Stability constants of some metal dithizonates

228

SHORT

Talanra. Vol

20, pp 228-232

Pergamon

STABILITY

COMMUNICATIONS

Press. 1973 Prmted m Great Br~tam

CONSTANTS

OF SOME METAL

DITHIZONATES

(Recewed 20 March 1972. Accepted 21 August 1972)

The mstabthty and poor solubihty of dithizone (diphenylthiocarbazone) m water makes the determmation of stab&y constants of its metal chelates difficult. For that reason, the values of the stabihty constants are almost unknown’ despite the excellence of dnhrzone as a chelatmg and extraction reagent We mvestigated that problem and found a satisfactory solution that is presented m this paper EXPERIMENTAL

Apparatus

All photometrrc measurements were made with the DU Beckman spectrophotometer, model 2400 (Fullerton, California). Quartz cells of l-cm pathlength were used throughout. An Orion model 801 pH-meter (A. H. Thomas Co., Philadelphia. Pa.), with combined glass and calomel electrode, was used for pH measurements Reagents

Perchlorates of the metals (G. F Smith Co., Columbus, Ohio) were used as 1M) x lo-‘M aqueous stock solutions. Dithrzone (Fisher Certified Reagent, Fan Lawn, NJ ) was purified by Welcher’s method.’ Its quality waschecked by paper chromatography with methanol-water 10 1 on Chrom AR500 paper (Mallmckrodt Chem. Works, St. Louis, Missouri) and by elemental analysis. The product was chromatographically pure. R, = 0.87 It was analysed with the followmg results: C 611)x, H 4 8%, N 21 7%. S 12 4%; required for C,sH,,N,S: C 60.9%. H 4.7x, N 21.9%, S 12.5%. The 1.00 x 10e3M stock solution of dithizone was prepared by dissolution of 6.4 mg of dithizone m 25.00 ml of O.lM sodtum hydroxide and O+lM hydroxylamme hydrochloride. The solution was kept m the dark at 25” It was used during the next three hours, then a new solution was prepared. Solutions of O.lM glycme + Oa5M hydroxylamme hydrochloride + 0.095M sodium perchlorate and 0.1 M sodium hydroxide were used to adlust the pH to 5 55-10.02. Measuremenr The sequence of buffer, metal ion and dithrzone was adhered to during the preparation of all solutions measured. The ionic strength was kept at O.lN (sodium perchlorate), the temperature at 25 k 1” The absorbance of solutions was measured agamst a buffer-reagent blank (or buffer-metal blank when an excess of metal ion was being used). RESULTS

AND

DlSCUSSlON

Stabkty of dithizone solutron

Hydrazme, ascorbic acid, sodium sulphite and hydroxylamme hydrochloride were mvestigated as stabihzmg agents for aqueous soluttons of dtthtzone. The last-mentioned reagent exhibits the most convenient properties It sufficiently stabihzes the solution of dithizone and its masking and reducmg effects are neghgible for the Table 1 Stability of dithizone stock solution* Time mm

2

8

10

12

17

24

38

At

1019

1019

1019

1019

1019

Af

0.985 0.952 0.786 0.419 0 323 0.286 0.273 0.265

48

1.019 1.019 1019

60

120

180

240

300

1.019 1019

1018

1015

1009

-

* Measured after dilution to 4.00 x 10e5M, at pH 6 50 and 470 nm t Stabilized solution. $ Unstabilized solution.

-

-

-

-

0997 -

SHORT

229

COMMUNICATIONS

concentrations of metal ions used The ttme-dependence of absorbance of the stabihzed and unstabihzed soluttons of dnhtzone IS shown tn Table 1 It can be seen that the stabthty of the stabihzed solutron IS qmte satisfactory during the first three hours. Stabdtt~~ ofelectroneutral

complexes

Metal dlthazonates are soluble at concentrattons of 10-5-10-6Y at pH > 5 3 Preclpttatlon occurs below that value Investigatmg the composition of complexes formed m the range of pH 5 5-6 5 by means of the contmuous vartatton method. we found that electroneutral complexes of ML, are formed m all cases, where the value of n corresponds to the charge on the metal ton. The occurrence of a smgle complex of ML, was checked by the method of Coleman et aL3 employmg the equation AA,/AA2

= AE.,,JAE,,,~, = const

where AA,, AA2 and A+,), AsE,,s, are the efiecttve absorbances measured at two different wavelengths We have

(1)

and molar absorpttvittes,

respectively,

AA, = A, - A,,,,

(2)

and

where A, and A,,,, are the absorbances of the complexing and hgand (drthtxone) solutton, respecttvely; E.,~,and cr,_,,are the molar absorpttvtttes of the complex ML, and of the hgand. Plottmg the dependence AA, =f(AA,) for equatton (1) we obtamed straight lures crossing the pomt AA, = AA, = 0 throughout. Knowmg the value of n for each complex, we determmed 1t.s overall stabthty constant j?., employmg the method of stolchtometrtc dilutton 4 Accordmg to that method we have b. = Q(Y) x;,,/(Y, n”cu”)

(4)

where akl,v, and aL,u, are the side-reactton coefftclents of the metal wtth the hgand Y of a masking reagent (and buffer) and of the hgand L with hydrogen tons, respectively; c,,, IS the total concentratton of the metal and (x - 1).+* yn = 2”(1 - r)(x - ty Table 2 Stability constants of some metal dlthizonates* I

Complex

AdBlL, CdL, CoL* CUL, FeL, HgL, MnL, NIL, PbLz PdLz SnL, ZnL,

pH 8.21 560 6.50 6 50 6 50 6 50 1002 6 50 6 50 650 6.50 5 55 6.50

nm 400 630 560 570 570 570 530 400 570 560 660 640 590

x 1.170 1 300t l.Ollt 1.014 1.034 2.050 1.005 1500 1.087 I.017 1012 1240 1.025

log au(v) 1 76 15.13: 046 0.59 6 58 0 16 25.254 002 2 36 092 7.68 0.397 1.01

log 8. 6.98 k 0.15 32.11 f 022 15.10 f 0.15 13.97 + 0 08 19.18 f 0.07 899 + 0.11 40 3 + 0.8 9.55 + 007 1417kO.05 14.16 + O-11 21 78 k 0.08 11.99 5 0 12 13.96 k 0.08

Ass x lO-3 cm’/mmole 405 21.6 24.1 22.6 16.5 19.5 10.6 18.0 21 5 21 5 8.8 32.9

u

9.77 59 2 280 # 20.0 49.0 127 24.9 28.0 52.8 212 5 52

-log v

-

log 81

-

1.22 7.81 f 0 11 2.06 7.52 + 0.09 1.72 9 35 k 0 11 1.57 4.78 + 0.15 1.98 2064 & 0.16 1.33 444 f O-09 1.67 7.42 + 0.11 1.86 7.31 f 009 2.01 11.39 +o 11 1.60 6.35 f 0 15 0.91 6.93 + O-09

* For determmatlon of j?. c,,, = 2M) x IO- 5M, glycme (cy = 0.08M) as maskmg reagent and a buffer were used for stolchlometric dllutlon; log zL,u, = 0 throughout c,., = 4GO x 10e4M, cr. = 403 x 10e5M and hexamme buffer of pH 6 20 were used for the determmatton of b1 The value of each stabihty constant IS an average of five determmatlons t CM= 500 X 10_6M : cv = O.OlM (EDTA) # CM= 125 x 10-2M. 4 cl = 0.08M (DTPA). c cv = 0lOM (chloride)

230

SHORT COMMUNICATIONS

where x = AA/(ZAA’)

and

f = (x/T) ‘(“+’ )

(6)

AA and AA’ are the effective absorbances measured for C~ = cJn and ch(’= c,‘/n respectlveiy (c,, = 2~~‘) Accordmg to the same method we have for the effective molar absorptlvrty A&, = AA(x - t&x(1

- t)]

(7)

and the value of E. IS calculated by means of equation (3) All results of this method are collected m Table 2. Stabrlity of complexes ML of hvalent ions The overall statnlity constants of those complexes can be determmed by employmg an excess of metal Ion Since the complexes of ML, are relatively stable, we may write m that case CL =

WI

+

2CML21=C&I& + V&-l)

where ch( and cL are the total concentrations of the metal and hgand. respectively. and chl P cL The expresstons m the square brackets designate the actual concentrations of the correspondmg species. p1 and pz are the overall stability constants of the complexes ML and ML* respectively The effectrve absorbance of that sotutron will be AA = AE,[ML] + As2[MLI] = c~[L](AE& + As,p,[L])

(9)

where AA=A-A,

(10)

and A&, = E, - .Q. AQ =

E2 -

cM

(lt)

A and A, are the absorbances of the complexmg and metal solutlons respecttvely, Ed, E* and Q, are the molar absorpttvtttes of the complexes of ML, ML, and metal solutlon respectively. The absorbance and molar absorptivlty of the ligand can be Ignored, owing to the small concentratton of the free lrgand Cornblning (8) and (9) by eiiminatton of [L] we obtain r = Z/3,,c1 r2/[/&‘(2k - l)c,,,] - (2k - 1)

(12)

where r = 1 - 2AAJ(hzc,)

(13)

k = AE~/AE~

(14)

and

Usmg cL’ = c J2, cMP= cMwe obtam instead of (12) and (13) r’ = &cLr’Z,[~1Z(2k - l)chl] - (2k - I)

(1%

r’ = 1 - 4AA’/(As2cL)

(16)

and

Combmmg (12) and (15) by ehmmatlon of k we get 2~~CJ(~,ZC~) = 1/[2(u - l)] 4 1/[2(u - I)]” = D

(17)

u = (r - r’) _ ’ - r’jr

(18)

where

In this manner we computed the normaltzed values of log u with a precision of I IO-21and cdcL = 10 and u = 100 as iimltmg condttIons, The case of AA = 2AA’ should be avolded RAE, = AsZ [when the equattons (8) and (9) complex (erther ML or MLZ) 1s formed Results of the determmatton of the stab&y

function of log v =I‘@). It is plotted m Fig 1 To obtain the to keep r - J 2 0.01, t( should be 2 105 and I 100 Assuming we obtam log B, ‘//& I 1.59 It occurs with Improper selection of either the wavelength tf become Identical] or the ratlo c&, and pH so that only one constants of complexes ML are also collected m Table 2

231

SHORT COMMuNlCATlONS

123456

7

Fig 1 Dependence of log u = f(u)

8

9 uxb

1 . b = 1, 2 . b = 0 1

Conclusions

Employmg spectrophotometry as the method of measurement, the use of stoichiometric dtlution enables us to keep the concentration of dithmone at a mmimum level and to perform the determination of overall stab&y constants of metal dithizonates in aqueous solution by the method described. In spite of that approach, dithizonates of bismuth and cadmium exhibit a great tendency to precipitate, so then stability constants should be determined at a total metal concentration c,., = 5.00 x lo-‘M. Mercury(H) dtthrzonate 1s so stable that the use of diethylenetriammopenta-acetic acid was necessary to adJust its elIecttve stabihty constant Results m Table 2 Indicate that the use of a stronger masking reagent would be destrable, but we could not find a better one Mono- and difunctional ligands containing sulphur donor groups frequently form mixed complexes wtth dithizone.’ Acknowledgement-The authors are grateful to the National Research Council (Ottawa, Ontario) for a grant that made thus work possible Department of Chemistry Unwersltp of Arizona Tucson. Arizona 85721, U.S.A

B W

Department of Chemzstq Unwersity of Waterloo Waterloo, Ontario, Canada

M. SAGAT

BUDESINSKY*

REFERENCES

1 L G. Sdlen and A E Martell. Stab&J Constants of Metal-Ion Complexes, 2nd Ed, p. 680. The Chemical Society, Spec Pub1 17. London. 1964. Supplement No 1, p. 709, Spec Publ. 25, 1971 2 F J Welcher. Orgamc Analytrcal Reagents. Vol 3, p 189. Van Nostrand, New York, 1953. 3 J S Coleman. L P Varga and S H Mastm, Inorg Chem 1970.9, 1015 4 B W Budesmsky. Z Anal. Chem.. 1972, 2% 186 5 J Star); and J Rtificka. Talanta. 1968. 15, 505 * To whom correspondence

should be addressed

232

SHORT

COMMUNlCATlONS

Summary-The overall stabthty constants of electroneutral drthtzonates of btsmuth, cadmmm. cobalt(H), copper(H), tron(II), lead(II), manganese(II), mercury(H), mckel, palladmm(II), stlver. tm(I1) and zmc were determined by means of stotchtometrtc dtlution m aqueous solution stabtlized by hydroxylamme hydrochlortde, at pH 5 50-1002. Stabthty constants of complexes ML of btvalent metals were determmed under simtlar condittons but with an excess of metal Zuaammwfaaaung-Dte Gesamt-Stabthtatskonstanten der elektrtsch neutralen Dtthrzonate von Wismut, Cadmium, Kobalt(II), Kupfer(II), Eisen(II), Blet(II), Mangan(II), Queckstlber(I1). Nickel. Palladium(II), Silber, Zmn(I1) und Zmk wurden durch stochtometrtsche Verdunnung m durch Hydroxylamm stabthsterter wagrtger Losung bet pH 5 50-10.02 bestimmt. Unter ahnhchen Bedmgungen, ledoch mrt Metallhberschul3, wurden dte Stabthtatskonstanten der Komplexe ML zwetwertrger Metalle ermtttelt. RCumi+-On a determine les constantes de stabtlite globales des dtthtzonates tlectromquement neutres de btsmuth, cadmium, cobalt(II), cutvre(II), fer(II), plomb(II), manganbe(II), mercure(I1). nickel, palladtum(II), argent, etam(I1) et zmc au moyen de dtlutton stoechtomitrtque en solutton aqueuse stabilisee par le chlorhydrate d’hydroxylamme, a pH 5,X%10,02. Les constantes de stabrliti de complexes ML de mttaux divalents ont ett dttermmees dans des condtttons semblables mars avec un excb de metal.

Talanta, Vol 20. pp 232-236

Pcrgamon Press, 1973 Prmted ,n Great Bntam

ENTHALPIMETRY

OF NON-AROMATIC

(Recemed 30 May 1972 Accepted

ALKENES

18 August 1972)

Enthalptmetry IS the analyttcal techmque of determmmg the amount of a substance present m an unknown sample by ailowmg rt to react quantttattvely wtth some reagent and measuring the amount of heat produced or taken up by the reactton. At constant external pressure, the heat effect 1s equal to the enthalpy change, hence the name. Although thermodynamtcs tells us that the AH of reactton may be postttve. negattve or zero, the last IS practically never the. case and some heat effect 1s observed for vtrtually all reacttons Because of the ubtqmtous heat of reactton, enthalptmetry is an extremely general method. requtrmg only that the substance sought undergo a quantttattve reaction wtth some reagent. Although thermochemtstry IS a very old sctence, enthalptmetric analysts has attracted wade attentton only during the past decade.‘-* In this work, we have combmed the prmctple of enthalptmetry wtth the preferred method of olelin determmatton, that of hydrogenation. Recent work has brought olefin determmatton by hydrogenatton to a high level of senstttvtty and accuracy. Brown’ has been able to analyse samples containing as httle 3 pmole of olefin by means of in stru generation of hydrogen and catalyst. He reported an error of about 4% for thts minute amount. Curran and co-workers have reported determmatron of between 1 and 2 pmole, usmg null-point pressurtmetry and volumetrtc addttton of sodtum borohydrtde to regenerate the hydrogen consumed lo More recently, they have determined less than 1 pmole wtth an error of about 69; by thetr techmque of substttutrng coulometrtc regeneration for the sodium borohydnde step ‘I The hydrogenatton reactton IS quantttattve and raped for most sample olefins.‘*.” hence we have used it as the basts of an enthalptmetrtc method for oletin determmatton. r4 The purpose of thts work was to modify the extstmg apparatus and procedure so as to take better advantage of the senstttvtty due to the large negattve AH of the hydrogenatton reaction We have been able to decrease the workmg range by more than an order of magmtude from our prevtous work, to about 2-20 gmole, wtthout sigmficant loss m accuracy Moreover, m another serves of expertments, we have decreased the amount of olefin taken to about 0.6 firnole,-although at thts level the accuracy suffers EXPERIMENTAL

Reagents

All olefins were obtamed from Chemtcal Samples Co. and certified to be more than 999; pure by the manufacturers Hexane was obtamed from Fischer Chemtcal Co and Matheson. Coleman and Bell supphed the 5’; palladtum catalyst on powdered charcoal