis of light-absorption studies on solutions of Crystal Violet (CV) molybdogermanate in acetone, the optimal pH conditions for quantitative formation of molybdogermanic acid (MGA) have been determined. Di. tri-, and tetra-salts of MGA have been formed and isolated. It has been shown that formation of the higher salts is favoured by lowering the acidity, but this increases also the amount of the solid co-product CV-isopolymolybdate. To overcome this inconvenience the surplus molybdate ions are masked by adding oxalate ions, thus allowing the separation of the corresponding solid tetra-salt up to pH = 6.5; the molar absorptivity of this compound in acetone solution is very high (4.2 x lo5 l.mole-‘.cm-I).
It is of great interest to study the composition and separation conditions of the compounds of basic dyes with various heteropoly acids, with a view to establishing their mechanism of formation and also elucidating the conditions favouring increase of the analytical sensitivity for the determination of the element
Preparation and separation of CV-MGA
compounds
forming the heteropoly acid. So far there are few examples of basic dyes being used as reagents for molybdogermanic acid (MGA).1-6 Among the triphenylmethane dyes the highest sensitivity is given by Brilliant Green (molar absorptivity 1.93 x lo5 l.mo1e-‘.cm-‘).2 The formation of CV-MGA compounds (CV = Crystal Violet) has been studied in fairly acidic solutions (06-0.7M nitric acid), and the di-salt of MGA obtained.* This paper presents a detailed investigation of the interaction of CV and MGA over a wide range of acidity and concentration of the reacting components, with the aim of establishing the optima1 conditions for the formation and separation of more highly substituted solid salts of MGA, and thus increasing the analytical sensitivity of the determination of germanium.
To a solution [containing a definite amount of Ge(lV)] in a conical centrifuge tube, a definite amount of molybdate was added followed by nitric acid until the optimum acidity (referred to as the initial acidity, pHi) required for the quantitative formation of MGA was reached, and the volume was made up to 5 ml with distilled water. The solution was stirred and left for 10-15 min for maximum formation of MGA. Then the optimum acidity (called the final acidity, pHr) for the separation of CV-MGA was established, and a certain amount of oxalate solution (if necessary) and the reagent dye were added and the volume was brought to 10 ml with distilled water. After mixing and formation of the considerable amount of precipitate. the mixture was’ centrifuged, the solution carefully decanted and the pH of the solution measured. The precipitate was washed in a test-tube with 2 ml of water, then separated by centrifuging and dissolved in 10 ml of acetone. The degree of combination of Ge(IV) in MGA and then in CV-MGA was estimated from the absorbance of the acetone solution. A blank test was performed to check the formation of CV-isopolymolybdate salts. The absorbance A of the solutions was measured at 595 nm. in l-mm cells. Solid compounds were separated by centrifuging for 1-2 min at 3000 rpm.
EXPERIMENTAL
RESULTS AND DISCUSSION
Reagenrs
The optimal
A 0.005M solution of Ge(lV) (pH 7.2) was prepared by dissolving the appropriate weight of (especially pure) GeOz in distilled water by adding small portions of sodium hydroxide solution, and was further diluted as required. Reagents used were sodium molybdate (purej, 0.024 and 0.012M solutions, CV (pure) 0.1% aqueous solution, sodium oxalate (pure) aqueous solution, nitric acid (especially pure. s.g. 1.41) acetone (pure). All the solutions were kept in polyethylene bottles.
* Lost in the post; copy received 17 June 1980.
conditionsjar the formation of MGA
In establishing the optimal acidity for the formation of MGA some authors’ have used the lightabsorption properties of MGA and tried to find the conditions under which the yellow colour of MGA is maximal. However, it is known that, depending on the acidity and the concentration of molybdate, various isomers of MGA (a- and @MGA) are formed, which differ in their stability and spectral characteristics.7-9 Therefore the maximum development of the yellow colour
1055
F. V. MIRZOYAN et al.
1056
1.0
2.0
30
40
w
Fig. I. Dependence of yield of MGA on acidity at various initial concentrations of molybdate ion. [Ge(IV)] = 1 x 10-‘M: [CV] = 1.2 x 10-4M; pH, = 0.8; [Moo:-]: I, 2.4 x 10-3M; 2, 1.2 x 10-3M; 3,0.7 x 10-3M.
of MGA does not unequivocally
indicate the quantitative formation of MGA. Consequently it is more convincing to investigate the formation of MGA not on the basis of its own light-absorption characteristics, but on that of the light-absorption of the salt of MGA with CV. It should also be taken into account that though its formation is very sensitive to the acidity of the medium, MGA (once formed) can exist in more acidic medium without being decomposed.*‘*” This allows the complete suppression of the formation of isopolymolybdates, by increasing the acidity to pH, = 0.8, at which decomposition of MGA does not take place, and accordingly a quantitative yield of CV-MGA is ensured. Figure 1 shows the dependence of absorbance on pHi for acetone solutions of CV-MGA at various initial concentrations of molybdate ions (pHi was measured in separate experiments). The acidity intervals given in Fig. 1 show the optimal conditions for MGA formation.* The validity of the considerations above is confirmed and the acidity interval for the formation of MGA is clearly indicated (pHi = 1.5-3.8 for 1.2 x lo-‘M molybdate and pH, = 1.2-3.8 for 2.4 x 10W3A4molybdate). Thus, the absorbance of the acetone solutions obtained under these optimal conditions is practically independent of the molybdate concentration and the acidity, in contrast to the behavior of MGA itself. Hence CV-MGA is of greater interest for analytical use. The oprimal conditions for separating pounds
CV-MGA
com-
Figure 2 shows the yield of CV-MGA compounds as a function of pHI at constant pHi. It shows that pHf = 0.7-0.9 ensures maximal and practically constant absorbance for the acetone solutions. * Estimated cm-‘.
on the basis of c = 4.2 x lo5 l.mole-’
.
The precipitates of CV-MGA are violet, characteristic of the singly-charged form of CV. If the pH is lowered the acetone solution decreases in absorbance, and the solid compound becomes green (acidity from pHI 0.4 to 4M nitric acid). From the colour of the precipitate it can be concluded that under these conditions MGA interacts with the green singly-protonated doubly-charged form of CV. An acetone solution of the product is violet, however, and presumably, when the precipitate is dissolved in acetone, the doubly-charged form of CV changes into the singlycharged form. Solutions of CV-MGA, at acidities from pH 0.4 to 2.OM nitric acid, have practically constant molar absorptivity (1.8 x 10’ l.mole- ‘.cm- I), which is indicative of the constancy of the composition of the CV-MGA product under these conditions. It therefore appears that the system stu$ed can give rise to compounds of various compositions. Figure 2 shows that the absorbance of an acetone solution of CV-MGA is a function of pHP but is independent of it over two ranges, in the vicinity of pHr = 0.1 and 0.8, the absorbance being greater at the higher pHI value. For both these pHI values the concentration ranges of the reactants that will give constant absorbance for the product have been determined. They also permit the pure CV-MGA compounds to be obtained. They are listed in Table 1. Use of higher concentrations of the components leads to appreciable separation of CV-isopolymolybdates and hence to decrease in the yield of CV-MGA compounds. The preparation of CV-MGA acidity conditions for MGA
compounds
at optimal
According to the results above the investigation of the interaction between MGA and CV at pH, > 1.0 is impossible because of the simultaneous precipitation of CV-isopolymolybdates (Fig. 2). which have similar spectral characteristics to those of the CV-MGA compounds. Hence it is necessary to mask the free
Molybdogermanic
1057
acid and Crystal Violet
Fig. 2. Dependence of yield of (1) CV-MGA and (2) CV-isopolymolybdate on pH,. [Ge(IV)] = = 1.2 x 10-3M. 1 x IO-5M; pH, = 2.5; [CV] = 1.2 x 10-4M; [Moo:-]
isopolymolybdate ions. Earlier this was done with oxalic acid at high acidity (0.60.7M nitric acid) which lowers the efficiency of the masking action. It was now found that the formation of the isopolymolybdates can be prevented by adding mineral acid to give pH < 1.0, without the need for the masking agent (Fig. 2). In establishing the optimal acidity for the formation of MGA. it was shown that the quantitative formation of MGA is possible at relatively low acidity (pHi 1.5-3.8). This makes it possible to form CV-MGA in this acidity range. with simultaneous masking of isopolymolybdates with oxalic acid. The effect of oxalate on the yield of CV-MGA as a function of pH is shown in Fig. 3. It can be seen that CV-MGA is formed from a 1.2 x 10m3M sodium molybdate/O.OlM oxalate solution in the acidity range pHr 0.5-7.0, quantitatively at pHr 1.5-2.5 or 4.0-6.0. The yield is lower at pHr 2.M.O because of the concurrent reactions of CV with both isopolymolybdates and MGA under these reaction conditions (Fig. 3. curve 1). According to Alexeyeva,” the HMo20; ion occurs in this acidity range, and this (or some similar species) appears to form a precipitate with CV. causing a significant rise in the absorbance of the blank solutions. An eightfold increase in the oxalate concentration (Fig. 3. curve 2) does not affect the quantitative separation of CV-MGA in the pH range 2.0-5.5 but completely eliminates the formation of CV-isopolymolybdates. Unfortunately MGA is stable for only 15 min under these conditions. Therefore it must be prepared and separated during that interval of time. The masking efficiency of the oxalate is mainly determined by the molybdate concentration. The yield Table 1. The optimal reagent concentrations
PH, 0.8
0.1
[Moo:-].
10-3M
0.8-1.8 0.848
of CV-MGA as a function of pH was studied at O.OSM oxalate concentration and fairly high concentrations of molybdate (Fig. 3. curve 3). Under these conditions the optimal range of acidity for formation of CV-MGA is expanded (pH 2.0-6.5). At pH 4.5 and constant concentration of molybdate (1.2 x 10e3M) and oxalate (1.0 x lo-‘M) the yield of CV-MGA was found to be quantitative with (0.9-5.0) x 10_4M cv. The correlation of these results with data obtained in the absence of oxalate indicates that the proposed variant of the method has markedly higher sensitivity. The acetone solutions of CV-MGA formed in presence of oxalate obey Beer’s law over the Ge(lV) concentration range 1 x lo-‘-l.6 x 10v5M. the molar absorptivity being 4.2 x 10’. This fact is unequivocally indicative of the change in composition of the CV-MGA compounds. The composition of CV-MGA cornpods The composition of the principal compounds formed between CV and MGA was determined by Job’s method (Fig. 4), at different final acidities: pHr 0.1 (curves 3 and 4) and pHr 0.8 (curves 1 and 2) and also at pHr 4.0 in the presence of oxalate as masking agent (curves 5 and 6). The maximum in the curves obtained at pH, 0.1 corresponds to the ratio CV : MGA = 2 : 1. Therefore. in more acidic solutions (acidity between pHr 0.4 and 2M nitric acid). according to the data in Fig. 2. the MGA salt precipitated contains the singly-protonated doubly-charged cationic form of the reagent dye (HCV”) (but see next paragraph). The formation of a compound with similar composition was reported for the preparation of CV-MGA compounds
[CV], 10-4M
[Ge(IV)], IOehM
Molar absorptivity. f.mole-‘.ctYL (595 nm)
1.1-2.2 1.1-3.0
0.2-16.0 0.3-I 8.0
3.2 x 10’ 1.8 x 10’
1058
F. V.
MIRZOYAN
er al.
Fig. 3. Dependence of yield of CV-MGA (1. 2, 3) and CV-isopolymolybdate (1’. 2’, 3’) on pHr, for various initial concentrations of oxalate ion. [Ge(IV)] = 1 x 10-5M; pH, = 2.2; [CV] = 1.2 x IO-“M; [Na2Mo0,]: curves I. 2-1.2 x 10-3M; curve 3.6.0 x 10-‘M. [Na2C204]: curve /.+.I.OlOM: curves 2, 3-0.080M.
earlier.’ in separation of CV-MGA compounds by flotation with organic solvent from 0.6-0.7N solutions of the acid. By the method proposed here, this compound is formed over appreciably wider ranges of concentration of the reactants. At pHr 0.8 the reacting ratio of MGA with CV is 1:3 (Fig. 4. curves 1 and 2). Therefore in the pH, range 0.8-1.0. according to the data in Fig. 2, the trisubstituted acid salt of MGA is formed, containing singly-charged CV cations. This raises the question of the nature of the 2:1 CV:GMA species referred to in the last paragraph. Arguments based on protonation of germanomolybdate would lead us to expect (CVf)r(H2GMAL-) rather than the (HCVZ+)r(GMA4-) which is postulated on the basis of the colour of the product. There seems to be no direct evidence either way. If the acidity is further decreased and oxalate is added, the MGA interacts with the dye in I:4 ratio (Fig. 4, curves 5 and 6) and the molar absorptivity of the acetone solution of the CV-MGA compound sharply increases (to 4.2 x lo5 I. mole-’ .cm- *). The reliability of the composition established for the CV-MGA compounds is confirmed by the fact that the molar absorptivity of the compound formed at pHr 0.1 is twice that of CV determined in acetone that of the (ecv = 1.05 x 10’ l.mole-l.cm-‘), pH,-O.8 compound is 3 x lcv and that for the oxalate system is 4 x cc,,. The absorbance of the CV-MGA compounds is evidently solely due to their CV content. The close correlation of the theoretically expected and experimental E values, and also the correlation of the practically observed and theoretically estimated* sensitivity of the determination of Ge clearly indicates that under these experimental conditions the CV-MGA compounds are quantitatively formed and separated. Therefore determination of the molybde* Note that if pH, = pH, = 0.8, MGA (and hence CV-MGA) will not be formed.
num content of the precipitates obtained at a definite Ge concentration should enable us to define clearly the inner co-ordination sphere of the compounds. The CV-MGA precipitate obtained under the conditions mentioned ([Moo:-] = 1.2 x 10e3M; [CV] = 1.2 x 10e4M; acidity given in Table 2) and containing 0.10 pmole of Ge(IV), after centrifuging and washing with water was dissolved in the same test-tube in 2-5 ml of concentrated sulphuric acid. The solution was diluted to volume with water in a 25ml standard flask, and its molybdenum content was determined by the thiocyanate method.13 To avoid spectral interference by the reagent dye, the molybdenum thiocyanate was extracted with 10 ml of butyl acetate and its absorbance measured at 460 nm. A blank determination was also done. A single extraction was separately shown to be quantitative. The calibration curve was linear. The mean recovery of molybdenum was identical (29; relative error) irrespective of pHr in the range 0.3-4.5 when pH, was 2.5, and irrespective of pHi (1.7-3.8) when pHr was 0.7,
0
40
cl4
06
0.2
I.6
20
Ceml
36
3.2
26
2.4
2.0
Ci,ml
Fig. 4. Continuous-variations series for the system CV-MGA. pH, = 2.5; [Moo:-] = 1.2 x l0-3M; pHf: curves 1, 2,-0.8; curves 3. h-0.1; curves 5. 64.0; [Na,C,O,] = 0.04M. Z[Ge(IV)] + [CV]: curves I, 4. 6-8.0 x lo-‘M; curves 2,3,54.0 x 10-5hf.
Molybdogermanic and corresponded to a ratio of Mo:Ge = 8: 1 in the CV-MGA compounds. It deserves attention that this is the stoichiometry of the MGA regardless of initial acidity in the range pHi = 1.7-3.8. Thus in addition to the well-known existence of the 12molybdogermanate and 1 l-molybdogermanate species in this acidity range,14,15 it can be stated that 8-molybdogermanate is also present. which appears to be stabilized as a result of the precipitation. Generalizing the results obtained, it can be conciuded that under the reaction conditions described, molybdogermanic acid (H4GeMos0aa) interacts with the dve reagent to form various ion-association compounds with different compositions, irrespective of the final acidity. The tetrasubstituted salt of MGA, which gives intensely coloured acetone solutions (E = 4.2 x 10’ I .mole” .cm-‘), is used in a highly sensitive and simple method for the photometric determination of germanium.
acid and Crystal
1059
Violet
The method has been used for determining Ge in various natural samples after its preliminary extraction and separ-
ation as GeC14.rh The results obtained were checked by the addition method. and the relative error was less than 3”,, at the
IO-‘M level.
REFERENCES
I. L. 1. Ganago and I. A. Prostak. Dokl.
Akad. Nauk SSR. 1964, 13;345. Idrm, Zh. Analit. Khim.. 1971. 26, 104. Gr. Popa and 1. Paralescu. Tdanra. 1964. IO, 315. Idem, Rrr. Chimir (Bucharer). 1970. 43, 21. V. P. Zhivopistsev and T. B. Tsheropanova. Zh. Andir. Khim., 1977. 32. 977. L. I. Ganago and I. A. Prostak. IX. Vhishikh Uchd-
Brlorousskoi
2. 3. 4. 5. 6.
0.5
nikh Zaledrnii. Khim. i Khim Teknol.. 1971, 14. 1165. 7. R. A. Chalmers and A. G. Sinclair. Anal. Chim. Acrcc. 1965, 33. 384. 8. F. Chauveau, P. Souchay and R. Schaal, Bull. Sot. Chim. France, 1959, 1190. 9. A. Hal&z and E. Pungor. Talantu. 1971. 18, 557. 565. 10. R. K. Motorkina. Zh. Ncorgan. Khim.. 1957. 2, 92. Il. Z. Ph. Shakhova and R. K. Motorkina, Merotli analix dkikh i rsrrmikh mural/or, p. 47. Izd. MGU. Moscow,
with nitric acid (the total volume must not exceed 9 ml at this point) and after mixing let the solution stand for 10
1956. 12. I. I. Alexeyeva.
min. If the volume is less than 9 ml. bring value with distilled water. then add 0.5 ml of oxalate. mix. then add 0.5 ml of O.l”,, CV shake the mixture until a precipitate appears. 3000 rpm. decant the liquid and dissolve the 10 ml of acetone. Measure the absorbance at termine the amount of Ge from a calibration
13. L. B. Zaychikova. Zarotlsk. Luh.. 1949. 15, 1025. 14. P. Souchay and A. Tchakirian. Ann. Chim.. 1946. 1, 232. 249. 15. M. Biquard and M. Lamache. Bd. Sot. Chin,. Fruncc,. 1971. 32. 16. V. A. Nazarenko. Analirichrska~u khimiu gwmaniu. p. 177. Izd Nauka. Moscow, 1973.
Procrtlurr
To the test solution containing 0.07-l 1.60 ldg of Ge, add ml of 0.024M sodium molybdate. adjust to pH 2.0-2.5
it up to this 0.2M sodium solution and Centrifuge at precipitate in 595 nm. Decurve,
Zh. Ntorgan.
Khim..
1967. 12, 1840.