Indian Journal of ChemistryVol. 33A, July 1994, pp. 644-650

Potentiometric and extractive spectrophotometric studies on mixed ligandcomplexes of copper(II) with 3-hydroxypicolinamide and a few aliphatic and

aromatic acids

Mohamed S Abu-Bakr", Mohamed M Khalil, Hassan Sedaira & Elham Y HashemDepartment of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt

Received 8 July 1993; revised and accepted 21 October 1993

The equilibria involved in the formation of the mixed-ligand complexes of the type CuAL, whereA= 3-hydroxypicolinamide (Hpa); L= phthalic (pha), salicylic (sa); thiosalicylic (tsa); anthranilic (ant),maleic (mal) and succinic acid (succ) have been studied by pH-titration technique in 40% (v/v) etha-nol-water medium at /=0.1 M (NaCl04) and 25°C. The formation constants of the binary and ter-nary complexes have been evaluated and the various complexation equilibria in solution are demon-strated and characterized. The discriminating and stability increasing properties of 3-hydroxypicolin-amide in the, mixed complexes are discussed. After considering all the parameters, the Cu-Hpa-tsaternary system has been utilized asa suitable, rapid and sensitive spectrophotometric method for themicrodetermination of copper. The method has been used in the determination of copper in somewhite metal alloys.

In recent years lot of work has been reported onthe metal complexes of picolinic and nicotinicacids I-J. In our earlier work", we reported the acidbase behaviour of 3-hydroxypicolinic acid and itscomplexation equilibria with ironrlfl). However,no solution studies on the acid-base properties of3-hydroxypicolinamide (Hpa) or its complexationreactions with copper(II) have hitherto been re-ported. In addition, the literature survey revealedthat the complex formation of copper(II) with3-hydroxypicolinamide (Hpa) in presence of someligands containing oxygen, sulphur or nitrogen asdonor ligands, has not been investigated. Hence,we thought it worthwhile to study the solution eq-uilibria and stability of the complexes of cop-per(II) with such bi-ligand systems.

In continuation of our studies on the complexa·tion equilibria of mixed ligand complexes in solu-non>", we report here the complexation reactionof copper(II) with 3-hydroxypicolinamide as a pri-mary ligand and phthalic acid (pha), salicylic acid(sa), thiosalicylic acid (tsa), anthranilic acid (ant),maleic acid (mal) or succinic acid (succ) as secon-dary ligands.

Materials and MethodsThe pH-titration technique of Irving and Ros-

sotti" and its modifications":" was employed todetermine the stability constants of the ternary

complexes formed. The measurements were madeat 25°C and an ionic strength of 0.1 M (NaCIO.Jin 40% (v/v) ethanol-aqueous solution. The opti-mum conditions for the spectrophotometric studyof the complexation equilibria of Cu(II)-Hpa-tsaternary system were determined. A convenientmethod has been developed for the determinationof copper content in white metal alloys.

The ligands 3-hydroxypicolinic acid (Hpa), sali-cylic acid (sa), and thiosalicylic acid (tsa) werepurchased from Aldrich. Spectrograde ethanolwas obtained from Merck. Other chemicals wereof AR grade. All the ligand solutions (2.5 x lO-J

M) were prepared in pure ethanol. A stock solu-tion of copper(II) perchlorate was prepared usingcopper carbonate and perchloric acid and stan-dardised by EDT All. Carbonate-free sodium hy-droxide solution (0.1 M) was prepared and stan-dardised by titration with potassium hydrogenphthalate. Sodium perchlorate (1 M) and perch-loric acid (0.1 M) standard solutions were alsoprepared. The working solutions were preparedby accurate dilution. De-ionized water was usedthroughout the present work. The acidity of solu-tions investigated was adjusted by the addition ofeither dilute perchloric acid or NaOH solution.The ionic strength was maintained constant at/= 0.1 M (NaCIOJ. Solutions of diverse ionsused for interference studies were prepared us-ing the nitrate, acetate, chloride or perchlorate

ABU-BAKR et al: STUDIES ON MIXED LIGAND COMPLEXES OF Cu(II) 645

salts of metal ions and potassium or sodium saltsof the anions to be tested.

.The pH tit rations were carried out using a Ra-diometer pH-meter (Model M63) equipped with aRadiometer combined glass electrode (GK 2301C). The correction of pH readings in ethanol-water medium were made as described else-where!". The absorption spectra of solutions wererecorded on a Perkin-Elmer (Lambda 38) spec-trophotometer in the wavelength range 300-500 nm using 1 em matched quartz cells. All thepotentiometric and spectrophotometric measure-ments were performed in 40% (v/v) ethanol-watermedium at 25°C and I= 0.1 M(NaCI04).

Standard ProcedureAn aliquot of a solution of copper(I1) contain-

ing 80 flg of copper(II) was introduced into a 25ml calibrated flask, then 5 ml of 2.5 x 10- 3 MHpa and 2.5 ml of 1M NaCI04 were added. Aftermixing with the requisite amount of pure ethanol,the pH of the solution was adjusted to 5.0and then 5.0 ml of 2.5 x 10-3 M tsa was added.The resulting solution was diluted to the requiredvolume with de-ionized water and was shaken for1 min to attain equilibrium. The absorbance ofsolutions was measured at 360 om against a re-agent blank similarly prepared but containing nocopper.

Determination of copper in some industrialsamples

White metal sample-A sample of - 150 mgwas weighed in a 250 ml beaker and to this wasadded cone. HN03 (10 ml), evaporated to dry-ness, treated with cone. H2S04 (3 ml) and heatedslowly to dryness. After cooling, the residue wasboiled with 20ml deionised water, filtered, washedand diluted to volume in 50.0 ml standard flask.The copper content was determined in a suitablealiquot.

Nickel-zinc alloy-A sample weighing 150 mgwas dissolved in minimum amount of aqua regiaand the contents were evaporated almost to dry-ness. To this was added 4-6 ml of cone, HCI andagain heated to dryness. It was cooled and theresidue was boiled with 20 ml of deionized water,filtered, cooled and dilut~d to appropriate volumein a 50 ml standard flask. This aliquot was ana-lysed for the copper content.

The dissociation constant pK ~A for Hpa, pha,tsa, mal or succ (pK ~,L)' sa and ant (pK ~L) weredetermined potentiometrically using Irving- Ros-sotti pH titration technique" along with their mo-difications?'!", The details regarding the poten-tiometric method have been reported earlier). Thestability constants for the binary metal complexeswere computed from titration curves in which themetal :ligand ratio was 1:3 at 25°C and 1=0.1 MNaCl04. The equilibrium constants for the ter-nary systems were computed from tit rations inwhich the total concentrations of Cu(II), Hpa andthe secondary ligand were in a 1:1: 1 molar ratio.The multiple titrations were carried out for eachsystem. The dissociation constants for the free li-gands and the stability constants for binary andternary complexes were calculated from the titra-tion data using a corrected version of the compu-ter programme SCOGSI3. The values of the dis-sociation constants for the free ligands and thestability constants for their binary complexes aregiven in Table 1.

Results and DiscussionThe potentiometric titration graph for Hpa in

the neutral form (HA) shows one inflection ata= 1 (where a is the number of moles of baseadded per mole of ligand). The constant pK ~A

corresponding to the dissociation of the ligand isgiven in Table}.

Table I-Acidity constants" of the ligands and stability constants of binary and mixed ligand complexes" of copper(lI)in etha-not-water medium

[Temp. = 25°C; 1= 0.1 MNaClO.j]

Ligand (L) pK~:L pKML log Kf~L. log K~~L, log (3~~L. log K ~~~L log (3~~AL Alog K(u

Hpa 8.0 8.37 7.81 16.18pha 3.89 6.20 11.32 10.63 2 a , 95 12.01 20.38 0.69sa 3.90 l3.86' 10.62 10.10 20.72 11.13 19.50 0.51tsa 4.50 9.35 7.60 8.03 16.40 0.41ant 6.30 5.75 4.90 10.65 6.12 14.49 0.37mal 3.81 6.10 5.08 4.50 9.58 5.41 13.78 0.33

succ 4.18 5.70 4.75 4.25 9.00 4.~9 13.36 0.24

"All constants are accurate to ± 0.02; "All constants are accurate to ± 0.05; 'Ref. 36.

646 INDIAN J CHEM, SEe. A, JULY 1994

The titration graph for a system containing 1:2molar ratio of Cu(U) and Hpa exhibits two inflec-tions at In = I and In = 2 (where In is the numberof moles of base added per mole of metal ion) in-dicating the formation of mono- and his-binarycomplexes. The corresponding equilibria may berepresented as follows:Cu2+ + HA;=:[CuAj"'"+ H+, Kf~iHl'a

and[CuA]+ + HA;=:[Cu(A),] + H+ KCulHpa! (2)- ,( uIHra" ...

The corresponding constants for the binarycomplexes of CU(Il) with the secondary ligandsused in this study, 'is depicted from titrationgraphs are included in Table 1.

... (1)

Ternary systemsThe potentiometric titration graphs for ternary

systems containing CU(Il), Hpa and sa or ant in a1: 1 : I molar ratio exhibit a single steep inflectionat In = 2. In ternary systems involving tsa, pha,malar succ as the secondary ligand, an inflectionis obtained at In = 3. The analysis of these curvesindicates that the 1: 1: 1 Cu-Hpa-L ternary com-plex is formed in all the cases (Table 1). The rela-tive stability of the ternary complexes as com-pared to that of the corresponding binary com-plexes can be quantitatively expressed in differentways. The most suitable comparison is in terms ofAlog K (ref. 14), which represents the differencein stabilities for the addition of ligands L to theI: 1 MA complex and the aquated metal ion asshown by Eqn. (3 ),Alog KM = log K ~-:;'L-log K tlL

= log KtllA -log KtlA ... (3)

The overall stability constant ~tlAL' which mustbe determined experimentally, is connected toK ~-:;.and K t1t by Eq~ (4) and (5) respectively.log K ~L = log PtlAL~ log K tlL (4)log K tltA= Pt1lA-log K tlL (5)

It is worthy to mention that the magnitude ofAlog K is strongly influenced by statistical differ-ence in the formation of each complex as well asdifferences in their bonding. Generally the experi-mental data show that the formation of the ter-nary complexes shifts the buffer region of the li-gand to lower pH values which indicate that theternary complexes are more stable than the binaryones.

According to our results, the complex equilibriafor the Cu-Hpa-L ternary complexes can be rep-resented by the following schemes:

For Cu(II)-HA-pha, tsa, mal or succ ternary sys-tem:Cu2+ + HA+ H2L;=:[Cu(A)L]- + 3H+, (3~~AL'" (6)[Cu(AW +H2l,;=:[Cu(A)L]- +2H+,K~~~L ... (7)

For Cu(II)-HA-sa ternary system:[Cu2+ + HA+ HL- ;=:[Cu(A)Lt + 2H+, ~f~AI

... (8)... (9)[Cu(AW + HL - ;=:[Cu(A)Lj- + H +, Kf~~L

For Cu(II)-HA-ant ternary system:Cu2 + HA+ HL;=:[Cu(A)L] + 2H+, ~f-~AL[Cu(AW +HL;=:[Cu(A)L]+ H+, K~~~L

... (10)

... (11)

It can be observed from the results (Table 1),that the values of Alog K is positive in all thecases which means that ternary complexes of cop-per(IJ) are more stable than the corresponding bi-nary ones. A comparison of the stability constantsof the ternary complexes indicates that the orderof stability in terms of the secondary ligand ispha > sa > tsa > ant> mal> succ.

The higher stability of pha complex than samay be explained I:; as follows: since the carboxyl-ate oxygen is not directly bound to the benzenenucleus, it therefore adjusts stereochemicallymore easily than the phenolate oxygen which isdirectly attached to the benzene nucleus. Thecoulombic repulsion between the end oxygens willbe more when both 0, ° donor atoms are phen-olic oxygens than when they are carboxylic oxy-gens. Thus, the Alog K should be higher when Lcoordinates through the two carboxylate oxygens(pha) than when L is sa, which contains one car-boxylate and one phenolate oxygen.

The stability order with respect to the aliphaticdicarboxylic acids is mal> succ. The higher stabil-ity of mal acid complex may be due to the pres-ence of double bond in the maleic acid which in-creases the stability of the complex due to exo-cyclic conjugation 10. Further, the d.n-pn interac-tion in Cu-mal complex makes the resulting ter-nary complex more stable.

The higher values of Alog K with pha and sa(aromatic 0,0 donor ligand) than mal and succ(aliphatic 0,0 donor ligand) may be due to thepresence of an aromatic ringl7,IH which alters thebonding properties of these aromatic carboxylicacids.

The data in Table 1 also show that the ~log Kvalues is more positive for tsa ternary complex(0, S donor ligand) than for ant (0, N donor) ter-nary complex. The observed trend may be ex-plained as follows: since sulphur is one of themost important donor atoms in vivo as well as in

ABU-BAKR et al.: STUDIES ON MIXED LIGAND COMPLEXES OF Cu(I1) 647

vitro, it is able to form both a and lr bonds". Theincrease in the stability of the ternary complexhas been attributed to dzr-d a; interactions of metald-orbitals to vacant d-orbitals available on sul-phur atom in tsa. The M-Slr interactions have al-so been considered to affect the formation con-stant of the ternary complex 20.

The lesser magnitude of positive Alog K valuewith ant (0, N donor ligand) may be due to thefact that ant is coordinated to the metal ion insuch a way that the benzene ring is coplanar withthe rest of the complex molecule since the aminoand COOH groups are in the ortho-positions21•

Spectrophotometric measurementsAcid-base behaviour of Hpa

To study the acid-base behaviour of Hpa in eth-anol-water medium (40% v/v) at /= 0.1 M NaC-10 ~, a method of graphical analysis of the absorb-ance- pH curves was used 22. In the pH range -0.5-10.5, the ligand Hpa exists in three differentforms lHi (pH 0.5-1.5), LH (pH 2.0-5.5) and L-(pH 6.0-10.5) having absorption maxima at 285nm, 295 nm and 330 nm respectively. The ab-sorbance vs pH graphs of Hpa indicate the pres-ence of two acid-base equilibria in solution withinthe pH range studied. The acid dissociation con-stants of Hpa were evaluated. The pKu, (LHi IlH)was found to be 0.9 whereas the value of pK,,,(LH/L -) was 8.0. The distribution curves for th~different species of Hpa computed from the aciddissociation constants of the reagent are shown inFig. 1.

'·02

~- 0.8-c••u.~ 0.6..or8s 0.4'j.s•• 0.2'0

3.0 5·0pH

Fig. l=Distribunon curves for the different acid-base formsof Hpa in 40% (vIv) ethanol-water at 25°C, I =0.1 MNaCIOJ, C[= 1.25 x 10-4 M; a=(I) [LH{YC[, (2) [LH~YC[

and (3) [LWYCL.

The solution spectra of 1:1 Cu2 + -Hpa binarycomplex measured against reagent blank as refer-ence are characterized by an absorption bandwith Amax near 330 nm. The spectra of Cu ' + -tsacomplexes are characterized by absorption bandat 340 nm (pH 6.0-7.0). However, the spectra ofthe ternary systems containing equimolar con-centrations of Cu 2 +, Hpa and tsa as a secondaryligand against a reagent blank containing the sameconcentrations of the two ligands exhibit a newabsorption band at 360 nm with maximum colourdevelopment at pH 6.0.

The formation of the ternary complex was ex-amined at different pH values using equimolarconcentrations of components. The variation ofabsorbance values with pH at A = 360nm (Fig. 2)indicate the existence of two basic equilibria with-in the pH range of study. The absorbance-pHgraphs were analysed graphically using the rel-ations derived earlier by Sommer et aF,·2'. Theanalysis of the two rising parts of these graphs,were performed assuming the following Scheme:Cu'" + HA~[CuA]+ + H + A[CuA] + + HL - ~[CuALJ- + H + B

(EI) (EJThe transformation (12) was used for the esti-

mation of the molar absorptivity (E ~) of the cop-per ternary complex.CM/M= t/E2 +(M- EICM)[H]qZ/MK*IE~C I

... (12)where Z = 1 + K,j[H], E I and E~ are the molar ab-sorptivities of the [CuA]" and lCuAL] com-plexes.

The transformation was linear assuming the re-lease of one proton (q = I) during the formationof the ternary complex according to Eq. (B). Thenumher of liberated protons (q) as well as thc eq-uilihrium constant of reaction (B) were ohtainedusing Eq. (13)log [(M- E1CM)Z/(E2CM - M)]

=qpH + log CL +10gK*z ... (13)

The results obtained for the molar absorptivit-ies, equilibrium constants and stability constantsof the binary and ternary chelates are given inTable 2. Job's method of continuous variation-v"was applied to find out the composition of theternary complexes. The results indicate that theoverall ratio of Cu-Hpa-tsa ternary complex is1:1:1. The stoichiometric ratio of the complexeswas also confirmed by applying the mole ratiomethod":

A linear calibration graph was obtained over

648

1.0

0.9

O.B

0.7

•• 0.6uc0

SJ..0

'" 0.5SJ-e

0.4

0.3

0.2

0.1

INDIAN J CHEM, SEC. A, JULY 1994

2. c 3.0 4.0 5.0 6.0 7.0

pH

B.O 11.0

Fig. 2-Absorbance vs pH graphs for Hpa, Cu(II)-binary and ternary complexes 40% (v/v) ethanol-water at 25°C 1=0.1 MNaClO •. (a) Hpa, CL = 1.25 x 10-· M, A.= 325 nm; (b) 1:1 Cu(lI)-Hpa, CL = CM = 2.5 x 10-· M, .1.=330 nm; (c) 1:1' Cu(lI)-tsa,

CL =CM = 2.5 x IO-~ M, A= 340 run; (d) 1:1:1 Cu(lI)-Hpa-tsa, Ceu =CHpa =C,,,, = 2.5 x 10-· M, A= 360 run.

1.0

Table 2- Mean values of equilibrium constants (Jog K *), sta-bility constants (log ~) and molar absorptivities (f) of cop-per(I!) mixed ligand complexes with Hpa and tsa in ethanol-

water medium

Equilibrium +

[Temp. = 25°C; 1=0.1 M NaCI04]

Constant log Constant MolarAbsorptivity

(f)I mol-I cm " '

[MAJ[HV[MlIHAI K*, ( - 2.0 ± 0.02)" 2.5 x 103

(A=325)

[MAL][HV[MAlHL] K*l (-3.2±0.01)' I x 1O~(.1.=360)

[MAI/[MIAI M (6.0)b

[MALV[MIAlL] ~i (20.3)<

• Charges are omitted.'From the absorbance vs. pH graphs for solutions with Cu(II)-Hpa-tsa of equimolar concentrations.blog M= log K ,* + pK'2Clog~l'= log K2* + log M+pKa2 (A) + pKa2 (L)

9.0 10.0

the concentration range 1.27-6.35 Jlg mt I ofcopper(II). The optimum range for accurate deter-mination as evaluated from a Ringbom plot?? was1.9-5.0 ug ml-I of copper. The molar absorptiv-ity and Sandell's30 sensitivity are 1 x 104 I mol- I

ern - I and 1.9 x 10 - 3 J1-g em - 2 respectively. Theprecision of the proposed method was checked byestablishing the concentration of ten samples,each containing 4 J1-glmlof copperfll). The meanrecovery was found to be 100.5% with a relativestandard deviation of 2.3%.

Effect of foreign ionsThe effect of foreign ions was studied by add-

ing different amounts of foreign ions to be investi-gated to a fixed amount of copper (0.08 mg) anddetermining the copper following the recom-mended procedure. The determination of cop-per(II) as a ternary complex was possible in thepresence of 10 mg of Li ", Na +, K +, Ag ", CaH,

Mg2+, Sr2+, Ba2+, SnH, Pb2+, AsH, La3+ ,Th4+,SO~-, NO), CI04, B40~-, CI-, Br-, F-, acetate

ABU-BAKR et al.:STUDIES ON MIXED LIGAND COMPLEXES OF Cu(lI) 649

Table 3-Determination of copper(lI) in synthetic samples and white metal alloys

Sample Composition of the synthetic Copper (,ugl25 ml)sample (,ugl25 ml)

Taken Found Recovery %

Mn(lI) 800, AI(III) 500, Ni(lI) 500 120.0 119.11 99.83100.0 100.1 100.1

2 Ni(lI) 50, Mn(lI) 50, Sn(lI) 50, 120.0 119.75 99.79Co(lI) 50, Sb(lI) 100 100.0 99.112 99.112

3 Zn(lI) 500, Pb(lI) 50, Fe(I1I) 50, 100.0 98.95 911.95Sn(ll) 50, Sb(I1I) 50 80.0 80.03 100.03

4 AI(I1I) 100, Ag(I) 50, Mg(lI) 50, 100.0 911.12 911.12Pb(II) 50, As(III) 100 80.0 79.54 99.42

5 Co(lJ) 50, Ni(lJ) 100, Mn(lJ) 50, 100.0 100.65 100.65Bi(I1I) 50 SO.O 79.18 98.97

Composition of alloy, % Copper Content %

Reported Found Recovery %1 White Metal Alloy Zn (1.38, Sn 84.0, Pb 3.9, Sb 7.5 4.10 4.0S 99.512 Nickel-Zinc Alloy Ni IS.2, Mn 0.27, Bi 0.6, Zn 25.8 55.12 55.10 ]()(J.() 3

and citrate. The determination of copper(lI) wasalso possible in presence of Sb3 ", Mn 2", Co 2+,Zn2+ Cd2+ Hg2+ H PO- (70 mg) and AP+, , '2 4 .(4.0mg) or Bi3+ (1.0 mg). Using the present ex-perimental conditions, it was observed that cop-per(II) could not be determined in the presence ofEDTA or CN-. Accordingly, they could not beused as masking agents. The interference causedby higher concentrations of Fe3 + (up to 6 mg)could be eliminated by adding ammonium fluo-ride solution ( - 50 fold excess) as masking agent.The tolerance criterion for a given ion was takenas the deviation of the absorbance value by morethan 2% from that expected for copper(II) alone.

ApplicationThe proposed method for spectrophotometric

determination of copper with Hpa and tsa is new,rapid and sensitive. The results are accurate andthe wide applicability of the method has beendemonstrated by the satisfactory analyis of syn-thetic mixtures and standard industrial samplesTable 3. The new method was found to have ad-vantages over the other sensitive methods-l=" forcopper due to its reasonable selectivity (most ionsdo not interfere under optimum conditions), goodreproducibility and highly stable reagent solutions.

References1 Casassas E & Fonrodona G, polyhedron, 7 (1988) 689.2 Jons 0 & Johansen E S, Inorg chirn Acta, 151 (1988)

129.

3 Seleim M, ldriss K A, Saleh M S & Sedaira H, Analyst.112(1987) 1685.

4 Abu-Bakr M S,] chern Technol Biotechnol, 1993 (accepted).5 Idriss K A. Seleim M M. Hassan M K Abu-Bakr M S &

Sedaira H, Analyst, ILO(1985)705.6 Seleim M M, ldriss K A, Abu-Bakr M S & Hassan M K

Indian] Chern, 27A (1988) 231.7 Abu-Bakr M S, Idriss K A, Seleim M M & Sedaira H. ]

Indian chem Soc, 68 (1991) 59.8 Irving H M & Rossotti H, ] chem Soc, (1953) 3397;

(1954) 2904.9 Chidambaram M V & Bhattacharya P K, ] inorg nucl

Chern, 32 (1970) 3271.10 Agrawal B H, Dwivedi K, Chandra M, Agrawala B &

Dey K A,] Indian chern Soc, 54 (1977) 931.11 Vogel A I, A textbook of quantitative inorganic analysis.

4th Edn, (EIBS, Longman, London) 1985.12 Douheret G, Bull Soc Chim Fr, (1967) 1412; (1968) 3122.13 Sayee L G, Talanta, 15 (1968) 1397.14 Fisher B E & Sigel H, Inorg Chern, 18 (1979) 425.15 Venkatnarayana G, Swamy S J & Lingaiah P, Indian J

Chern, 27A (1988) 613.16 Nepal J K & Dubey S N, ] Indian chem Soc, 66 (1989)

469.17 Clayton J B & McClure S D, J Arn chern Soc, LOI (1979)

2335.18 Clayton J B & McClure S D,] Arn chem Soc. 101 (1979)

2430.19 Sigel H, Greisser R & McCormic D B, Inorg chim Acta,

6 (1972) 559.20 Joshi J D, PanchaJ B R & Bhattacharya P K, J inorg nucl

Chern, 35 (1973) 1685.21 Bisht J P S, Bisht N P S & Singh S P, Indian J Chern,

28A(1989)812.22 Kuban V & Havel J, Acta chem Scand, 27 (1973) 528.23 Sommer L, Kuban V & Havel J, Folia Fac Sci Nat Univ

Bmo Chemia, 7 (1970) Part I, p. 33.24 Vozinca P, Havel J & Sommer L, Colln Czech chem

Commun, 45 (1980) 54.

650 INDIAN J CHEM, SEe. A, JULY 1994

25 Hrdlicka A & Langova M. Colin Czech chem Commun.45 (1980) 1502.

26 JobP,AnnChim(Rome),9(1928) 113.27 Shirif F G & Awad A M, J iCIOTgnucl Chern, 24 (1962)

79.2H Yoe J H & Jones H L, Indian Engng Chem Anal Ed, 16

(1944) Ill.29 Ayres GH,Anaiyt Chern, 21 (1949)652.30 Sandell E B, Colorimetric determination of traces of me-

tals. 3rd Edn (Interscience, New York) 1959, p. 97.

31 Marczenco Z, Spectrophotometric determination of ele-ments(John Wiley, New York) 1976.

32 ReddyTS&RaoSB, Cu"Sc~48(1979)439.33 Lal K & Malhotra S R, J Indian chem Soc, 60 (1983)

308.34 Shrivastava M & Pandey G S, Ann Chim, 76 (1986) 69.35 Arya S P. Malia J L & Slathia V. Talanta. 34 (1987) 293.

36 Ramakrishna R S & FernandopulJe M E. J inorg nuclChern, 33 (1971) 1940.