52
Anal. Chem. 1982, 5 4 , 52-55
N,N’-Diarylbenzamidines for Extraction of Molybdenum in the Presence of Thiocyanate Khageshwar Slngh Patel, Rajlv Mohan Verma, and Rajendra Kumar Mlshra” Department of Chemistry, Ravishankar University, Ralpur-4920 10, Madhya Pradesh, India
A method Is described for the spectrophotometric determlnation of mlcroamounts of molybdenum wlth N,N’-diarylbenzamldlnes (HA) and thlocyanate, based on extraction of molybdenum-thiocyanate specles wtlh dimers amldlne (HA,) as 1:21 (Mo:SCN:H,A,) complexes Into benzene over a wide acldlty range. The molar absorptlvltlesof the complexes Ile In the range of (1.85-1.65) X lo4 L mol-’ cm-’ at A, 485-470 nm. The extraction of the metal wlth N,N’-dlphenylbenzamldlne hydrochlorlde (DPBAH) has been studled In detall. The metal Ions whlch are generally assoclated wtlh the the metal do not Interfere. The method has been accurately applied to ore, alloy-steels, and synthetlc mlxtures.
A number of methods have been reported (1-13) for the spectrophotometric determination of molybdenum in various complex materials. However, none is free from interference by elements such as Fe, Cu, Ni, Co, V, or W and many other serious drawbacks. Of these, the method using the determination of the metal with SCN-SnC12 is the oldest and most commonly used for the spectrophotometric determination of molybdenum. Even this method suffers from a number of limiting factors such as variaton of absorbance with respect to concentration of Fe, SCN, SnClz and H+, unstability of color reaction, etc. (1,Z). The dithiol, 2-mercaptobenzothiopyrone, phosphotriamide, chloromazine hydrochloride, potassium ethylxanthate, nitrone, and tetraphenylarsonium chloride are sensitive methods (1-13) used for microamounts of the metal. Of these, the first four methods suffer from an interference of Fe, Cu, W, etc. The potassium ethylxanthate method is complicated, V and W interfere, and the absorbance of reagent blank is relatively large. The nitrone is probably the most selective method but the presence of iron is necessary for the rapid and complete extraction of the metal. In addition, a long time (20 min) is required for the complete formation of chelates. The 2-mercaptobenzo-r-thiopyronemethod is also quite sensitive but suffers from an interference of Fe, Co, Ni, and variation Cu, F-, or SZOS-, absorption of reagent at ,,A, of the color intensity of complex with respect to amount of reagent. The tetraphenylarsonium chloride method is also complicated because Ti(II1) is required for full color development and W, Nb, or Ni (>lo-fold) interfere. In this paper, we report a very useful method for determination of molybdenum, which is highly selective, sensitive, and reproducible, based on the reduction of Mo(V1) into Mo(V) with ascorbic acid and then reaction with thiocyanate in strong hydrochloric acid media, formation of Mo(V)-SCN complex and subsequent extraction into benzene with N,N’-diarylbenzamidines. The method is free from most limiting factors of solvent-extraction methods such as interference of the metals which are commonly associated with molybdenum, variation of position of ,A,, and absorbance of the complex with respect to reagent concentration, order of addition of reagents, nonlinearity of Beer’s law, etc. Moreover, the absorbance of the reagent a t ,A, is also negligible. Amidines (HA) dimerize (14) through intermolecular hy-
drogen bonding to give rise to a dimer (H2A2)which acts as a potential univalent, N,N-bidentate chelating agent in a fashion similar to carboxylic acids (15). These compounds have been found to possess several noteworthy features as an analytical reagent such as stability toward heat, light, and air, simple and easy preparation, and long storage time without deterioration. Moreover, their color reaction toward molybdenum in the presence of thiocyanate is highly selective and sensitive. The simplest, N,N‘-diphenylbenzamidine hydrochloride (DPBAH) and its seven derivatives, Figure 1, have been prepared and used as new reagents for selective extraction of molybdenum. Of these, N,”-diphenylbenzamidine hydrochloride (DPBAH) has been studied in detail. EXPERIMENTAL SECTION Apparatus. Spectrophotometric measurements were made with a Carl-Zeiss Specord ultraviolet-visible spectrophotometer and an ECIL UV-VIS spectrophotometer, Model GS-865, equipped with l-cm matched quartz and silica cuvettes. The pH values were determined by a Systronic pH meter, type-322. Chemical and Reagents. All the chemicals used were of analytical grade. A stock solution of Mo(V1) was prepared by dissolving 2.04 of ammonium molybdate (E. Merck) in double distilled water. The molybdenum content of solution was determined gravimetrically by using oxime (16). Amidines were prepared by the condensation of an equimolar ratio of N-phenylbenzimidoyl chloride with the appropriate aromatic amine in ether media (17). The resulting hydrochlorides were filtered and recrystallized from absolute ethanol containing a few drops of concentrated hydrochloric acid. They gave satisfactory C, H, and N analysis. A 2.63 M (20% (w/v)) ammonium thiocyanate solution, 0.57 M (10 (w/v)) ascorbic acid, and 10 M HC1 acid were used for extraction work. A 0.003 M (0.1% (w/v)) solution of HA in benzene was prepared by addition of liquior ammonia dropwise till reagent is dissolved and excess ammonia is removed by boiling. All the solutions used were presaturated with benzene. Procedure. An aliquot of solution containing 50 wg of Mo was taken into a 100-mL separatory funnel. To this, was added 5 mL of ascorbic acid and 3 mL of thiocyanate solution and acidity was adjusted to 4 M in a total 25 mL aqueous phase. The metal was extracted with 25 mL of benzene solution of reagent for 2 min. The benzene layer was collected over anhydrous sodium sulfate (2 g) in a 50-mL beaker and absorbance was measured at ,A, against a suitable reference. RESULTS AND DISCUSSION Absorption Spectra. The absorption spectra of extracts of complexes formed by the MO-SCN species with amidines and also the spectra of DPBAH in benzene are shown in Figure 2. The reagent did not show any significant absorption of thiocyanate mixed between 450 and 700 nm. The A,, chelate of Mo with HA occurred at 465-470 nm and there was no shift in the wavelength of maximum absorption when either the acidity was varied from 0.5 to 8.0 M HC1 or the molar ratio of Mo to HA or SCN was varied. The position of ,A, is not affected with respect of substitution in HA, but molar absorptivity is effected, Table I. Choice of Solvents, The thiocyanato mixed-ligand chelates of Mo formed with HA are extractable into various organic solvents like chloroform, benzene, carbon tetrachloride, xylene, 2,4-dichlorobenzene, etc. with different values for
0003-2700/82/0354-0052$01.25/00 1981 American Chemicai Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982 53
Table I. Spectral Dati; of Mo-SCN-H,A, no.
--
X
4
H H H H
5 6
H H
7 8
H p-c1
1
2 3
r
Complexes in Benzene
amidine Y H P-CH, p-CH,CH, v-Cl 2 954 Cl), H a-C1 H
2
optimum acidity range, M (HCl)
H H H H H 2,6-(CH3), 2.6-ICH,), 2;3-(CH;j;
2.5-5.0 1.2-5.4 1.2-5.0 3.0-5.2 1.0-5.0 2.5-5.0 2.5-6.5 3.5-7.0
1
-@
CI-
L
J
Figure 1. Graphical reprssentatlon of amidines.
WAVELENBTH n m
Absorption spectra of Mo-SCN-H,A, complexes rand N,N’dlphenylbenzamidine in benzene: [H2A2],0.003 M; [SCN], 0.3 M; [ascorbic acid], 0.1 M; [HCI], 4 M; ( 0 )Mo-SCN-N-p-chloroFlgure 2.
phenyl-N’-phenylbenzamldlne; (0) Mo-SCN-N-p-ethylphenyl-N’phenylbenzamindine; (A)Mo-SCN-N,N’dlphenylbenzamidine; (+) N,N’diphenylbenzamidina in benzene, 0.009 M.
sensitivity but the same values for A,-. ‘ f i e sensitivity is high in aromatic hydrocarbons due to preferable high extractability of thiocyanato complexes in them. In the present investigation, benzene is chosen because of its llow cost and high distribution coefficient of reagent. Other ciolvents like alcohols, ketones, esters, etc. are found to be unsuitable for extraction work due to either low absorbance or uinstability of complex in them. Effect of Various Reducing Agents. Various reducing agents like ascorbic acid, stannous chloride, and cuprous chloride were tried. Of these, ascorbic acid was found more useful for reduction and catalyst purposes. In addition, the concentration of ascorbic acid was not critical unlike stannous chloride and cuprous chloride. Effect of Acidity. The acidity of aqueous phase was maintained with 10 M hC1 acid. The optimum working range lies between 1.2 and 7.0 M HC1 as shown in Table I. A 100% extraction of the metal was also observedl with H2S04acid and
h,,,
nm
465 470 470
E,
L mol-’ cm-’ 1 7 000 17 500
470 470
16 500 1 7 300 16 800 18 500
470 470
18 000 18 200
465
Sandell’s sensitivity, pg Mo cm-* 0.005 64 0.005 48 0.005 8 1 0.005 54 0.005 7 1 0.005 18 0.005 33 0.005 27
optimum activity ranges were found to be between 2.0 and 6.0 M H2S04,having the same values for molar absorptivity as do HCl acids. However, the extraction of the metal was comparatively more rapid and selective in HCl media. Therefore it was preferred for the detailed study. The position remains intact with respect to the kind of acids or of A,, variation of acidity of aqueous phase. Other acids like HN03 and CH3COOH were not suitable for extraction work due to low color absorbance of metal chelates. The extraction rate of the metal is slow at highest and lowest acidity limits due to protonation of amidine and incomplete reduction of Mo(V1) into Mo(V) and vice versa, respectively. Therefore, an acidity of 4 M is chosen for all extraction work. The effect due to electron repellling groups like -CH3, -CzHs, etc. in the N-ring is very much evident in the optimum acidity range, which lowers the limit to 1.2 M as seen in compounds 2 and 3. With electron-withdrawing groups like -C1, etc. the reverse effect is observed as in compound 4, in which the lower limit is shifted toward higher value, 3.0 M HC1. Effect of Reagent Concentration. The effect of the amount of HA or SCN on the distribution coefficient of metal as a ternary complex was examined by varying the molar ratio of reagents to metal by keeping the other variables constant. The results obtained showd that at least a 25-fold molar excess of HA and 0.05 M SCN are necessary for full color development. The addition of excess HA up to 200-fold molar excess caused no adverse effect on the nature of complex. The optimum concentration of SCN for the 100% extraction of the metal must be between 0.05 and 0.90 M. The sequence in which reagents were mixed is not critical. The substitution The in HA has practically no effect on the position of Am,. substitution a t N-ring with aryl substitution showed a hyperchromic shift, the order being p-tolyl > p-chlorophenyl > phenyl > p-ethylphenyl. However, the introduction of bulky groups, e.g., -(CHJ2, etc. in the ”-ring evidently affected the molar absorptivity of the complex to higher values (6-8). Effect of Electrolyte, Time, Temperature, Volume Ratio, and Standing Time. The equilibrium period necessary for complete extraction of Mo-SCN-H,A,-complex in benzene was examined and found to be 1 min. The color intensity of complex is not affected by addition of electrolytes such as NaC1, KC1, LiC1, NH4C1, or (NH4)2S04up to 2 M, as the distribution coefficient of metal chelate is high. The nature of the complex is not affected by variation in temperature from 20 to 40 O C and volume of aqueous phase from 15 to 60 mL. The complex is stable a t least for 40 h a t room temperature. Optimum Concentration Range, Molar Absorptivity, Sensitivity, a n d Precision of Method. Adherence to Beer’s law was tested and confirmed for each reagent. Beer’s law was obeyed in the range of 0.4-5.4 ppm of Mo with DPBAH. The optimum concentration range for the effective spectrophotometric determination of Mo, evaluated by Ringbom’s plot (18) was 0.6-5.0 ppm of the metal. The molar absorptivities and sensitivities of other complexes along with respective A,, are listed in Table I. The precision of method
54
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
Table 11. Effect of Diverse Ions in Determination of 2 ppm Mo at 4 M HCl ion Fe( 11) Fe( 111) Ni( 11) Co(11) Cu(11) Mn( 11) Zn( 11) Cd(11) Fb(I1) Hg(II) Pd( 11) Al( 111)
TI(111) La(111) Cr( 111) Bi(II1) Nb(V) Taw Sb( 111) TWV) Ti(IV) Zr(1V)
vv1
U(V1) W( VI ) fluoride iodide thiosulfate oxalate tartrate citrate phosphate arsanate EDTA
added as FeS0,~(NH4),S0;7H,0 Fe(NO,),.SH,O NiSO, Co(Nb,), CuS0;5HZO MnC1,.4H,O ZnSO, CdSO;
tolerated amt,a ppm 9000 2000 12000 500 500 10000\ 5000 2000 4000
Pb(N03)2
HcCL pic1; Alz(S04),~(NH,),S0,24H,0 TlC1,
2000 500 7000 1000
4000 8000 niobium citrate tantalum citrate K(SbO)C,H,O, T W O 314 K ,Ti O(C,O J,. 2H ,O ZrOC1, ",(VO,) UO,(NO,),~GH,O (NH,), WO 4.2H,O NaF KI sodiuk potassium tartrate sodium citrate Na,PO, Na,HAsO;7 H,O Na,EDTA
a Error causing
