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Spectrophotometric study of the chromium(III) azide reaction
J. Inotg. Nu¢I. Chem., 1962, Vol. 24, pp. 1373 to 1379. l ~ l l m l o n Prma Ltd. Printed in F-naland
SPECTROPHOTOMETRIC STUDY OF THE CHROMIUM(Ill) AZIDE REACTION F. G. SHERIF, W. M. OgAeY and Hussen~ SADEK Faculty of Science, Alexandria University, Alexandria, Egypt (Received 18 January 1962; in revised form 20 March 1962)
Abstract---Conditions for the use of the azide as a spectrophotometric reagent for Cr(IH) are investigated. The system obeys Beer's law in the range of 4-320 p.p.m. The colour is affected by acid concentration; maximum colour development is attained in a pH range of 4.2-5.2. Interference due to CuZ+, Fe3+ and UO22+ ions can be reduced by adding a few ml of EDTA. The logarithmic method confirmed the existence of a monoazidochromimn(III) ion in dilute solutions. In concentrated solutions, Job's method of continuous variation revealed the existence of a series of complexes between chromium and azide ions varying in composition from 1: 1 (green) to 1:6 (violet) chromium to azide. IN connection with studies entailing the reaction of Cr0II) (violet form) with sodium azide in aqueous solution and the characterisation of the structure of the possible complex ions,O) it became necessary to study the use of the coloured solutions for the spectrophotometric determination of minute amounts of Cr(III), Only a limited amount of data is available in the literature concerning the determination of Cr(III) by colormetric methods. Other spectrophotometric determinations are generally based on the oxidation to Cr(VI) in aqueous solutions prior to measurement. This is probably due to the slow reaction o f the strongly hydrated Cr(III) ions with many ligands. However, oxidation requires severe conditions for accurate results. The azide reaction has been used recently for the determination of a few transition elements, such as Fe3+,(2) Cu2+(3) and UO22+. (4) The azide ion N], similar to the thiocyanate, is a member of the pseudohalogenoid group and forms highly intense coloured complexes. Sodium azide, a strong electrolyte, has no absorption in the visible range: this is an advantage in spectrophotometric measurements. In spite of the high sensitivity of this reagent towards Cr(III), its use is limited to weakly acidic solutions. Strong mineral acids decompose the colour owing to the displacement of the weakly dissociated hydrazoic acid. Nevertheless, the determination can always be carried out in solutions with adjusted pH values and traces of chromium can thus be determined. Conditions for this procedure are discussed in the present investigation. The method of continuous variation of Joe(5) was applied to chromium azide in relatively concentrated solutions, so as to indicate the possible formation of different compleaed species in the same solution. a~ F. G. SNmtrf and W. M. OPo~Y, J. lnorff. Nu¢l. Chem. 17, 152 (1961). ~2~ H . K . EL-SRAMYand F. G. Snm~, Egypt. J. Chem. 1, 35 (1958); 2, 217 (1959). c3~G. S ~ I and G. OSTOC~.~ J. Inarg. Nucl. Chem. 8, 346 (1958). ~4~F. G. S ~ and A. M. AWAD,J. Inorg. Nucl. Chem. 19, 94 (1961). c5~p. Joe, C.R. Acad. Sci., Paris, 180, 928 (1925).
1373
1374
F . G . SFvmn,, W. M. Og,dm,,,and H ~
SAV~
EXPERIMENTAL The preparation and analysis of the different solutions used, together with the experimental proc~ures are given in a previous article.a) The hydrated violet form of chromium nitrate was used. Colour intensities were measured by a Unieam speetrophotometer S.P.500. The pH values were measured using a Cambridge pH meter, model G. RESULTS
Effect of concentration of reagents The absorbancy of one series of solutions containing 0.008 M Cr(III) and 0.02 N HC104 and varying amounts of I M azide solution (25 ml total volume) was measured at the wavelength 440 m/z. From the data presented in Table 1, it appears that the maximum intensity of the colour was reached when the concentration of the reagent reached 0"2 M. Further addition of azide resulted in turbidity and eventually basic chromium azide was precipitated. However, in concentrated solutions the precipitate dissolved after 48 hr and the solution turned violet. In another series of solutions containing the above concentration of Cr(III) and 0.2 M HaNs, varying amounts of 1 N HCIO4 were added and the total volume adjusted to 25 ml. Maximum absorbancy was reached when the concentration of acid was 0.06 N. Higher concentration of the acid resulted in a decrease in intensity and the solutions smelled strongly of hydrazoic acid. The pH values of this series of solutions were measured. TABLeI.--E~CT OFX~OBNTSON COLOURINTENSITYCr(IlI) = 0.008 M Conch. of Nin moles3 Absorbancy (H + = 0.02 N) 440 mtt 0"04 0"08
0"12 0"16 0"20
0"27 0"54 0-61 0"65 0"67
Conc. of HCIO4 N (N] = 0.2 M) 0"01 0"02
0-03 0"04 0"06
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0"24
0'64
pH 5"48 5"39 5"22 5"05 4"77
Absorbancy 440 mtt 0"64 0"67
0"75 0"81 0"84
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0-08 0"10 0"14
4"46 4"24 3"94
° .........
0"82 0"80 0-58
The results are shown in Table I. It was found that the optimum pH range for the maximum development of colour for analytical determinations lay between 4.2-5.2. Subsequent measurements were carried out using solutions within this acid concentration. Small changes in acid concentration in this range of pH have negligible effects on the colour, because the solutions are buffered by the HN3-NaN3 mixture formed. Acetate and phosphate buffer solutions could not be used to adjust the pH values as the Cr(III) ion forms interfering complexes with them. When increasing amounts of different acids were added to chromium azide solutions prepared from NaN3 andchrominm perchlorate, nitrate, sulphate, chloride and acetate respectively, the colour intensity decreased irrespective of the anion of the acid.
Speetrophotometric study of the chromium(HI) azide reaction
1375
In order to study the effect of anions, solutions of chromium azide were prepared from different salts of 0.02 M Cr(III) and 0.02 M azide with no adjustment of pH. It was found that the perchlorate solution was the most intense and the acetate was the least. The order of colour intensity was CIO~4>NOi>CI->SO~->Ch3COO-. This is in agreement with the well known fact that the perchlorate ion is the least complexing, giving enough free Cr(III) ions to react with the azide, while the chromium acetate complex is the most stable.
Stability of colour The absorbancy of a solution which was 0.008 M with respect to Cr(III) and contained the optimum concentrations of reagents was measured periodically at 440 m# against a reference solution containing the reagents. The temperature of the solution was kept at room temperature (24-28 °C). The rate of the reaction between the chromium and azide ions was found to be very slow. The absorbancy value was doubled in one hour. Then it increased slowly and the deep green colour of the complex seemed to be fully developed after two hours from mixing and did not change for at least 6 hr. The absorbancies of some solutions were found to be unchanged after 2 days.
Effect of temperature and light Solutions containing excess of azide were found to be stable to temperature changes in a range of 15-45°C. There was a negligible difference between the intensifies of solutions at varying temperatures and those kept at room temperature as long as the colour was already fully developed. No difference was found between two solutions containing the same chromium concentration and reagents, one being kept in the dark and the other exposed to daylight over the same period of time (2 days).
Conformity to Beer's law A number of solutions containing varying amounts of chromium and the optimum amounts of azide and perehloric acid were prepared and their absorhancies measured at 440 m/~ after 2 hr from mixing, using a reagent blank. A plot of absorbancy values against Cr(III) concentration gave a straight line passing through the origin, showing that Beer's law is obeyed in a range of 4-320 p.p.m. Cr(III), with a sensitivity of 2 p.p.m. This method has a distinct advantage of providing a wide range of applicability. The lower limit could be extended si~ificantly by the use of longer cells. This is especially promising as the absorption of the reagent itself is negligible.
Effect of diverse ions In order to study the effect of diverse ions, solutions containing 0.008 M Cr3 +, 0.2 M N 3- and 0.06 N H + were prepared and solutions of different ions were then added in a titration fashion. The absorbancy of the mixture was measured after each addition, allowing for colour development of the original solution prior to measurements. The results are corrected for volume changes. A change of -t-0.01 of absorbancy unit is considered to be interference. The results are given in Table 2. The interference of Cu2+ and UO22+ ions can be l'educ~d appreciably by the addition of 1 ml of 0.05 M solution of EDTA. Interference of Fe3 + ions could only be decreased by the addition of 2 ml of EDTA and measurement at 500 m/~. The phosphate ions form a precipitate after the addition of few fractions of a ml of the solution. Thorium behaves similarly, but suitable increase of acid concentration will prevent precipitation.
F. G. St~Jr, W. M. O~mY and Hus~JN S~a3eg
1376
TAm.e 2.--MAXlJm~ TOI,BRANCEOF INTERFERINGIONS Cr3+ = 0~08 M; N3- = 0"2M; H + = 0 ~ 6 N ; ~ = 440p
Ion
Added as
Cu2+
CuCI2
U022+
UO2(NO3)2
Fe 3+ Th4 + Ni 2+ Mn 2+ Ce3+ NH4 + n82+ VO3-
Fe(NO3)3
F-
0.3 (35) (34) 0.1 (33)
6
Th(NO3)4
MO70246WO42Cr2072P O 4 3-
Maximum tolerance (p.p.m.)
NiCI2 MnCI2 Ce(NO3)3 NH4NO3 HgC12 NaVO3 (NH4)6M07024 Na2WO4 K2Cr207 Na3PO4 NaF
46 96 130 110 24 240 12 32 300 170 300
160
Values in parentheses for Cu2+ and UO22+ are taken in presence of 1 ml of 0.05 M EDTA. The value for Fe3+ is in presence of 2 ml of EDTA at Z - 500 g. Structure o f the chromium azide complex in dilute solutions
It was shown previously that a monoazidochromium(III) ion, CrN32+, can be formed in dilute aqueous solutions containing chromium nitrate and sodium azide,(1) that have two absorption maxima at 440 and 605 m#. The logarithmic method o f BENT and FRENCH(6) was applied h e r e to further confirm these results. In one series of solutions the total concentration of chromium was kept constant at 0.024 M and that o f azide was varied from 0.002 to 0.012 M, keeping the ionic strength constant at 0.156 by adding the appropriate amounts of sodium perchlorate solution. The absorbancy was measured at 440 rap. The logarithm of the absorbancy was a linear function o f the logarithm o f the N] ion concentration (Fig. 1). In a second series of solutions the concentration of sodium azide was kept constant at 0.012 M and that of chromium was varied from 0-004 to 0.020 M, keeping the ionic strength the same as before. The logarithmic relation was similarly obtained. The reaction between chromium and azide ions can be represented by the equation: m Cr3+ + n N] = [Crm(N3)~](3"'~) +
(1)
The logarithm of absorbancy will be proportional to log [CrmN3~] in the logarithmic expression: Log [CrmN3,~ ----m log [CrS+]+n log [ N i l + l o g K
(2)
The experimental points were found to fall on the theoretical line with the assumption that m --: n = 1.
(e) H. E. BE~rr and C. L. F~NC~ J'. Amer. Chem. Soc. 63, 568 (1941).
Spectrophotometrk study of the chromium(HI) azide reaction
I /
- 1.9
/ .~/
1377
I
I
I
I
-2.1
/ !
~ -2.3 _1
/ S
/
I
/
S
I
S J
Ao
/ -2.5
-I "5
S
S
I S
-2-7
S
S
-I "3
-I-I Log
-0"9
-0.7
Qb$orboncy
FIG. 1.--Limiting logarithmic method at wavelength 440 rap. Cr3+:0~24 M and azide conen. varied from 0.002 to 0.012 M. Ionic strength = 0"156. The circles are the experimental points. The dotted lines represent the theoretical values for n = 0.5, 1 and 2, where n is the number of azide ions associated with one Cr3 + ion in the complex.
Structure of the complex in concentrated solutions The method of continuous variations of Joe(5) was appfied at different wavelengths and at different total concentrations. The contours of the curves obtained in every case would be indicative of the formation of different coloured species with different stabilities and absorption properties. Deformed curves might be obtained in the stepwise formation of complex ions formed from a metallic ion and the same ligand, if the formation constants and maximum absorptions are not widely separated from each other. In our work, the observed absorbancy values were plotted against the mole fraction of the metallic ion rather than Y values (Y = difference between observed values and those calculated for no reaction) as stated by Joe, since it was found that there is no difference between the contours of the curves in both cases. The blank used was a chromium nitrate solution of the same concentration as that present in the test solution. Three different runs with total concentrations of 0.04, 0.16 and 0-32 M were carried out using equimolar solutions. In our previous report,(z) it was stated that a 1"1 compound, Cr N32+ is formed in dilute solution, at a total concentration of 0.008 M. In Fig. 2, the irregularity of the curves near the maximum can be smoothed to one sharp maximum at a mole fraction of Cr(III) of 0-333 suggesting the formation of a 1:2 complex, Cr(N3)2 +. When the rules of VOSaUt~OH and COOPER(7) were applied, the solid curve was obtained, showing one definite maximum at 1:2 molar ratio, assuming that the 1 : 1 complex was first formed. At a (7) W. C. VOSBtmOHand G. R. Coopmt, J. Amer. Chem. Soc. 63, 437 (1941).
1378
F. O. SHm~, W. M. Otv,nY and H L m ~ SAD~
O'7
o
(~t
O.Z
'
'
0.3
04
Mole
'
o
fraction,
0
o.8
0-9
Cr '3
l~o. 2.--Continuous variation method; total concentration of Cr(III) and azide ions is 0.04 M. Y' = Observed values minus those calculated for a 1:1 complex. total concentration of 0.16 M, the results show that the prevailing species in solution are the 1 "3 complex. In solutions having a total concentration of 0.32 M (Fig. 3) there is sufficient contribution from the 1:1 form to produce a definite asymmetry
f'6
1.4
1.2
~
1.0
o 0.8
0.6
0"4
0"2
0
Oil
0"2
0
Mole
0-5
fraclion,
0'7
0.9
Cr °3
Pro. 3.--Continuous variation method; total concentratioa of Cr(III) and azide ions is 0.32 M. Y"--Observed values minus throe calculated for 1:I C"r:n~ide.
Speetrophotometric study of the chromium(IIl) azide reaction
1379
in the curves, as shown by the shoulder at a mole fraction of Cr(llI) of 0.45. However, there is a clear maximum at longer wavelengths, 480 and 500 m/~, indicating the formation of a 1:6 compound Cr(N3)~-. As we approach shorter wavelengths, e.g. 460 mp, the maximum falls at a ratio of 1:4, Cr(N3)4-. By applying the rules of VOSBURGH and COOPER and plotting the difference between the observed values and those calculated for 1 ~1, the interference in the different curves due to the 1 : 1 form disappears, and a curve with one maximum at a ratio of 1 "6 is obtained. These data show that the increase in concentration of reagents leads to the formation of azide-rich complexes. Higher complexes are easier to identify as we approach longer wavelengths. These results are confirmed by observing a hypsochromic effect produced upon the gradual increase in azide ion concentration with respect to a solution 0.04 M Cr(Ill). The colour changes gradually from deep green to distinct violet, and the maxima change from 440 and 605 m/~ to 490 and 655 m/t respectively.
SPECTROPHOTOMETRIC STUDY OF THE CHROMIUM(Ill) AZIDE REACTION F. G. SHERIF, W. M. OgAeY and Hussen~ SADEK Faculty of Science, Alexandria University, Alexandria, Egypt (Received 18 January 1962; in revised form 20 March 1962)
Abstract---Conditions for the use of the azide as a spectrophotometric reagent for Cr(IH) are investigated. The system obeys Beer's law in the range of 4-320 p.p.m. The colour is affected by acid concentration; maximum colour development is attained in a pH range of 4.2-5.2. Interference due to CuZ+, Fe3+ and UO22+ ions can be reduced by adding a few ml of EDTA. The logarithmic method confirmed the existence of a monoazidochromimn(III) ion in dilute solutions. In concentrated solutions, Job's method of continuous variation revealed the existence of a series of complexes between chromium and azide ions varying in composition from 1: 1 (green) to 1:6 (violet) chromium to azide. IN connection with studies entailing the reaction of Cr0II) (violet form) with sodium azide in aqueous solution and the characterisation of the structure of the possible complex ions,O) it became necessary to study the use of the coloured solutions for the spectrophotometric determination of minute amounts of Cr(III), Only a limited amount of data is available in the literature concerning the determination of Cr(III) by colormetric methods. Other spectrophotometric determinations are generally based on the oxidation to Cr(VI) in aqueous solutions prior to measurement. This is probably due to the slow reaction o f the strongly hydrated Cr(III) ions with many ligands. However, oxidation requires severe conditions for accurate results. The azide reaction has been used recently for the determination of a few transition elements, such as Fe3+,(2) Cu2+(3) and UO22+. (4) The azide ion N], similar to the thiocyanate, is a member of the pseudohalogenoid group and forms highly intense coloured complexes. Sodium azide, a strong electrolyte, has no absorption in the visible range: this is an advantage in spectrophotometric measurements. In spite of the high sensitivity of this reagent towards Cr(III), its use is limited to weakly acidic solutions. Strong mineral acids decompose the colour owing to the displacement of the weakly dissociated hydrazoic acid. Nevertheless, the determination can always be carried out in solutions with adjusted pH values and traces of chromium can thus be determined. Conditions for this procedure are discussed in the present investigation. The method of continuous variation of Joe(5) was applied to chromium azide in relatively concentrated solutions, so as to indicate the possible formation of different compleaed species in the same solution. a~ F. G. SNmtrf and W. M. OPo~Y, J. lnorff. Nu¢l. Chem. 17, 152 (1961). ~2~ H . K . EL-SRAMYand F. G. Snm~, Egypt. J. Chem. 1, 35 (1958); 2, 217 (1959). c3~G. S ~ I and G. OSTOC~.~ J. Inarg. Nucl. Chem. 8, 346 (1958). ~4~F. G. S ~ and A. M. AWAD,J. Inorg. Nucl. Chem. 19, 94 (1961). c5~p. Joe, C.R. Acad. Sci., Paris, 180, 928 (1925).
1373
1374
F . G . SFvmn,, W. M. Og,dm,,,and H ~
SAV~
EXPERIMENTAL The preparation and analysis of the different solutions used, together with the experimental proc~ures are given in a previous article.a) The hydrated violet form of chromium nitrate was used. Colour intensities were measured by a Unieam speetrophotometer S.P.500. The pH values were measured using a Cambridge pH meter, model G. RESULTS
Effect of concentration of reagents The absorbancy of one series of solutions containing 0.008 M Cr(III) and 0.02 N HC104 and varying amounts of I M azide solution (25 ml total volume) was measured at the wavelength 440 m/z. From the data presented in Table 1, it appears that the maximum intensity of the colour was reached when the concentration of the reagent reached 0"2 M. Further addition of azide resulted in turbidity and eventually basic chromium azide was precipitated. However, in concentrated solutions the precipitate dissolved after 48 hr and the solution turned violet. In another series of solutions containing the above concentration of Cr(III) and 0.2 M HaNs, varying amounts of 1 N HCIO4 were added and the total volume adjusted to 25 ml. Maximum absorbancy was reached when the concentration of acid was 0.06 N. Higher concentration of the acid resulted in a decrease in intensity and the solutions smelled strongly of hydrazoic acid. The pH values of this series of solutions were measured. TABLeI.--E~CT OFX~OBNTSON COLOURINTENSITYCr(IlI) = 0.008 M Conch. of Nin moles3 Absorbancy (H + = 0.02 N) 440 mtt 0"04 0"08
0"12 0"16 0"20
0"27 0"54 0-61 0"65 0"67
Conc. of HCIO4 N (N] = 0.2 M) 0"01 0"02
0-03 0"04 0"06
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0"24
0'64
pH 5"48 5"39 5"22 5"05 4"77
Absorbancy 440 mtt 0"64 0"67
0"75 0"81 0"84
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0-08 0"10 0"14
4"46 4"24 3"94
° .........
0"82 0"80 0-58
The results are shown in Table I. It was found that the optimum pH range for the maximum development of colour for analytical determinations lay between 4.2-5.2. Subsequent measurements were carried out using solutions within this acid concentration. Small changes in acid concentration in this range of pH have negligible effects on the colour, because the solutions are buffered by the HN3-NaN3 mixture formed. Acetate and phosphate buffer solutions could not be used to adjust the pH values as the Cr(III) ion forms interfering complexes with them. When increasing amounts of different acids were added to chromium azide solutions prepared from NaN3 andchrominm perchlorate, nitrate, sulphate, chloride and acetate respectively, the colour intensity decreased irrespective of the anion of the acid.
Speetrophotometric study of the chromium(HI) azide reaction
1375
In order to study the effect of anions, solutions of chromium azide were prepared from different salts of 0.02 M Cr(III) and 0.02 M azide with no adjustment of pH. It was found that the perchlorate solution was the most intense and the acetate was the least. The order of colour intensity was CIO~4>NOi>CI->SO~->Ch3COO-. This is in agreement with the well known fact that the perchlorate ion is the least complexing, giving enough free Cr(III) ions to react with the azide, while the chromium acetate complex is the most stable.
Stability of colour The absorbancy of a solution which was 0.008 M with respect to Cr(III) and contained the optimum concentrations of reagents was measured periodically at 440 m# against a reference solution containing the reagents. The temperature of the solution was kept at room temperature (24-28 °C). The rate of the reaction between the chromium and azide ions was found to be very slow. The absorbancy value was doubled in one hour. Then it increased slowly and the deep green colour of the complex seemed to be fully developed after two hours from mixing and did not change for at least 6 hr. The absorbancies of some solutions were found to be unchanged after 2 days.
Effect of temperature and light Solutions containing excess of azide were found to be stable to temperature changes in a range of 15-45°C. There was a negligible difference between the intensifies of solutions at varying temperatures and those kept at room temperature as long as the colour was already fully developed. No difference was found between two solutions containing the same chromium concentration and reagents, one being kept in the dark and the other exposed to daylight over the same period of time (2 days).
Conformity to Beer's law A number of solutions containing varying amounts of chromium and the optimum amounts of azide and perehloric acid were prepared and their absorhancies measured at 440 m/~ after 2 hr from mixing, using a reagent blank. A plot of absorbancy values against Cr(III) concentration gave a straight line passing through the origin, showing that Beer's law is obeyed in a range of 4-320 p.p.m. Cr(III), with a sensitivity of 2 p.p.m. This method has a distinct advantage of providing a wide range of applicability. The lower limit could be extended si~ificantly by the use of longer cells. This is especially promising as the absorption of the reagent itself is negligible.
Effect of diverse ions In order to study the effect of diverse ions, solutions containing 0.008 M Cr3 +, 0.2 M N 3- and 0.06 N H + were prepared and solutions of different ions were then added in a titration fashion. The absorbancy of the mixture was measured after each addition, allowing for colour development of the original solution prior to measurements. The results are corrected for volume changes. A change of -t-0.01 of absorbancy unit is considered to be interference. The results are given in Table 2. The interference of Cu2+ and UO22+ ions can be l'educ~d appreciably by the addition of 1 ml of 0.05 M solution of EDTA. Interference of Fe3 + ions could only be decreased by the addition of 2 ml of EDTA and measurement at 500 m/~. The phosphate ions form a precipitate after the addition of few fractions of a ml of the solution. Thorium behaves similarly, but suitable increase of acid concentration will prevent precipitation.
F. G. St~Jr, W. M. O~mY and Hus~JN S~a3eg
1376
TAm.e 2.--MAXlJm~ TOI,BRANCEOF INTERFERINGIONS Cr3+ = 0~08 M; N3- = 0"2M; H + = 0 ~ 6 N ; ~ = 440p
Ion
Added as
Cu2+
CuCI2
U022+
UO2(NO3)2
Fe 3+ Th4 + Ni 2+ Mn 2+ Ce3+ NH4 + n82+ VO3-
Fe(NO3)3
F-
0.3 (35) (34) 0.1 (33)
6
Th(NO3)4
MO70246WO42Cr2072P O 4 3-
Maximum tolerance (p.p.m.)
NiCI2 MnCI2 Ce(NO3)3 NH4NO3 HgC12 NaVO3 (NH4)6M07024 Na2WO4 K2Cr207 Na3PO4 NaF
46 96 130 110 24 240 12 32 300 170 300
160
Values in parentheses for Cu2+ and UO22+ are taken in presence of 1 ml of 0.05 M EDTA. The value for Fe3+ is in presence of 2 ml of EDTA at Z - 500 g. Structure o f the chromium azide complex in dilute solutions
It was shown previously that a monoazidochromium(III) ion, CrN32+, can be formed in dilute aqueous solutions containing chromium nitrate and sodium azide,(1) that have two absorption maxima at 440 and 605 m#. The logarithmic method o f BENT and FRENCH(6) was applied h e r e to further confirm these results. In one series of solutions the total concentration of chromium was kept constant at 0.024 M and that o f azide was varied from 0.002 to 0.012 M, keeping the ionic strength constant at 0.156 by adding the appropriate amounts of sodium perchlorate solution. The absorbancy was measured at 440 rap. The logarithm of the absorbancy was a linear function o f the logarithm o f the N] ion concentration (Fig. 1). In a second series of solutions the concentration of sodium azide was kept constant at 0.012 M and that of chromium was varied from 0-004 to 0.020 M, keeping the ionic strength the same as before. The logarithmic relation was similarly obtained. The reaction between chromium and azide ions can be represented by the equation: m Cr3+ + n N] = [Crm(N3)~](3"'~) +
(1)
The logarithm of absorbancy will be proportional to log [CrmN3~] in the logarithmic expression: Log [CrmN3,~ ----m log [CrS+]+n log [ N i l + l o g K
(2)
The experimental points were found to fall on the theoretical line with the assumption that m --: n = 1.
(e) H. E. BE~rr and C. L. F~NC~ J'. Amer. Chem. Soc. 63, 568 (1941).
Spectrophotometrk study of the chromium(HI) azide reaction
I /
- 1.9
/ .~/
1377
I
I
I
I
-2.1
/ !
~ -2.3 _1
/ S
/
I
/
S
I
S J
Ao
/ -2.5
-I "5
S
S
I S
-2-7
S
S
-I "3
-I-I Log
-0"9
-0.7
Qb$orboncy
FIG. 1.--Limiting logarithmic method at wavelength 440 rap. Cr3+:0~24 M and azide conen. varied from 0.002 to 0.012 M. Ionic strength = 0"156. The circles are the experimental points. The dotted lines represent the theoretical values for n = 0.5, 1 and 2, where n is the number of azide ions associated with one Cr3 + ion in the complex.
Structure of the complex in concentrated solutions The method of continuous variations of Joe(5) was appfied at different wavelengths and at different total concentrations. The contours of the curves obtained in every case would be indicative of the formation of different coloured species with different stabilities and absorption properties. Deformed curves might be obtained in the stepwise formation of complex ions formed from a metallic ion and the same ligand, if the formation constants and maximum absorptions are not widely separated from each other. In our work, the observed absorbancy values were plotted against the mole fraction of the metallic ion rather than Y values (Y = difference between observed values and those calculated for no reaction) as stated by Joe, since it was found that there is no difference between the contours of the curves in both cases. The blank used was a chromium nitrate solution of the same concentration as that present in the test solution. Three different runs with total concentrations of 0.04, 0.16 and 0-32 M were carried out using equimolar solutions. In our previous report,(z) it was stated that a 1"1 compound, Cr N32+ is formed in dilute solution, at a total concentration of 0.008 M. In Fig. 2, the irregularity of the curves near the maximum can be smoothed to one sharp maximum at a mole fraction of Cr(III) of 0-333 suggesting the formation of a 1:2 complex, Cr(N3)2 +. When the rules of VOSaUt~OH and COOPER(7) were applied, the solid curve was obtained, showing one definite maximum at 1:2 molar ratio, assuming that the 1 : 1 complex was first formed. At a (7) W. C. VOSBtmOHand G. R. Coopmt, J. Amer. Chem. Soc. 63, 437 (1941).
1378
F. O. SHm~, W. M. Otv,nY and H L m ~ SAD~
O'7
o
(~t
O.Z
'
'
0.3
04
Mole
'
o
fraction,
0
o.8
0-9
Cr '3
l~o. 2.--Continuous variation method; total concentration of Cr(III) and azide ions is 0.04 M. Y' = Observed values minus those calculated for a 1:1 complex. total concentration of 0.16 M, the results show that the prevailing species in solution are the 1 "3 complex. In solutions having a total concentration of 0.32 M (Fig. 3) there is sufficient contribution from the 1:1 form to produce a definite asymmetry
f'6
1.4
1.2
~
1.0
o 0.8
0.6
0"4
0"2
0
Oil
0"2
0
Mole
0-5
fraclion,
0'7
0.9
Cr °3
Pro. 3.--Continuous variation method; total concentratioa of Cr(III) and azide ions is 0.32 M. Y"--Observed values minus throe calculated for 1:I C"r:n~ide.
Speetrophotometric study of the chromium(IIl) azide reaction
1379
in the curves, as shown by the shoulder at a mole fraction of Cr(llI) of 0.45. However, there is a clear maximum at longer wavelengths, 480 and 500 m/~, indicating the formation of a 1:6 compound Cr(N3)~-. As we approach shorter wavelengths, e.g. 460 mp, the maximum falls at a ratio of 1:4, Cr(N3)4-. By applying the rules of VOSBURGH and COOPER and plotting the difference between the observed values and those calculated for 1 ~1, the interference in the different curves due to the 1 : 1 form disappears, and a curve with one maximum at a ratio of 1 "6 is obtained. These data show that the increase in concentration of reagents leads to the formation of azide-rich complexes. Higher complexes are easier to identify as we approach longer wavelengths. These results are confirmed by observing a hypsochromic effect produced upon the gradual increase in azide ion concentration with respect to a solution 0.04 M Cr(Ill). The colour changes gradually from deep green to distinct violet, and the maxima change from 440 and 605 m/~ to 490 and 655 m/t respectively.