Spectrophotometric study of n,n'-bis (2-sulphoethyl)dithio-oxamide as a reagent for palladium

ANALYI’ICA

512

SPECTROPHOTOMETRIC

STUDY

DITHIO-OXAMIDE A.

AS

GOEiVICNND,

J~cpu~ltmwI

CHIMICA

M.

OF

ACTA

N,N’-BIS(z-SUEPHOETHYL)-

A REAGENT HERMAN

AND

FOR Z.

PALLADIrJM

EECKHAUT

uf Geneva1 Clrentislry, The Uniuevsiiy ojG/rolt (Rcccivcd

February

roth,

(Uclgicw)

I 963)

In recent investigations several substituted dithio-oxamidesi-b have been proposed as reagents for palladium. JACOBS AND YOEO have examined the complex formation of N,N’-bis(z-sulphoethyl)dithio-oxamide with copper(H), cobalt(II) and nickel(H). It has been found that this reagent forms different yellow, water-soluble chelates with palladium(H). The color sensitivity and stability of some of these chelates, formed between I mole of palladium and 2 moles of the reagent, compare favourably with those of several reactions with other substituted clithio-oxamides. The reaction can be carried out in strong hydrochloric solution as well as in buffer-cd weak acid solution, and does not require strict control of variables. The reagent has the further advantage of being water-soluble. In this paper the spectrophotometric determination of palladium with the above reagent is studied.

Absorbance curves were recorded with a Beckman Moclel DKI spectrophotometcr. Matched silica cells of I cm light path were used. Absorbances at a definite wavelength were measured with a Hilger Uvispek spectrophotometer. Reagents Standard palladiccm solution. This was prepared by dissolving 4 g of palladium(H) chloride (Carlo Erba, R. P.) in 200 ml of distilled water, with sufficient hydrochloric acid to give a final solution about 0.9 M in the acid. The palladium content was determined gravimetrically by precipitation with dimethylglyoxime. The standard solution contained 11.74 mg of palladium per ml. Working solutions were prepared as needed, by volumetric dilution of this stock solution. Reagent solution. A 0.02 M solution of N,N’-bis(z-sulphoethyl)dithio-oxamide diammonium salt was prepared by dissolving 7.410 Q in 1000 ml of distilled water. The stock solution is stable for at least 2 months. All other solutions were made by diluting this standard solution. Buffer solutions. Buffer solutions according to Sorensen were made by mixing Altal.

Chim. Acta, 28 (1963)

512-518

N.N’-BIS@SULPHOETHYL)-DITHIO-OXAMIDE

FOR

513

Pcl

adequate amounts of a 0.1 M citric acid solution with a 0.2 M solution of sodium dihydrogen phosphate as given by KORDATZKI~. Solutions of foreign ions. Stock solutions containing I mg per ml of the ion were prepared from nitrates or chlorides of the metal. Comfilex formation between ~alladi~cm(Il) and N,N-bis-(z-szdphoethyl)dithio-oxamide Absorbance curves obtained in strong hydrochloric acid solution (6 M) are shown in Fig. I. The reagent shows an absorption maximum at 302 rnp and a negligible extinction above 400 rnp. Sharp maxima at 386 rnp and 280 rnp are found when I mole of palladium(T1) is added to I mole of reagent. For an I : z ratio a flat maximum

II

300

coo WAVE/. ENGTH

300

500

Fig. I. Absorbance curves for Pd-rcagcnt mixtures in strong hydrochloric acid solution. A. rcngcnt garo-6 M; U. rcngcnt 5.x0-G M-Pd Cla 2.5~ro-~M;C,rcclgcnt5~ro-5M-J?dC125~ra-~M.

400 WAVELENGTH

500

Fig.

2. Absorbance curves for Pd-rcngcnt mixA. rcagcnt 5*10-6 M; B, rcturcs at PH = 6.6. agent 5.10-b M-Pd Cl, 2.5.x0-~ M; C, rcngcnt .j’ro-“M-T%1 Clz 5.10-6 M.

occurs at 425 rnp and a sharp one at 283 rn+ The curves obtained in a buffered weak acid solution at PH 6.6 are represented in Fig. 2. In this solution the maxima are shifted to the UV region. For an I: I ratio of the metal and ligand, the maximum is found at 275 rnp; for one mole of Pd with 2 moles of reagent, a flat maximum occurs at 382 rnp and a second very pronounced maximum at 281 rnp. The composition of the complex formed was established by the continuousvariationsmethod of JOB as modified by VOSBURG AND COOPERSand the mole ratio method of YOE AND JONES~. In strong acid solution, absorbance measurements were made at 425 m,u and 386 mp. It can be seen from Figs. 3 and 4 that two complexes are formed, the first correAI&. Ctrim. Acln, 28 (1963) 512-518

‘A.

5x4

COEMINNE,

M. .HERMAN,

2.

EECKHAUT

spending to a ratio rPd:1R, the second to a ratio IPd: zR in the presence of excess reagent. In buffered weak acid solution, measurements were made at 382 rnp and 350 mp. Figs. 5 and 6 show that also in this medium the two complexes rPd: IR and 1Pd:zR are formed. It can be shown that the I : z complex is the most stable and is formed always in the presence of excess reagent. It is therefore the most suitable for the spectrophotometric determination of palladium.

14

x0.

0

Fig. 3. Continuous A - 425 mp;

variations 13, A =

method.

386

A.

mp.

9

MOLES

PH

6.6

variations method and 350 mp.

(A) Determination

of palladium

at

PER

I UOLE

Fig, 4. Molt ratio method. A, A = A = 386 mp.

NdLhS

Pig. 5. Continuous

AhAGENl

REAOENT

Fig. G. Mole ratio method

at

PER

:OLE

PH

6.6

Pd

425 mp;

B,

ld

and 382 mp

in strong Irydvochloric acid solution

The absorbance curve (Fig. I) shows a maximum at 425 mp. As the reagent solution has a slight extinction at this wavelength all absorbances were measured against a reagent blank of the same acidity. /l,zal.Ckim. A&n, 28 (x963) 512-518

FOR Pd

N,N'-BIS(2-SULPHOETHY+)-DITHlO-OXAMIDE

515

Effect .of hydrochloric acid concentration. In order to, examine. the. effect of acid concentration, different samples were prepared, each containing 5 ml of 5.10-4 M reagent and 2.5 ml of 5-10-4 M PdCL and different amounts of a IO M hydrochloric acid solution. The samples were diluted ,to 50 ml with twice-distilled water, the final concentration of acid being varied from 0.5 to 8 M.

Fig.

7. Effect of hydrochloric

acid concentration

on Pd complex

at

425

m/c.

Fig. 7 shows that the absorbance does not rise remarkably above a hydrochloric acid concentration of 5 M. For practical reasons, a final concentration of 6 M was chosen. Effect of reagent concentration. Samples were prepared containing 5 ml of 5.10-4 M PdCle. 30 ml of IO M hydrochloric acid, different amounts of reagent solution, and distilled water up to 50 ml. The final concentrations of the reagent and the corresponding absorbances are given in Table I. It can be seen that for constant absorbances, a four-fold or greater excess of reagent is required.

TABLE1 EFFECT

OF

Pd I;‘iml

Yengcl,l I.O’IO’4 1.910-4 2.0*x0-4

4.0*10-J 12.0*x0-4 20.0.10-4

REAGENT concn. concn.

CONCENTRATION g*ro-6

M

Absorbance

(425 nap)

0.597 0.637 0.042 .0.643 0.644

0.643

Effect of time on COZOUY formation. Maximum absorbance is reached after 30 min, and remains constant for 30 min. After this time the absorbance decreases very slowly. This can be seen from Table II where the absorbances are noted, measured after different times against a reagent blank. Conformity to Beer’s law and sensitivity. Beer’s law was examined using a ,serics of samples with a reagent concentration of 2.~0 -3 M, and was found to be obeyed over the range I to 12 #g per ml. The average molar extinction coefficient was found to be E = 12,860 with a mean deviation of o.qq%. This corresponds to a sensitivity of 0.0083 ,pg/rnl as defined by SANDELL~~. .'

Anal. Cl&n.

Acta,

28 (1963)

512-518

516

A.

GOEMINNE,

M. HERMAN,

Z. EECKHAUT

Recommended procedure and standard deviation. To the sample, containing I to IO ,ug of palladium, 30 ml of IO M hydrochloric acid is added, followed by 5 ml of z-10-2 M reagent solution. The sample is diluted to 50 ml with twice-distilled water, and the absorbance is measured after 30 min at.425 rnp against a reagent blank containing the same concentration of acid.

STABILITY

OP

Ti?tlC (win)

nt.~orbuncc o&40

G IO

15 2:

0.645 0.648 0.64g 0.64g

go I20

0.647 o.G4G

150 180

0.643 0.639

TABLE EFFECT

Amount ion (XJ

Foreign

OF

Ratio X:Pd 0.2

RhJ+ Ir3+ Run+ Alla+

5.0 7.5 ::;f

PC3

X53.0 *

Nia+ Cr3+ Cu a-c. I-l&p+ Nitrate Sulpl1ntc

-

III

FOREIGN

IONS

of Pd in tlic rcfercncc solution:

1w+

c0a+

COLOUR

0.2

38.0 3.8 O.IG 191.0

280.0 280.0

267 pg

. -. 272 280

+ -t

276 273 271

: 2.2 -t- 1:s

260

-

272 271 271 278 267 268 268

1.9 4.8

2.7 + x.9 -t- Is.5 + 1.5 +

3-8

f

0:4 OS4

+

Ten samples were measured in this way. The standard deviation was 0.004 absorption units, which corresponds to 0.033 pg of palladium per ml. Effect of foreign ions: To test the influence of foreign ions on the colour, samples were prepared containing 5 ml. of ~-IO- 3 M reagent, 30 ml of IO M hydrochloric acid and 267 pg of palladium. Varying amounts of suitable solutions of the diverse ions were added mdividually, and the solutions were diluted to 50 ml. Anal. Chinr. A ctn, 28 (x963)

512-518

N,N’-snS(z-S~JLPHOETHYL)-DITHIO-OXAMIDE

FOR

Pd

517

Table III gives the palladium concentration found, and the percentage error for different ratios of foreign ion to palladium. Cu 2+, Pt*+, Ru3+, Fe3+, and Au3+ interfere badly. Less interfering are Rh 3+, Ir3+ and Craf, while Ni 2+, COB+, Hg2+, nitrate and sulphate ions can be present in a fifty-fold excess without great interference. (B)

Determination of palladium in hffeered weak acid solution In buffered weak acid solution the absorbance curve (Fig. z) shows a maximum at 382 m,u. As the reagent blank shows an extinction of 0.549 at this wavelength, all measurements are carried out against a reagent blank in the same buffer solution at 392 rnp where the extinction is lower. Effect of pH and coCor stability. In the presence of excess reagent, the absorbance remains practically constant in the PH range from 3 to 8. Therefore strict control of acidity is not required, and all measurements are made in a sodium phosphate-citric acid buffer at PH 6.6. At this value maximum absorbance is reached in less than IO min. The color intensity remains constant for at least 5 IL Conformity to Beer’s law and sensitivity. Beer’s law is obeyed over the range 0.5 to 8.5 pg Pd per ml. An average molar extinction coefficient E = 10,840 was found The corresponding sensitivity, as defined by with a mean deviation of 0.35%. SANDELL, is 0.0098 pugper ml. Recommended pvocedzcre and standard deviation. To the sample, containing I to 8 pg of palladium, 20 ml of the buffer PH 6.6 is added, followed by 5 ml of 2.10-3 M reagent solution. The sample is diluted to 50 ml with twice-distilled water, and the absorbance is measured at 392 rnp against a reagent blank containing the same buffer. Ten samples were measured in this way. The standard deviation was 0.004 absorption units, corresponding to 0.039 ~6 of Pd. Effect of_ foreign ions. The effect of foreign ions was studied as described above. _ The results are summarized in Table IV. It can be seen that the interference at pH 6.6

TAULE TWFISCT

-

FOREIGN

XONS

I\mount of RI in the rcfcrcncc solution: ---_-_---

_----_...---.-. Foreign

OF

I\’

ion (X)

1w+ lih3+ It-s+ IluJ+

X: Pd

Ruiio

__,___-_____-___--_..-~~

1.1 x.8 1.5

0.X

.4113+

I.5

coa+

0.4 0.4

I?@+

Ni’J+ cl-a+

cu=+

I-igs+

Nitrate Sulphatc

X.0

3.7 0.2

3.2

300.0 300.0

2G7 ~16

fJalfudiarna (I&

fcuold

o/u Error

.-.-_--

270

f

270 270 276 =7r

-1_ I.1 + I.1 -I- 3.4 -i- 1.5

273 284 27” 270

+ f f +

272 278

-I- 1.9 + 4-r

Anni:

I.1

2.1

G-3 1.1 I.1


Chinr.

Acta,

28

(XgGg)

5x2-518

518

A. GOEMINNE,

M. HERMAN,

2. EECKHAUT

@f most cations is greater than for the reaction in strong acid solution. Therefore the l&%&: medium must be preferred for practical applications, although the rate of colour formation is less. SUMMARY N,N’-bis(z-Sulphocthyl)dithio-oxamide forms two water-soluble yellow chelates with palladium(H), in strong hydrochloric acid solution or in buffered weak acid solution; with excess of reagent, a xl?d:2R complex is always obtained. The absorption maximum occurs at 425 rnp in G M hydrochloric acid and at 382 rnp in buffered weak acid solution (pn 6.6). The colour is formed more slowly in strong acid solution, but there arc fewer interferences. Relatively high concentrations of Co2+, Nil+ and l-igz+ can bc tolcratcd, as well as smaller concentrations of I++, Rha+ and Cr3+. The molar absorption coefficients arc 12,8Go and 10.840 respectively in 6 M hydrochloric acid and buffered weak acid solution. The standard deviation in both casts is 0.004 absorbancc units, i.e. respectively 0.033 and 0.039 ~8 of Pd.

Lc N.N’-bis(n-sulphocthyl)dithio-oxamidc formc avcc Ic palladium aussi bicn cn milieu acidc chlorhydriquc conccntrd (G M HCI) qu’en solution tampon dc PH 6.6 deux complexes jaunts, solubles dans l’cau. En prescncc d’un excbs dc rdactif on obticnt toujours lc complcxc 1l?d:2R. Cc dernicr complcxc montre un maximum d’absorption h 425 rnp en 6 M HCI ct % 382 rnp en solution tampon dc pt~ G.G. Lcs coefficients d’absorption sont rcspcctivcmcnt 12,8Go et ~0,840, ZUSAMMENFASSUNG N,N’-bis(2-SuIphocthyl)dithio-oxamide bildet mit Palladium sowohl in stark salzsaurcm Medium (6 M HCI) wit in eincr schwach sauren Pufferl6sung zwci wasserltislichc. gelb gcflrbtc Komplexc Mit einem Ubcrmasz Rcagenz bckommt man immer den Komplcx 1Pd:zR. dcr tin Absorptions. maximum hat bci 425 rnp in HCl Medium und bci 382 rnp in eincr schwach snurcn PuffcrlGsung. Der molar-c Extinktionskocffinicnt ist rcspclctiv 12,8Go und 10,840. REFERENCES 1 W. D. JACOI~S, Anal. Chcm.. 32 (rg60) 512. a W. D. JACOBS, Anal.Chem., 33 (rgbr) 1279. 3 W. D. JACOBS, C. M. WIIIZELER AND W. M. WAGONNER, Talarrla, g (19G2) 243. 4 J. T. PYLE AND W. D. JACOUS, Talanfa, g (rgG2) 761. 0 A. A. JANSSENSAND M. A. HERMAN, Bull. Sot. Chittr. &?lges, 70 (rgGr) 597. Q W. D. JACOBS AND J. H. YOE, Proc. Symp. Chem. Co-ord. Compounds, Agru, ZtJdia, 1959 Part III, Nat. Acad. Sci. India, rgGo, p. 22G. 7 W. KORDATZKI, 'Tascltenbuch clerPraktischen pFZ-Messfrrrg, Miillcr-Stcinickc Vcrlag, Miinchcn 1941. 0 W. C. Voseunc AND G. R. COOPER, J. Am. C/rota.SOC.. G3 (x941) 437. D J. H. Yen AND A. L. JONES, Znd. Etrg. Chena., Atral. Ed., xd (1944) 3. 1~ E. 13. SANDDLL, Calorimetric Determinafion of Traces ofMetals, 3rd cd., Intcrscicncc, New York 1959, p. 83. Arta!. Chittt. A&a,

28 (1963)

512-5xE