Direct heterometric microtitration of copper with oxine in the presence of other metals

464

ANALYTICA

13IRIZC-l.

HETEROMETRIC OXINE

IN

THE

CHIMICA

ACTA

VOL.

MICROTITRATION PRESENCE

iM. BOBTELSKY

AND

OF

Y.

OF OTHER

COPPER

10

(1954)

WITH

METALS

\VEL\VART

INTRODUCTION

In a previous report a method was presented for a quick heterometric determination of micro quantities of copper with oxine in the absence of interfering cations. This report deals with the determination of copper in the presence of an excess of other metals. It was intended to investigate whether a direct determination of copper may be carried out in the prescncc of complex binders such as tartrate or citrate. The amount of copper in the solutions analysed is about 5 I mg and is about 5n/ of the total in the presence of the light metals Mg and Al, about 2% in the presence of other metals such as Zn, Mn, Co, Ni, Cd and Cr and about o.z% in the presence of Pb. This investigation is in no way complete. Further investigation will show to what cxtcnt the concentration of the citrate or tartrate and consequently of the metals present, may be increased without interfering with the determination of the copper. Only in the case of iron did we not succeed in obtaining a clear endpoint (no maximum was obtained). The time required for a complctc titration is 10-20 minutes and the error is o-1.0 y/ml. No interference due to adsorption, which would shift the endpoint of the titration, was noticed. This conforms with the results of heterometric investigations in general. This may be explained by the fact that the Primury particles which form the suspension generally do not adsorb. On the other hand, the point of initial precipitation is postponed in many cases, especially in more dilute copper solutions, by the presence of an excess of certain metals (in complex form). The latter may be explained by the influence of the metal complexes on the solubility of copper oxinate or it may be caused by supersatnration. In a somewhat higher concentration of copper this phenomenon disappears. The position of the initial point, however, is without influence on the endpoint of the titrations. TECHNIQUE

All the titrations

were ma&

with a heterometer

(photometer

for the titration

VOL.

10

(1954)

DIRECT

HETEROMETRIC

~IICROTITRATION

OF

COPPER

4%

of suspensions). Generally a cylindrical vessel was used which had an inner diameter of 3.8 cm. Titrations number 4. II and 12 were made in a vessel of diameter 5.0 cm in order to decrease the optical density. In all titrations with oxinc a deep orange filter was used. Only the solutions containing iron or cobalt were titrated with a red filter. No additional filters wcrc required in the presence of other coloured salts as the colour of the salt solution did not change during the titrations. A microburct (O.OI ml) was used. The maximum density point was obtained from enlarged curves. A line was drawn through the maximum points obtained and the point of intersection of this line with the last portion of the density curve was taken as the endpoint of the titration. In some cases where a convex optical density curve was obtained, the first contact point of the density curve with the maximum density line was taken instead. The reagents wcrc either analytical

reagents or chemically

pure.

RESULTS

Table I presents a complication of the composition and results of titrations of copper in the presence of other metals. Figs. I and 2 show the course of the

08

I 1.5

I

I

a?

25

I I ml

?O

Fig. x. Titration curves of co per in the presence of otEer metals.

466

M.

BOBTELSKY,

Y.

WELWART

VOL.

10

(x954)

VOL.

10

(1954)

DIRECT

HETEROMETRIC

MICROTITRATION

OF

COPPER

467

same titrations (the numbers of the curves correspond to the numbers of the experiments in Table I). The titrations were made either in citrate or tartratc solutions. No fundamental difference was observed in either case, although in many instances only in the presence of one of them could good results be obtained. c

4

I

QC

I

I

OA

Fig. 2. Tit ration curves of co per in the prcscncc of ot K cr metrrls.

0

0.5

1.5

2

2.5

ml

Cd’s gave a precipitation in the presence of citrate and a tartrate solution was therefore used instead; the highest concentration of Cd’s used could be only 0.05 molar. In some cases (exp. g and 12) the maximum density values were too low and higher concentrations of copper were used instead. The titrations which proceeded with difficulty are marked in the Table with “d”. Generally, the accuracy is the same in the presence of either of the complex binders. The error lies between 0.0-1.0 y/ml. Only in a few cases (which were not used as analytical methods) were the errors higher. The results presented are not complete and in each case further detailed investigation is rcquircd. Some

remarks

concevnixg

the resatlts.

In the titration of copper with oxine in the presence of large excesses of other metal salts, we meet with the following difficulties: the point (on the abscissa) of the initial precipitation and the maximum density vaZzrc may both change with the added salt. The former may shift very near to the end of the titration

hi.

468

BOUTELSKY,

Y.

WELWART

VOL.

10

(1954)

and the latter may be very depressed. These phenomena depend on the salt added and on the complex binder used. The above difficulties may be overcome either by incrcnsing (doubly) the nmount of copper OI by changing the com@ex binder. Analysing Table I we see the following: The blank in the presence of a basic tartratc solution (cxp. I) gives a point of initial precipitation at 1.4 ml or at 2/3 of the titration. This point is only slightly changed in the presence of trivalent metals. Mn‘C2, Cdf2 and Pb’2 shift the point towards the beginning of the titration, but the maxima values obtained (Cd+2, Pb+2) are depressed. Even more complicated are the phenomena in the presence of citrate. The initial point of the blank (cxp. 2) occurs, in comparison with tartrate, more at the beginning of the titration. Mgek2 and Zn+2 (CdSC2gives a precipitate with citrate) hardly influence the results. The trivalent metals, Mn+2 and Co+2 shift the point of the initial precipitation towards the end of the titration, but the maxima values are not changed. Contrary to this, the maxima values are greatly reduced in the cast of Ni+2 (exp. g) and Al+3 (exp. IG). It is interesting to note that, using the higher copper concentration, the depression of the maximum value disappears (exp. IO) and the blank value is obtained again. These phenomena require an individual study. METHOD

General tccltnique. The solution must be vigorously stirred during the titration. A microburct (& 0.01 ml) or a syringe is used (the end of the buret must be immersed in the solution). The readings on the galvanomctcr are taken only when they become constant: this takes generally a few seconds to a minute. The titrations are made at room temperature and last IO-20 minutes. &loys. The metal is dissolved in nitric acid or in a mixture of nitric acid and hydrochloric acid. Excess acid must be evaporated. The analysed solution must be neutral. Detcnnination The solution

of copper contains

in the @vxelzcc

an cxccss

of Mg,

of sodizrm citrate Mn,

Co, Ni,

Al

OY Cr.

To 18 ml of a solution which contains S I mg copper (as nitrate or chloride), are added 2 ml of x.oJl sodium citrate. The solution is titrated with o.oriU-o.osM oxine (50°jo alcohol). If the solution contains an excess of Zn, Al or Ni, the amount of copper must be doubled. Error o.-1.0 y/ml. Determination 7%

soldow

of copper contains

in the fivesoncc

an excess

of basic potassium

(or

sodium)

tar&ate

of AZ, &In, Cd or Pb

To IG ml of a solution which contains S I: mg copper (as nitrate or chloride), are added z ml of x.onr potassium tartrate and 2 ml of I.OM NH,. The solution is titrated with 0.0xil~-0.02~1~ oxinc (50% alcohol). In the presence of an excess of Cd+2 the amount of copper must be doubled. Error o-1.0 y/ml.

VOL.

10

(1954)

DIRECT

HETEROMETRIC

MICROTITRATION

OF

COPPER

469

SUMMAR-Y A new hetcrometric method is presented for a rapid micro-determination of copper with oxine. s I mg of copper in 20 ml of solution may be determined with of o-r .o microgram per ml. The solution may contain 95% Mg or Al, an accurac is made at room 98% Zn. n? n, Co, Ni, Cd or Cr and go.8O/e Pb. The determination temperature and takes 10-20 minutes.

Unc mdthode heteromdtrique est prescntde pour la micro-determination rapidc du cuivre avec l’oxine. On peut doscr s I mg de cuivre clans 20 ml de solution ar ml. La solution peut contenir 95% avec une precision de o-1.0 microgram Pb. Cette determination est Mg ou Al, 98% Zn, Mn, Co, Ni, Cd ou E:r et 99.8% faite 8 la temperature ordinaire et dure environ 10-20 minutes. ZUSAMMENFASSUNG Eine heterometrische Methode zur raschen Milcrobestimmung von Kupfer mit Oxine wird presentiert. s I mg Kupfer in 20 ml Liisung lcann mit einer Genauigkeit von o-1.0 Microgramm per ml bestimmt Werdcn. Die zu untersuchende LCisung kann enthalten 95% Mg oder Al, 98% Zn, Mn. Co, Ni, Cd oder Cr, sowic g9.8%, Pb. Die Bestimmung wird bci gewohnlicher Tempcratur ausgefiihrt und dauert etwa 10-20 Minuten. Received November 6th, 1953