- Email: [email protected]
Spectrophotometric study of the reaction of the uranyl ion with 4-(2-thiazolylazo) resorcinol
Talama,
1967, Vol. 14. pp. 171 to 185.
Pergamon Press Ltd.
Printed in Northem
Iteland
SPECTROPHOTOMETRIC STUDY OF THE REACTION OF THE URANYL ION WITH 4-(2-THIAZOLYLAZO) RESORCINOL Department of Analytical Chemistry, 3. E. PurkyniZ University, Bmo, Czechoslovakia
V. M. IVANOV Department of Analytical Chemistry, V.M. Lomonosov University, Moscow, U.S.S.R. @eceived 2 June 1966. Accepted 8 August 1966) Samnary-The uranyl ion forms only 1: 1 chelates with 4-(2-thiazolylazo)resorcinol (TAR) in solution, UO&fAR)H+ being formed below pH 3 and UO,(TAR) above pH 35. The latter complex may also be precipitated at pH > 3. The quantitative formation of UO*(TAR) at pH 75-7-8 in solutions containing a small excess of reagent and some ~e~ol~ine as buffer can be used for the sensitive spectrophotometric determination of uranium, Several interfering ions can be masked with a mixture of sodium fluoride, cyclohexanediaminetetraacetic acid and 5-sulphosalicylic acid. TAR is slightly less sensitive than 4-(2-pyridylazo)resorcinol as a reagent for uranium but is more selective.
THE ~substitut~ azo-dyes and bisazo-dyes are among the most sensitive reagents for spectrophotometric determination of uranium in aqueous solution. Heterocyclic azo-dyes with an aromatic nucleus containing a nitrogen atom orrho to the azo group also react with the uranyl ion with great sensitivity over a wide range of pH. Of the pyridylazo dyes, the most common, 1-(2-pyridylazo)-2-naphthol (PAN) and ~(2-p~dyl~o~resorcinol (PAR),I--I0 have been used frequently for the spectrophotometric dete~nation of uranium, and their complexation equilibria have been widely studied. PAR is soluble in water and so are its metal chelates, which gives it an advantage over PAN, the metal chelates of which are usually extracted into organic solvents. Little information is available about the reaction of uranyl ion with thiazolylazo dyes,except for f-(2-thiazolylazo)-2-hydroxy-4-dimethylaminobe~ene,112-(2-thiazolyl~o)-l-hydroxy-S-diethyla~nobe~enelz and ~(2-thi~olyl~o)resorcinol (TAR).la Because of its similarity to PAR, TAR aroused our interest in it as a possible analytical and complex-forming reagent. Its reaction with the uranyl ion is described in detail in this paper, and it is shown that TAR is a very useful reagent for the spectrophotometric determination of uranium. EXPERIMENTAL Reagents Uranyl nitrate solution. A stock solution was made by dissolving 49.8 g of uranyl hexahydrate in 100 ml of 1M perchloric acid and diluting the solution to 1 1. It was standardized gravimetrically
172
L.
SOhiMER
and V. M. IVANOV
via triuranium octaoxide, and contained 23.83 mg of uranium/ml. To prevent hydrolysis, the perchloric acid concentration must not fall below O*lM. TAR. A sample from Lachema, Brno, was recrystallized twice from ethanol in the presence of activated carbon, and dried over calcium chloride hemihydrate to constant weight. The purity of the sample was checked by elemental analysis, ash determination, and potentiometric titration in dimethylformamide-hydrochloric acid medium with chromous chloride.14 Reagent solutions in ethanol or in dimethylformamide were used, but the concentrations of these solvents in the hnal solutions never exceeded 4 and 30 % v/v respectively. Control of PH. The pH was adjusted by addition of dilute sodium hydroxide, ammonia, or perchloric acid solutions, and buffers were used only in the determination of uranium. The ionic strength was kept constant at O-1 by addition of sodium perchlorate. All chemicals used were analytically pure. Apparatus Spectrophotometer. Model SF-4 (USSR) with 1 cm cells. pH Meter. Radiometer PHM 3k, precision &to*02 pH units.
THE
REACTION
OF
THE
URANYL
ION
AND
TAR
At pH < 3 a soluble red uranyl chelate is formed; the colour of the reagent is yellowish orange in these conditions. At pH > 3.5 the colour of the chelate solution is red-violet, and is intensified by a small excess of ligand or a large excess of metal ion. The chelating equilibria and the composition and stability of the chelates were studied spectrophotometrically by graphical and numerical analysis of pH-absorbance curves for solutions containing various ratios of metal ion and ligand,15m16by the method of continuous variations,l’ by the slope-ratio method,l* and the mole-ratio method at selected pH values.la All the results in this paper are for solutions containing not more than 4% v/v of ethanol; this amount of ethanol does not affect the results. The pH, values are valid for 30% v/v dimethylformamide solution and have not been recalculated for true hydronium concentration. Acid-base equilibria of TAR The concentrations of the ligand species present in solution (HsR+, H,R, HR-, Ra-) are determined by the acid dissociation constants pK,, = 0.96, pKBfL= 6.23, pKaS = 9N20 Only the first two proved to be important, and the wavelengths of maximum absorption were 488 rnp for H3R+ and 410440 rnp for H,R. The molar absorptivities of these species at 530 rnp were 1.15 x 104 and 2.4 x lo2 mole-l. cm8 respectively. Absorption spectra of the uranyl chelates The absorption curves for solutions containing an excess of metal ion, and for equimolar mixtures of the reagents, at pH < 5, have a maximum at 530 rnp and an isosbestic point at 470475 rnp (Figs. 1 and 2). An additional inflexion in the spectrum of the chelate corresponds to deformation of the ligand absorption maximum at 440 rnp and is similar to that previously found for metal chelates of TAR.2O For solutions with a small (l- to 5-fold) excess of reagent, the absorption maximum of the chelate is shifted to 540-545 rnp at pH > 5 (after correction for the absorbance due to the free ligand).
Uranyl ion reaction with TAR
I
340
FIG. 1.-Absorption c, pH: I-2.03;
400
460
520
A nm500
spectra for UO,*+ and TAR in 4 % aqueous ethanol. = 5.0 x lo-“M; ca = 5.0 x lo-SM Z-2.32; 3-2.55; 63.08; J-3.55; 6-3.98.
*i
FIG. 2.-Absorption
spectra for equimolar UO,*+ and TAR in 20% aqueous ethanol. C’, = Ca = 2.5 x 10-&M pHk: I-3.26; Z-3.45; 3-3.94; 44.10; 54.29; W.42; 7-5.65.
173
174
L.
tt&SMER
iSId
v.
M.
hANiW
~~~~~~~~~o~~~~~g me&J ion irt excess atpH < 3.5. Plots of absorbance EW.pH give curves which are ~~~~~~towards lower pH values as the concentration ratio of metal ion to reagent is increased, and the length of the linear portion of the curves is §im~~t~eousiy decreased. The curves show a distirxt change of slope with increase
CR= 5 x 1O-6M; C&a:
14;
S-10; 3-M; #-2s;
S--50; 6-80: 7-100.
that above pH 3 the in pH if the eon~~~~~ ratio is high @Gg. 3)* xt is blasted E&R+ and H,R forms of the reagent react ~~o~~~g to the e~ui~b~a f H3R+ +%KQRH+ -I- 2W UC&w+ lZ&R%SUQ,RH f l-I+-
m m
To &mk tf;li~~~~~~~~~ the curves were added g~aph~~y by means of the and log X =flog C,, pH), where C, and C, are ~~~f~~~~io~s CAA =fmf the a~~~~ GO~~~~~~~~~ of reagent a& metal iOn r~$~~~V~~~~ A is the:~~orb~~ and [H] is the hydrogen ion concentratian. In addition, the molar absorptivity of the complex and the e~~l~bri~ constant were calcrdated from two points on the curve fabsorbances A, and 83 by means af the fo~~o~~~ set of e~~ai~~~s, where cl3 is the molar abs~~tivity of the complex, cnl and 6Bgare the molar abso~t~v~ti~ ofthe species IT&R+and HaR respectively, Mrepxesents UO,, the constants are d&&d by the equations, and the charges are omitted.
175
and HsR+ are present but onZy H,R+ reacts, by equilibrium (I) k
=
WWW12 [MIF-bRl [Ml + NW
12
CM =
N [Ml
CR= [J&RI+ NN + WW A=
+ +d-W
&M=l
+ dJW1
WI2 Gdc%HGt- 4
(1)
aJW2 A2- a2Wl124 ‘lH = C,(a,[H],2 - a2[H]2)
(2)
b. =
CR/A = -!_ + A;‘;‘“, M
%H
loge
=
log
0, =
Ad1
Ca ~HR-A
(3) 12 1H
k,, + log CM + 2pH +
JW%
-
(4)
CR(ER~
+
~R&J[WJ.
(5)
H,R and H3R+ are present but om’y H,R reacts, according to equilibrium (II) k
=
n
N-U
MW[Hl= DM-W
(6) cMa(EIHcR
-
M-W2 - a2W124 ‘I= = &da#‘% - a2W12) CR/A = 2
lois
A - A,, %HCR
-
+
4W AC&AH
4
(7)
(8)
= log CM + pH + log k,,
(9)
A A
=
CR@RI[HI
0
+
ER2&1)
[HI +&I
*
(10)
H,R ody is present
CR =
WW
+ F-&RI
A = +2[H2Rl + &&MRH] CR/A =-L %H A
-
ER2CR =
logs ~HR-A c
f
(A -
ERB'%[HI
(11)
ACm'bH
log CM + log k,, + pH.
(12)
176
L. SOWER and V. M. IVANOV
The values for &I= and k,, calculated from equations (6) and (7) agree well with other values for these constants (Table I), and confirm that equilibrium (II) occurs at pH > 1.82 and C, = 5 x 1O-3M. The splitting off of one proton was further confirmed by the straight line obtained for equations (8) and (9) when the value of &iu calculated from equation (6) (Fig. 4) was used. When equilibrium (I) was considered, the dispersion of values of k,, calculated from equation (1) was larger
FIG. 4.-Analysis of linear portions of pH-absorbance curves. CR = 5 x 10-6M, &/CR: I-100; 2-80; 3-50; 4-25; 5-16 Open points: y = log [(A - E&~)/(E~&~ - A)] Filled points: y = log (A - AO)/(eIHCB - A) where Ao = G&BJHI + ~&,~)/(lHl + L).
(k,, = 0.07-0*21 for CE4= 2.5 x 1O-3 - 5 x 103M and C, = 5 x 10-5M). The existence of H,R+ can be neglected at pH > 2.5 and CM < 2.5 x 10-3M in the interpretation of the pH-absorbance curves. The points on the straight lines for the log functions calculated from equation (11) coincide well with those obtained from equation (9) (Fig. 4). For CM > 2.5 x 1(k3M the deviation of both lines increases rapidly. By comparison of the family of pH-absorbance curves for various &-values, put in the form pH =f(_4 - eu.&YR)/Cnor pH = log (A - AJ/(eIuCM - A), a linear plot (Fig. 5a) can be derived for corresponding solutions with equal values of the apparent molar absorptivity : pH,, = (--log Cna>m/x+ constant
(12)
where pH,-,i represents the pH values for corresponding solutions, m is the number of metal ions bond in the chelate and x is the number of protons split off during chelation. The results (Fig. 5b) confirm the loss of one proton and the formation of mononuclear MRH at pH 2-3.5, C&r2.9 x lO-L3*7 x 10-3M and C, 5 x 10-5M (Le.,
Uranyl ion reaction with TAR
2
3
177
4pH
(4
(b)
FIG. Sa.-The
pH dependence of the apparent molar absorptivity of the uranyl-TAR complex in 4 % aqueous ethanol. C, = 5 x 10-6M; C&B: I-25; 2-16; 3-10; 44 y = (A - Q&T)&, FIG. %-Plot of pH =f(-log Cd for the corresponding solutions. x = (A - ea&J/Ca.
over the linear lower part of the pH-absorbance curves). In the upper part of the pH-absorbance curves a different equilibrium exists which under certain conditions corresponds to UO,RH+ 6 UOeR + H+
(III)
and the equations k, =
1
[MRI/Hl NW
CR= W’W + [MRI A = +n[MRH] + er[MR] (13) (14) (1% (lo) where E, is the molar absorptivity of MR. For the calculation of&r and k,’ the values of &rucalculated for the equilibrium (II) postulated as existing for the lower part of the pH curves were used for solutions with
178
L.
!~OMMER
and V. M. IVANOV
the same concentration of metal ion. The graphical transformations (14) and (15) were most useful for calculating E~simply, and equations (13) and (16) were used for calculation of k,’ (Fig. 6). Consistent values of er and k,’ were obtained only over limited ranges of pH and metal ion concentration (see Table I): C, 5 x 10-5M; pH 340-4*08, CM 5 x 10-3M; pH 3*70-450, Cnl 4 x 103M; pH 3.85-4.70, C, 2.5 x 1O-3M.
FIG. 6.-Plot
of A us. clCB - (A - ~~&)[Hl/k~ for the second linear portion of the pH-absorbance curves. CR = 5 x lo-6M; CM/CR: 1-100; 2-80; 3-50.
Below these ranges, MRH is also often present and causes a large deviation in the calculated values, e.g., at pH < 2.7 and CM/CR = 100, the value for al, 2.33 x 104, was practically the same as that of zzrn, and a negative value was obtained for k,‘, which could have no physical significance. In such a case formation of MRH according to equilibrium (II) must predominate. Equimolar solutions and solutions with excess of &and. The pH-absorbance curves for solutions with CM 2.5 x 1O-s and C, 2.5 x 10+-7.5 x 10”’ were shifted towards lower pH values as the ligand concentration increased, but generally reached a horizontal at pH values >5. Measurements were made at 570 rnp to reduce the absorbance due to the excess of ligand, and corrections were applied for this absorbance. Because of the limited solubility of the ligand in ethanolic medium, 30 % v/v aqueous dimethylformamide was used as solvent. From the linear parts of the curves for C,/C, ratios of 10, 20 and 30, it was confirmed that equilibrium (II) took place. For this purpose the equations used were CM = [M] + [MRH]
C, = &/A
W&l + [MRHI- [H,Rl
= L &lH
1%
E
=
+ e ‘kHlc 1H
11
(17)
R
pH + log CR + log kll.
The molar absorptivity calculated from the intercept on the graph of equation (17) was used to obtain log kl, from equation (17), Fig. 7. The values of &1nobtained
Uranyl ion reaction with TAR
179
agreed well with those for solutions containing excess of metal ion, when the conversion factor Am/A,,,, = l-08 was applied. The higher values of k,, were partly influenced by the dimethylformamide present.
The MR complex was formed to a negligible extent under the given conditions. The shape and position of the pH-absorbance curves for nearly equimolar solutions
FIG. 7.-Graphical
analysis of the rising portion of pH-absorbance curves according to equation (17). 30 % aqueous dimethylformarm ‘de; 57Omp c, = 2.5 x lo-6M; C&aa: I-10; 2-20; J-30.
(CR/CM Q 6) were similar to those for a large excess of ligand but the absorbance was about twice as great (Fig. 8). This indicated the existence of the MR species,
formed according to the equilibria M+H,RfMR+2H+ M+HR-%MR+H+
FIG. 8.-pH-Absorbance
curves for uranyl-TAR solutions in 8 % aqueous ethanol/4 % aqueous dimethylformamide. C, = 2.5 x 10-6M, C&&: I-l ; 2-2; 34; 4-6.
(Iv) (V)
180
L. SOMMERand V. M. IVANOV
at pH > 4. The large dispersion of the results for ~~(I.15 x 104 - 46-l x lOa) provides further evidence for the existence of a mixture of MRH and MR species under these conditions. Other evidence for the 1: 1 complex Continuous variation, slope-ratio and mole-ratio plots (Figs. 9, 10,ll) all indicated the existence of a 1: 1 complex over the pH range 3.5-7.8. The mole-ratio plot for solutions at pH < 3.3 no longer shows a break on the curves, but this is taken as
Fro. 9.-Job curves for equimolar solutions of UOaa+ and TAR at 530 rnp. CM = 4.5 x 10-6M; pH: l-7.8 (no buffer); 2-6*00 (0*2M triethanolamine) CM = 3-O x 10-6M; pH: 3-7.8 (no buffer); 4-6.02 (0.2M triethanolamine).
FIO. lO.-Slope ratio plots. pH 6.67 (0.2M triethanolamine); l-530 rnp; 2-570 rnp pH 3.50 (0.2M formate buffer); 3-530 rnp; k-570 rnp.
181
Uranyl ion reaction with TAR
A
Q3
1
2
3I$&
FIG. Il.-Mole ratio plots for increasing concentrations of TAR. C, = 25 x 10-6M; pH 3.63; I-570 m,u; Z-530 ~IJJ; pH 6.95: 3-570 m,u; 4-530 m,u.
evidence of another form of 1: 1 complex with a higher absorptivity (occurring when CR/CM > 1) rather than of the presence of a higher complex. A green-violet crystalline product was obtained by treating 100 ml of 1.25 x 10-sM uranyl nitrate in IM perchloric acid with 50 ml of 1.25 x 10e2M TAR in ethanol, and adding 1M ammonia dropwise with continuous stirring. The precipitate which appeared at pH 3 was filtered off on a sintered glass crucible, washed with small amounts of ethanol and water, and dried over calcium chloride hemihydrate in a vacuum desiccator. The solid was analysed (found: C 19.6%; H 2.0%; N 7.5 %; U Ml %; H,O 7.5%; calculated for U02R.2H20: C 20.5%; H 1.7%; N 8.0%; U 45.2 %; H,O 6.8 %) and the ratios U: C and U: N were 1: 8.8 and 1:2*9, compared with 1: 9 and 1: 3 expected theoretically. The compound strongly adsorbs water, so the water content is always higher than the theoretical value. There was no evidence for the formation of a I:2 complex in the solid state. Constants for the uranyl-TAR chelate We may summarize the various equilibrium constants for the system: k, =
PfRWF-WWW,Rl
k,, = [MRHIW12/PfI[H,Rl k,’ = [MR][H]/[MRH] k2 = kr& KIH
=
WW/M~HR*l
= WK,,*
KI = [MRl/[MI[Rl= k’4d&*
L. SOMMERand V. M. IVANOV
182
where Ka2* is the dissociation constant for the hypothetical dissociation
HO
and Ka3* is the dissociation constant for the hypothetical dissociation
-d
-6
These hypothetical constants are required for the evaluation of KIH and Kl because in the formation of the complex MRH the proton of the o-hydroxy group is displaced, whereas it is thep-hydroxy group that dissociates first in the absence of a complexforming cation. It is not possible to measure the values of these “reversed” dissociation constants, and for simplicity it is assumed zl*nathat they have the same values as the normal constants, i.e., pK,,* = pK, and pK,s* = pK,,. The values for particular equilibrium constants and absorptivities are given in Table I. There is good agreement between the values obtained by different methods. The values for the other constants then become: log k,, = -0.65; log KIH = 9.8; log Kl = 11.35. TABLE I.-MOLAR MRH &1x x lo-”
2.4; 2.3 (7) 2.46; 23; 2.31; 2.4(10) 2.3 (17)
AND ABSORPTIWIIES
EQUILIBRIUM
MR El x lo-”
3.45; 3.4; 3.3 (14) 3.3 (15)
CONSTANTS
log kl
log k,
0.28 (6) 0.30 (11) 0.28@(9) 0.34; 0.40 (18)$
-45 (13) -4.5 (16)
Numbers in brackets are those of the equations used. The results for equations (6), (9), (11). (13) and (16) are the means of 3, 5, 5.4 and 4 values respectively, for various excesses of l&and and metal ion. Absorbances were corrected for the absorbance of excess of l&and. * Calculated by Ed = A/C, or E~ = A/& from Job plots at pH > 6 with excess of ligand or metal ion (mean of 9 values). t From linear plots of A =f(CR> for solutions with excess of metal ion at pH 6.95 (slope ratio method). 2 Values obtained for solutions in 30 % v/v aqueous dimethylformamide.
The agreement of the values for the molar absorptivity calculated from Job plots, slope-ratio plots, pH-absorbance curves and equations (14) and (15) is further evidence that only the MR species is formed at pH > 6, and not MR,. SPECTROPHOTOMETRIC
DETERMINATION
OF
URANIUM
Of the 1: 1 chelates formed under various conditions, the UO,R complex is the most suitable for spectrophotometric use. Although this complex is the sole species formed at pH > 5 if the ligand is present in excess, pH 7-5-8 was chosen because at this pH the complex is formed quantitatively in the presence of only a small excess of ligand and several interfering ions can be masked.
Uranyl ion reaction with TAR
183
Reaction conditions Sodium acetate, triethanolamine, tris(hydroxymethyl)amiminomethane and urotropine were used as buffers, for the pH range 6-8. None of them influenced the absorbance of the chelate when they were present in 0*2_0*4M concentration. For further work 0.1M triethanolamine was used; a 1M stock solution was adjusted to pH 7.5-8 by addition of perchloric acid. For determination of 20-120 ,ug of uranium, 5 ml of 4 x 1WM TAR in 96% ethanol or 0.1M ammonia were used. If the ammoniacal reagent was used, a few drops of 0-M perchloric acid were added until the red colour of the solution changed to yellow, and then 5 ml of 96% ethanol were added to keep the reagent in solution. Variation of the ionic strength over the range 0*1-O-3 (by addition of sodium perchlorate) was without influence on the absorbance. The absorbance increases slightly in the first 60 min and then begins to decrease. The optimal time for measuring the absorbance is O-10 min after the solution has been prepared. Beer’s law is obeyed over the range 044.8 ppm of uranium at pH 3-8 in the absence of a buffer or at pH > 6 in the presence of 0*2M triethanolamine, 0.04M urotropine or O-02M sodium acetate. The molar absorptivity calculated from several points on the calibration curve was 3.30 x l@ (mean of 9 values), in good agreement with the values given in Table I. InfIuence of masking agents Table II shows the effect of various common masking agents. Carbonate and oxalate were found to interfere strongly. Fluoride, cyclohexanediaminetetra-acetic TABLEII.-EFFE~
OF SOMEMASKINOAGENTSON THBD-ATION
Masking agent EDTA CHDTA Sodium tluoride 5-Sulphosalicyclic acid Sodium thiosulphate sodium acetate Potassium sodium tartrate Diammonium hydrogen phosphate Hydrogen peroxide
OF 60 /Jg OF
Concentration ratio agent/uranium
Relative error in determination, %
20 3ooo 5ooo 3100 16000 6ooo 400 420 4200
-2.0 -0.8 +3-o -3.0 -0.7 +3*3 -5.8 +2*8 -4.4
(CHDTA) and 5-sulphosalicylic acid are very useful masking agents for a number of interfering ions, and the mixture of them recommended by Florence and Farrarr’ was used in this work. Even when a large excess of this mixture was added there was less than 3 % change in the absorbance of the uranyl-TAR complex. The mixture used contained 25 g of cyclohexanediaminetetra-acetic acid, 2-5 g of sodium fluoride and 65 g of 5-sulphosalicyclic acid dissolved in water to give a solution which was neutralized with ammonia (1 + l), filtered, adjusted to pH 7-5-8 with dilute ammonia, and diluted to 11. acid
Procedure To the slightly acid sample solution (in a 50 ml volumetric flask) containing 20-120 pg of uranium add 10 ml of masking mixture, 5-6 drops of phenolphthalein
184
L. SOMMERand V. M. IVANOV TABLEIII.-DETERMINATION OF 60 rug OF OF VARIOUS
Salt added Al(Nr% MnSO, Cd(NG3, Bi(CIO,)I Fe(Clo& Be(NGI), InCl, GaCl, Th(NG3, La(ClG3, Y(ClG.), Dy(ClG& Yb(ClG& Pr(NG3, ZrcNG*)* VOSOI Hg(NG3, CUSOI coo, Ni(NG& Zn(NG& K&rO, Ce(SGJ, Na,MoO, Na,WO,
URANIUM
IN THE
PRESENCE
IONS
Concentration of metal ion, mglml
Relative error of determination, %
0.111 0.130 1.168 0.105 0.326 0.053 0.186 0.049 0.154 0.300 0.063 0.107 0.099 0.220 0.114 0.010 0.104 0.194 0.024 0.057 0.735 0.027 0.018 2.70 0.488
1-1.3 +0*7 +6*6 +2.3 -4.4 -5.8 +1*3 +0*7 +2*8 +5*7 +1*3 +2.8 -4.3 -2.8 +2.2 +2.9 +1*3 +2.8 +2*2 -2.8 -2.8 $-3-o +3.5 -4.4 -2.8
and first ammonia until a pink colour appears and then just enough 1M perchloric acid to discharge the colour. Then add 5 ml of 4-O x 10e4M TAR in 96% ethanol, and 5 ml of 1M triethanolamine buffer, pH 7.5-7.8. Dilute the mixture to the mark, mix, and measure the absorbance within 10 min in l-cm cells at 530-545 rnp against a reagent blank of the same composition and pH. The sensitivity is 0.072 pg/cm2 for an absorbance of 0.01, and the relative error is less than 2 %. Effect of various ions
Table III shows the effect of various ions on the determination of uranium by the procedure given. Acknowledgement-Thanks
are due to V. Svoboda who supplied the sample of TAR.
Zusanunenfassung-Das Uranylion bildet nur 1: l-Chelate mit 4-(2Thiazolylazo)resorcin (TAR) in Losung, UO,(TAR)H+ bei pH < 3 und UO,(TAR) bei pH > 3,5. Der zuletzt genannte Komplex kann such bei nH > 3 aeftillt werden. Die quantitative Bilduna von UO,(TAR)‘bei pH 7,5-7,8 in Wsungen, dik einen kleinen ReagensCiberschuh und Triathanolamin als Puffer enthalten. kann zu emnfindlither spektrophotometrischer Bestimmung von U&n dienen. M&rere stiirende Ionen k&men mit einer Mischung von Natriumfluorid, Cyclohexandiamintetraessigsaure und 5-Sulfosalicylslure maskiert werden. TAR ist als Uranreagens etwas weniger empfindlich als 4-(2Pyridylazo)resorcin aber selektiver. R&n&--L’ion uranyle ne forme que des chelates 1: 1 avec le 4-(2thiazolylazo) r&sorcinol (TAR) en solution, UO1(TAR)H+ se formant en-dessous de pH 3 et UO,(TAR) au-dessus de pH 3,5. Le demier complexe peut aussi &re pr&pitb ii pH 3. On peut utiliser la formation
Uranyl ion reaction with TAR
185
quantitative de UOn(TAR) B pH 7,5-7,8 en solutions contenant un faible ex& de reactif et un peu de triethanolamine comme tampon pour le dosage spectrophotometrique sensible de I’uranium. On peut dissimuler differents ions g&rants par un melange de fluorure de sodium, acide cyclohexanediaminotdtrac&ique et acide S-sulfosalicylique. Le TAR, en tant que reactif de l’uranium, est legerement mains sensible que le 4-(2-pyridylazo) resorcinol, mais il cst plus selectif. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
K. L. Cheng, Anal. Chem., 1958,30,1027. I&m, Takmta 1962,9,739. S. Shibata, Anal. Chim. Actu, 1960,22,479. I&m, Repts. Govt. Id. Res. Inst., Nagoya, 1960,9, 174. Yu. A. Zolotov, I. P. Alimarin and V. V. Bagreev, Tr. Komis. po Analit. Khim., Akad. Nauk SSSR, 1965,15,59. M. R. Hayes and J. S. Wright, Talanta. 1964, 11,607. F. H. Pollard, P. Hanson and W. J. Geary, Anal. Chim. Actu, 1958,20,26. A. I. Busev and V. M. Ivanov, Vestnik Moskov Univ. Ser. II, 1960, 15, No. 3, 52. T. M. Florence and Y. Farrar, And. Chem., 1963,35,1613. W. J. Geary, G. Nickless and F. H. Pollard, Anal. Chim. Actu, 1962, 27,71. E. Sorensen, Actu Chem. Scat&., 1%0,14,965. Hun Suj-tse and Den Sin-tsjan, Jua Ntzynen, No. 7, 1964,612; Khim-Re$ Zh., 1965,13094. N. Kaneniwa, F. Yoshizawa and J. Homma, Kanazawa Daigaku Yakugakubu Kenkyu Nempo 1960,10,42; Chem. Abstr., 1961,55,23155. V. M. Ivanov, D. HalovS. and L. Sommer, 2. Anal. Chem., in press. M. Hnilii?kov&, Thesis, University of Bmo, 1966. L. Sommer, J. KuL(erova, H. Prochazkova and M. Hnilickovl, Publ. Fat. Sci. Univ. Erno, No. 464,1965,249. P. Job, knn. Chim., 1928, (lo), 9, 113. A. E. Harvev and D. L. Mannine. J. Am. Chem. Sot.. 1950.72.4488. J. H. Yoe and A. L. Jones, Znd. &g. Chem., Anal. Ed., 1944,16,11. M. Hnili~kova and L. Sommer, Tuluntu 1966,13,666. M. HniliEkova and L. Sommer, Collection Czech. Chem. Commun., 1961,26,2781. A. Conini, I. M-L. Yih, Q. Fernando and H. Freiser, Anal. Chem., 1962,34,1090.
1967, Vol. 14. pp. 171 to 185.
Pergamon Press Ltd.
Printed in Northem
Iteland
SPECTROPHOTOMETRIC STUDY OF THE REACTION OF THE URANYL ION WITH 4-(2-THIAZOLYLAZO) RESORCINOL Department of Analytical Chemistry, 3. E. PurkyniZ University, Bmo, Czechoslovakia
V. M. IVANOV Department of Analytical Chemistry, V.M. Lomonosov University, Moscow, U.S.S.R. @eceived 2 June 1966. Accepted 8 August 1966) Samnary-The uranyl ion forms only 1: 1 chelates with 4-(2-thiazolylazo)resorcinol (TAR) in solution, UO&fAR)H+ being formed below pH 3 and UO,(TAR) above pH 35. The latter complex may also be precipitated at pH > 3. The quantitative formation of UO*(TAR) at pH 75-7-8 in solutions containing a small excess of reagent and some ~e~ol~ine as buffer can be used for the sensitive spectrophotometric determination of uranium, Several interfering ions can be masked with a mixture of sodium fluoride, cyclohexanediaminetetraacetic acid and 5-sulphosalicylic acid. TAR is slightly less sensitive than 4-(2-pyridylazo)resorcinol as a reagent for uranium but is more selective.
THE ~substitut~ azo-dyes and bisazo-dyes are among the most sensitive reagents for spectrophotometric determination of uranium in aqueous solution. Heterocyclic azo-dyes with an aromatic nucleus containing a nitrogen atom orrho to the azo group also react with the uranyl ion with great sensitivity over a wide range of pH. Of the pyridylazo dyes, the most common, 1-(2-pyridylazo)-2-naphthol (PAN) and ~(2-p~dyl~o~resorcinol (PAR),I--I0 have been used frequently for the spectrophotometric dete~nation of uranium, and their complexation equilibria have been widely studied. PAR is soluble in water and so are its metal chelates, which gives it an advantage over PAN, the metal chelates of which are usually extracted into organic solvents. Little information is available about the reaction of uranyl ion with thiazolylazo dyes,except for f-(2-thiazolylazo)-2-hydroxy-4-dimethylaminobe~ene,112-(2-thiazolyl~o)-l-hydroxy-S-diethyla~nobe~enelz and ~(2-thi~olyl~o)resorcinol (TAR).la Because of its similarity to PAR, TAR aroused our interest in it as a possible analytical and complex-forming reagent. Its reaction with the uranyl ion is described in detail in this paper, and it is shown that TAR is a very useful reagent for the spectrophotometric determination of uranium. EXPERIMENTAL Reagents Uranyl nitrate solution. A stock solution was made by dissolving 49.8 g of uranyl hexahydrate in 100 ml of 1M perchloric acid and diluting the solution to 1 1. It was standardized gravimetrically
172
L.
SOhiMER
and V. M. IVANOV
via triuranium octaoxide, and contained 23.83 mg of uranium/ml. To prevent hydrolysis, the perchloric acid concentration must not fall below O*lM. TAR. A sample from Lachema, Brno, was recrystallized twice from ethanol in the presence of activated carbon, and dried over calcium chloride hemihydrate to constant weight. The purity of the sample was checked by elemental analysis, ash determination, and potentiometric titration in dimethylformamide-hydrochloric acid medium with chromous chloride.14 Reagent solutions in ethanol or in dimethylformamide were used, but the concentrations of these solvents in the hnal solutions never exceeded 4 and 30 % v/v respectively. Control of PH. The pH was adjusted by addition of dilute sodium hydroxide, ammonia, or perchloric acid solutions, and buffers were used only in the determination of uranium. The ionic strength was kept constant at O-1 by addition of sodium perchlorate. All chemicals used were analytically pure. Apparatus Spectrophotometer. Model SF-4 (USSR) with 1 cm cells. pH Meter. Radiometer PHM 3k, precision &to*02 pH units.
THE
REACTION
OF
THE
URANYL
ION
AND
TAR
At pH < 3 a soluble red uranyl chelate is formed; the colour of the reagent is yellowish orange in these conditions. At pH > 3.5 the colour of the chelate solution is red-violet, and is intensified by a small excess of ligand or a large excess of metal ion. The chelating equilibria and the composition and stability of the chelates were studied spectrophotometrically by graphical and numerical analysis of pH-absorbance curves for solutions containing various ratios of metal ion and ligand,15m16by the method of continuous variations,l’ by the slope-ratio method,l* and the mole-ratio method at selected pH values.la All the results in this paper are for solutions containing not more than 4% v/v of ethanol; this amount of ethanol does not affect the results. The pH, values are valid for 30% v/v dimethylformamide solution and have not been recalculated for true hydronium concentration. Acid-base equilibria of TAR The concentrations of the ligand species present in solution (HsR+, H,R, HR-, Ra-) are determined by the acid dissociation constants pK,, = 0.96, pKBfL= 6.23, pKaS = 9N20 Only the first two proved to be important, and the wavelengths of maximum absorption were 488 rnp for H3R+ and 410440 rnp for H,R. The molar absorptivities of these species at 530 rnp were 1.15 x 104 and 2.4 x lo2 mole-l. cm8 respectively. Absorption spectra of the uranyl chelates The absorption curves for solutions containing an excess of metal ion, and for equimolar mixtures of the reagents, at pH < 5, have a maximum at 530 rnp and an isosbestic point at 470475 rnp (Figs. 1 and 2). An additional inflexion in the spectrum of the chelate corresponds to deformation of the ligand absorption maximum at 440 rnp and is similar to that previously found for metal chelates of TAR.2O For solutions with a small (l- to 5-fold) excess of reagent, the absorption maximum of the chelate is shifted to 540-545 rnp at pH > 5 (after correction for the absorbance due to the free ligand).
Uranyl ion reaction with TAR
I
340
FIG. 1.-Absorption c, pH: I-2.03;
400
460
520
A nm500
spectra for UO,*+ and TAR in 4 % aqueous ethanol. = 5.0 x lo-“M; ca = 5.0 x lo-SM Z-2.32; 3-2.55; 63.08; J-3.55; 6-3.98.
*i
FIG. 2.-Absorption
spectra for equimolar UO,*+ and TAR in 20% aqueous ethanol. C’, = Ca = 2.5 x 10-&M pHk: I-3.26; Z-3.45; 3-3.94; 44.10; 54.29; W.42; 7-5.65.
173
174
L.
tt&SMER
iSId
v.
M.
hANiW
~~~~~~~~~o~~~~~g me&J ion irt excess atpH < 3.5. Plots of absorbance EW.pH give curves which are ~~~~~~towards lower pH values as the concentration ratio of metal ion to reagent is increased, and the length of the linear portion of the curves is §im~~t~eousiy decreased. The curves show a distirxt change of slope with increase
CR= 5 x 1O-6M; C&a:
14;
S-10; 3-M; #-2s;
S--50; 6-80: 7-100.
that above pH 3 the in pH if the eon~~~~~ ratio is high @Gg. 3)* xt is blasted E&R+ and H,R forms of the reagent react ~~o~~~g to the e~ui~b~a f H3R+ +%KQRH+ -I- 2W UC&w+ lZ&R%SUQ,RH f l-I+-
m m
To &mk tf;li~~~~~~~~~ the curves were added g~aph~~y by means of the and log X =flog C,, pH), where C, and C, are ~~~f~~~~io~s CAA =fmf the a~~~~ GO~~~~~~~~~ of reagent a& metal iOn r~$~~~V~~~~ A is the:~~orb~~ and [H] is the hydrogen ion concentratian. In addition, the molar absorptivity of the complex and the e~~l~bri~ constant were calcrdated from two points on the curve fabsorbances A, and 83 by means af the fo~~o~~~ set of e~~ai~~~s, where cl3 is the molar abs~~tivity of the complex, cnl and 6Bgare the molar abso~t~v~ti~ ofthe species IT&R+and HaR respectively, Mrepxesents UO,, the constants are d&&d by the equations, and the charges are omitted.
175
and HsR+ are present but onZy H,R+ reacts, by equilibrium (I) k
=
WWW12 [MIF-bRl [Ml + NW
12
CM =
N [Ml
CR= [J&RI+ NN + WW A=
+ +d-W
&M=l
+ dJW1
WI2 Gdc%HGt- 4
(1)
aJW2 A2- a2Wl124 ‘lH = C,(a,[H],2 - a2[H]2)
(2)
b. =
CR/A = -!_ + A;‘;‘“, M
%H
loge
=
log
0, =
Ad1
Ca ~HR-A
(3) 12 1H
k,, + log CM + 2pH +
JW%
-
(4)
CR(ER~
+
~R&J[WJ.
(5)
H,R and H3R+ are present but om’y H,R reacts, according to equilibrium (II) k
=
n
N-U
MW[Hl= DM-W
(6) cMa(EIHcR
-
M-W2 - a2W124 ‘I= = &da#‘% - a2W12) CR/A = 2
lois
A - A,, %HCR
-
+
4W AC&AH
4
(7)
(8)
= log CM + pH + log k,,
(9)
A A
=
CR@RI[HI
0
+
ER2&1)
[HI +&I
*
(10)
H,R ody is present
CR =
WW
+ F-&RI
A = +2[H2Rl + &&MRH] CR/A =-L %H A
-
ER2CR =
logs ~HR-A c
f
(A -
ERB'%[HI
(11)
ACm'bH
log CM + log k,, + pH.
(12)
176
L. SOWER and V. M. IVANOV
The values for &I= and k,, calculated from equations (6) and (7) agree well with other values for these constants (Table I), and confirm that equilibrium (II) occurs at pH > 1.82 and C, = 5 x 1O-3M. The splitting off of one proton was further confirmed by the straight line obtained for equations (8) and (9) when the value of &iu calculated from equation (6) (Fig. 4) was used. When equilibrium (I) was considered, the dispersion of values of k,, calculated from equation (1) was larger
FIG. 4.-Analysis of linear portions of pH-absorbance curves. CR = 5 x 10-6M, &/CR: I-100; 2-80; 3-50; 4-25; 5-16 Open points: y = log [(A - E&~)/(E~&~ - A)] Filled points: y = log (A - AO)/(eIHCB - A) where Ao = G&BJHI + ~&,~)/(lHl + L).
(k,, = 0.07-0*21 for CE4= 2.5 x 1O-3 - 5 x 103M and C, = 5 x 10-5M). The existence of H,R+ can be neglected at pH > 2.5 and CM < 2.5 x 10-3M in the interpretation of the pH-absorbance curves. The points on the straight lines for the log functions calculated from equation (11) coincide well with those obtained from equation (9) (Fig. 4). For CM > 2.5 x 1(k3M the deviation of both lines increases rapidly. By comparison of the family of pH-absorbance curves for various &-values, put in the form pH =f(_4 - eu.&YR)/Cnor pH = log (A - AJ/(eIuCM - A), a linear plot (Fig. 5a) can be derived for corresponding solutions with equal values of the apparent molar absorptivity : pH,, = (--log Cna>m/x+ constant
(12)
where pH,-,i represents the pH values for corresponding solutions, m is the number of metal ions bond in the chelate and x is the number of protons split off during chelation. The results (Fig. 5b) confirm the loss of one proton and the formation of mononuclear MRH at pH 2-3.5, C&r2.9 x lO-L3*7 x 10-3M and C, 5 x 10-5M (Le.,
Uranyl ion reaction with TAR
2
3
177
4pH
(4
(b)
FIG. Sa.-The
pH dependence of the apparent molar absorptivity of the uranyl-TAR complex in 4 % aqueous ethanol. C, = 5 x 10-6M; C&B: I-25; 2-16; 3-10; 44 y = (A - Q&T)&, FIG. %-Plot of pH =f(-log Cd for the corresponding solutions. x = (A - ea&J/Ca.
over the linear lower part of the pH-absorbance curves). In the upper part of the pH-absorbance curves a different equilibrium exists which under certain conditions corresponds to UO,RH+ 6 UOeR + H+
(III)
and the equations k, =
1
[MRI/Hl NW
CR= W’W + [MRI A = +n[MRH] + er[MR] (13) (14) (1% (lo) where E, is the molar absorptivity of MR. For the calculation of&r and k,’ the values of &rucalculated for the equilibrium (II) postulated as existing for the lower part of the pH curves were used for solutions with
178
L.
!~OMMER
and V. M. IVANOV
the same concentration of metal ion. The graphical transformations (14) and (15) were most useful for calculating E~simply, and equations (13) and (16) were used for calculation of k,’ (Fig. 6). Consistent values of er and k,’ were obtained only over limited ranges of pH and metal ion concentration (see Table I): C, 5 x 10-5M; pH 340-4*08, CM 5 x 10-3M; pH 3*70-450, Cnl 4 x 103M; pH 3.85-4.70, C, 2.5 x 1O-3M.
FIG. 6.-Plot
of A us. clCB - (A - ~~&)[Hl/k~ for the second linear portion of the pH-absorbance curves. CR = 5 x lo-6M; CM/CR: 1-100; 2-80; 3-50.
Below these ranges, MRH is also often present and causes a large deviation in the calculated values, e.g., at pH < 2.7 and CM/CR = 100, the value for al, 2.33 x 104, was practically the same as that of zzrn, and a negative value was obtained for k,‘, which could have no physical significance. In such a case formation of MRH according to equilibrium (II) must predominate. Equimolar solutions and solutions with excess of &and. The pH-absorbance curves for solutions with CM 2.5 x 1O-s and C, 2.5 x 10+-7.5 x 10”’ were shifted towards lower pH values as the ligand concentration increased, but generally reached a horizontal at pH values >5. Measurements were made at 570 rnp to reduce the absorbance due to the excess of ligand, and corrections were applied for this absorbance. Because of the limited solubility of the ligand in ethanolic medium, 30 % v/v aqueous dimethylformamide was used as solvent. From the linear parts of the curves for C,/C, ratios of 10, 20 and 30, it was confirmed that equilibrium (II) took place. For this purpose the equations used were CM = [M] + [MRH]
C, = &/A
W&l + [MRHI- [H,Rl
= L &lH
1%
E
=
+ e ‘kHlc 1H
11
(17)
R
pH + log CR + log kll.
The molar absorptivity calculated from the intercept on the graph of equation (17) was used to obtain log kl, from equation (17), Fig. 7. The values of &1nobtained
Uranyl ion reaction with TAR
179
agreed well with those for solutions containing excess of metal ion, when the conversion factor Am/A,,,, = l-08 was applied. The higher values of k,, were partly influenced by the dimethylformamide present.
The MR complex was formed to a negligible extent under the given conditions. The shape and position of the pH-absorbance curves for nearly equimolar solutions
FIG. 7.-Graphical
analysis of the rising portion of pH-absorbance curves according to equation (17). 30 % aqueous dimethylformarm ‘de; 57Omp c, = 2.5 x lo-6M; C&aa: I-10; 2-20; J-30.
(CR/CM Q 6) were similar to those for a large excess of ligand but the absorbance was about twice as great (Fig. 8). This indicated the existence of the MR species,
formed according to the equilibria M+H,RfMR+2H+ M+HR-%MR+H+
FIG. 8.-pH-Absorbance
curves for uranyl-TAR solutions in 8 % aqueous ethanol/4 % aqueous dimethylformamide. C, = 2.5 x 10-6M, C&&: I-l ; 2-2; 34; 4-6.
(Iv) (V)
180
L. SOMMERand V. M. IVANOV
at pH > 4. The large dispersion of the results for ~~(I.15 x 104 - 46-l x lOa) provides further evidence for the existence of a mixture of MRH and MR species under these conditions. Other evidence for the 1: 1 complex Continuous variation, slope-ratio and mole-ratio plots (Figs. 9, 10,ll) all indicated the existence of a 1: 1 complex over the pH range 3.5-7.8. The mole-ratio plot for solutions at pH < 3.3 no longer shows a break on the curves, but this is taken as
Fro. 9.-Job curves for equimolar solutions of UOaa+ and TAR at 530 rnp. CM = 4.5 x 10-6M; pH: l-7.8 (no buffer); 2-6*00 (0*2M triethanolamine) CM = 3-O x 10-6M; pH: 3-7.8 (no buffer); 4-6.02 (0.2M triethanolamine).
FIO. lO.-Slope ratio plots. pH 6.67 (0.2M triethanolamine); l-530 rnp; 2-570 rnp pH 3.50 (0.2M formate buffer); 3-530 rnp; k-570 rnp.
181
Uranyl ion reaction with TAR
A
Q3
1
2
3I$&
FIG. Il.-Mole ratio plots for increasing concentrations of TAR. C, = 25 x 10-6M; pH 3.63; I-570 m,u; Z-530 ~IJJ; pH 6.95: 3-570 m,u; 4-530 m,u.
evidence of another form of 1: 1 complex with a higher absorptivity (occurring when CR/CM > 1) rather than of the presence of a higher complex. A green-violet crystalline product was obtained by treating 100 ml of 1.25 x 10-sM uranyl nitrate in IM perchloric acid with 50 ml of 1.25 x 10e2M TAR in ethanol, and adding 1M ammonia dropwise with continuous stirring. The precipitate which appeared at pH 3 was filtered off on a sintered glass crucible, washed with small amounts of ethanol and water, and dried over calcium chloride hemihydrate in a vacuum desiccator. The solid was analysed (found: C 19.6%; H 2.0%; N 7.5 %; U Ml %; H,O 7.5%; calculated for U02R.2H20: C 20.5%; H 1.7%; N 8.0%; U 45.2 %; H,O 6.8 %) and the ratios U: C and U: N were 1: 8.8 and 1:2*9, compared with 1: 9 and 1: 3 expected theoretically. The compound strongly adsorbs water, so the water content is always higher than the theoretical value. There was no evidence for the formation of a I:2 complex in the solid state. Constants for the uranyl-TAR chelate We may summarize the various equilibrium constants for the system: k, =
PfRWF-WWW,Rl
k,, = [MRHIW12/PfI[H,Rl k,’ = [MR][H]/[MRH] k2 = kr& KIH
=
WW/M~HR*l
= WK,,*
KI = [MRl/[MI[Rl= k’4d&*
L. SOMMERand V. M. IVANOV
182
where Ka2* is the dissociation constant for the hypothetical dissociation
HO
and Ka3* is the dissociation constant for the hypothetical dissociation
-d
-6
These hypothetical constants are required for the evaluation of KIH and Kl because in the formation of the complex MRH the proton of the o-hydroxy group is displaced, whereas it is thep-hydroxy group that dissociates first in the absence of a complexforming cation. It is not possible to measure the values of these “reversed” dissociation constants, and for simplicity it is assumed zl*nathat they have the same values as the normal constants, i.e., pK,,* = pK, and pK,s* = pK,,. The values for particular equilibrium constants and absorptivities are given in Table I. There is good agreement between the values obtained by different methods. The values for the other constants then become: log k,, = -0.65; log KIH = 9.8; log Kl = 11.35. TABLE I.-MOLAR MRH &1x x lo-”
2.4; 2.3 (7) 2.46; 23; 2.31; 2.4(10) 2.3 (17)
AND ABSORPTIWIIES
EQUILIBRIUM
MR El x lo-”
3.45; 3.4; 3.3 (14) 3.3 (15)
CONSTANTS
log kl
log k,
0.28 (6) 0.30 (11) 0.28@(9) 0.34; 0.40 (18)$
-45 (13) -4.5 (16)
Numbers in brackets are those of the equations used. The results for equations (6), (9), (11). (13) and (16) are the means of 3, 5, 5.4 and 4 values respectively, for various excesses of l&and and metal ion. Absorbances were corrected for the absorbance of excess of l&and. * Calculated by Ed = A/C, or E~ = A/& from Job plots at pH > 6 with excess of ligand or metal ion (mean of 9 values). t From linear plots of A =f(CR> for solutions with excess of metal ion at pH 6.95 (slope ratio method). 2 Values obtained for solutions in 30 % v/v aqueous dimethylformamide.
The agreement of the values for the molar absorptivity calculated from Job plots, slope-ratio plots, pH-absorbance curves and equations (14) and (15) is further evidence that only the MR species is formed at pH > 6, and not MR,. SPECTROPHOTOMETRIC
DETERMINATION
OF
URANIUM
Of the 1: 1 chelates formed under various conditions, the UO,R complex is the most suitable for spectrophotometric use. Although this complex is the sole species formed at pH > 5 if the ligand is present in excess, pH 7-5-8 was chosen because at this pH the complex is formed quantitatively in the presence of only a small excess of ligand and several interfering ions can be masked.
Uranyl ion reaction with TAR
183
Reaction conditions Sodium acetate, triethanolamine, tris(hydroxymethyl)amiminomethane and urotropine were used as buffers, for the pH range 6-8. None of them influenced the absorbance of the chelate when they were present in 0*2_0*4M concentration. For further work 0.1M triethanolamine was used; a 1M stock solution was adjusted to pH 7.5-8 by addition of perchloric acid. For determination of 20-120 ,ug of uranium, 5 ml of 4 x 1WM TAR in 96% ethanol or 0.1M ammonia were used. If the ammoniacal reagent was used, a few drops of 0-M perchloric acid were added until the red colour of the solution changed to yellow, and then 5 ml of 96% ethanol were added to keep the reagent in solution. Variation of the ionic strength over the range 0*1-O-3 (by addition of sodium perchlorate) was without influence on the absorbance. The absorbance increases slightly in the first 60 min and then begins to decrease. The optimal time for measuring the absorbance is O-10 min after the solution has been prepared. Beer’s law is obeyed over the range 044.8 ppm of uranium at pH 3-8 in the absence of a buffer or at pH > 6 in the presence of 0*2M triethanolamine, 0.04M urotropine or O-02M sodium acetate. The molar absorptivity calculated from several points on the calibration curve was 3.30 x l@ (mean of 9 values), in good agreement with the values given in Table I. InfIuence of masking agents Table II shows the effect of various common masking agents. Carbonate and oxalate were found to interfere strongly. Fluoride, cyclohexanediaminetetra-acetic TABLEII.-EFFE~
OF SOMEMASKINOAGENTSON THBD-ATION
Masking agent EDTA CHDTA Sodium tluoride 5-Sulphosalicyclic acid Sodium thiosulphate sodium acetate Potassium sodium tartrate Diammonium hydrogen phosphate Hydrogen peroxide
OF 60 /Jg OF
Concentration ratio agent/uranium
Relative error in determination, %
20 3ooo 5ooo 3100 16000 6ooo 400 420 4200
-2.0 -0.8 +3-o -3.0 -0.7 +3*3 -5.8 +2*8 -4.4
(CHDTA) and 5-sulphosalicylic acid are very useful masking agents for a number of interfering ions, and the mixture of them recommended by Florence and Farrarr’ was used in this work. Even when a large excess of this mixture was added there was less than 3 % change in the absorbance of the uranyl-TAR complex. The mixture used contained 25 g of cyclohexanediaminetetra-acetic acid, 2-5 g of sodium fluoride and 65 g of 5-sulphosalicyclic acid dissolved in water to give a solution which was neutralized with ammonia (1 + l), filtered, adjusted to pH 7-5-8 with dilute ammonia, and diluted to 11. acid
Procedure To the slightly acid sample solution (in a 50 ml volumetric flask) containing 20-120 pg of uranium add 10 ml of masking mixture, 5-6 drops of phenolphthalein
184
L. SOMMERand V. M. IVANOV TABLEIII.-DETERMINATION OF 60 rug OF OF VARIOUS
Salt added Al(Nr% MnSO, Cd(NG3, Bi(CIO,)I Fe(Clo& Be(NGI), InCl, GaCl, Th(NG3, La(ClG3, Y(ClG.), Dy(ClG& Yb(ClG& Pr(NG3, ZrcNG*)* VOSOI Hg(NG3, CUSOI coo, Ni(NG& Zn(NG& K&rO, Ce(SGJ, Na,MoO, Na,WO,
URANIUM
IN THE
PRESENCE
IONS
Concentration of metal ion, mglml
Relative error of determination, %
0.111 0.130 1.168 0.105 0.326 0.053 0.186 0.049 0.154 0.300 0.063 0.107 0.099 0.220 0.114 0.010 0.104 0.194 0.024 0.057 0.735 0.027 0.018 2.70 0.488
1-1.3 +0*7 +6*6 +2.3 -4.4 -5.8 +1*3 +0*7 +2*8 +5*7 +1*3 +2.8 -4.3 -2.8 +2.2 +2.9 +1*3 +2.8 +2*2 -2.8 -2.8 $-3-o +3.5 -4.4 -2.8
and first ammonia until a pink colour appears and then just enough 1M perchloric acid to discharge the colour. Then add 5 ml of 4-O x 10e4M TAR in 96% ethanol, and 5 ml of 1M triethanolamine buffer, pH 7.5-7.8. Dilute the mixture to the mark, mix, and measure the absorbance within 10 min in l-cm cells at 530-545 rnp against a reagent blank of the same composition and pH. The sensitivity is 0.072 pg/cm2 for an absorbance of 0.01, and the relative error is less than 2 %. Effect of various ions
Table III shows the effect of various ions on the determination of uranium by the procedure given. Acknowledgement-Thanks
are due to V. Svoboda who supplied the sample of TAR.
Zusanunenfassung-Das Uranylion bildet nur 1: l-Chelate mit 4-(2Thiazolylazo)resorcin (TAR) in Losung, UO,(TAR)H+ bei pH < 3 und UO,(TAR) bei pH > 3,5. Der zuletzt genannte Komplex kann such bei nH > 3 aeftillt werden. Die quantitative Bilduna von UO,(TAR)‘bei pH 7,5-7,8 in Wsungen, dik einen kleinen ReagensCiberschuh und Triathanolamin als Puffer enthalten. kann zu emnfindlither spektrophotometrischer Bestimmung von U&n dienen. M&rere stiirende Ionen k&men mit einer Mischung von Natriumfluorid, Cyclohexandiamintetraessigsaure und 5-Sulfosalicylslure maskiert werden. TAR ist als Uranreagens etwas weniger empfindlich als 4-(2Pyridylazo)resorcin aber selektiver. R&n&--L’ion uranyle ne forme que des chelates 1: 1 avec le 4-(2thiazolylazo) r&sorcinol (TAR) en solution, UO1(TAR)H+ se formant en-dessous de pH 3 et UO,(TAR) au-dessus de pH 3,5. Le demier complexe peut aussi &re pr&pitb ii pH 3. On peut utiliser la formation
Uranyl ion reaction with TAR
185
quantitative de UOn(TAR) B pH 7,5-7,8 en solutions contenant un faible ex& de reactif et un peu de triethanolamine comme tampon pour le dosage spectrophotometrique sensible de I’uranium. On peut dissimuler differents ions g&rants par un melange de fluorure de sodium, acide cyclohexanediaminotdtrac&ique et acide S-sulfosalicylique. Le TAR, en tant que reactif de l’uranium, est legerement mains sensible que le 4-(2-pyridylazo) resorcinol, mais il cst plus selectif. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
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