A new liquid membrane electrode for the selective determination of perrhenate

0039-9140/88$3.00+ 0.00 Copyright 0 1988Pergamon Press plc

Talanta, Vol. 35, No. 5, PP. 361-364, 1988 Printed in Great Britain. All rights reserved

A NEW LIQUID MEMBRANE ELECTRODE FOR THE SELECTIVE DETERMINATION OF PERRHENATE SAAD S. M. HASSAN* and M. A. HAMADA Department of Chemistry, Faculty of Science, Qatar University, Doha, Qatar (Received 5 October 1987. Accepted 10 December

1987)

Stunmary-A new perrhenate ion-selective electrode has been developed, incorporating a nitrobenxcne solution of nitron perrhenate as a liquid membrane. The electrode gives near-Nemstian response to 3 x 10-s-10-2M perrhenate over the pH range 3-g. Most common anions (except for periodate and perchlorate) give little interference. The electrode has been satisfactory for direct potentiometric determination of as little as 10 pg/ml rhenium. The average recovery and standard deviation were 99% and 2.1%, respectively. Measurements of the solubility products of some sparingly soluble perrhenates gave results that agreed closely with those recorded in the literature and obtained by other procedures.

Rhenium is used for the production of some hard corrosion-resistant alloys and as a catalyst for industrial hydrogenation and dehydrogenation reactions.’ Most of the procedures for its determination involve conversion with hydrogen peroxide into perrhenate, which can be assessed by gravimetric,” spectrophotometric,H indirect atomic-absorption spectrometric,’ polarographic,” and isotopic dilution” methods. Most of these techniques involve several time-consuming manipulation steps and are neither selective nor sensitive. It has been reported that the commercial perchlorate? and nitrate” ion-selective electrodes with membranes containing nickel bathophenanthroline perchlorate and tributyloctadecylphosphonium nitrate, respectively, can be used, after appropriate membrane conditioning, as perrhenate sensors. Electrodes responsive to perrhenate as a primary ion have been based on the ion-pair complexes of perrhenate with tetraoctylammonium,‘4 tetradecylamm~nium,‘~ triheptyldodecylammonium,16 tetratriaquo-oxahexakis(stearato)phenylarsonium,“*r8 trichromium(III)‘9 and some basic dyeGo dispersed in lipophilic solvents or polymer matrices. Many of these electrode systems, however, suffer from serious interference by Cl-, NO<, SCN-, I-, ClOh, We-, VOj- and MoOi- ions.r%1%15.17.19 Recently we described some membrane electrode systems for IO:,21 Cl0~,~~ BF;23 and SCN-24 based on the ion-pair complexes of these ions with nitron. In this investigation, the nitron-perrhenate ion-pair complex was prepared and tested as electroactive species for use in an electrode responsive to perrhenate. A liquid membrane consisting of a nitrobenzene solution of nitron perrhenate was found to respond to 3 x 10-5-10-2M ReO; over the pH

*To whom correspondence

should be addressed.

range 3-8 with minimal anions.

interference

from many

EXPEEIMENTAL Apparatus

All EMF measurements were made at 25 & 1” with the nitron perrhenate liquid membrane electrode and a Corning pH/ion meter (Model 135). An Orion Ag/AgCl double junction reference electrode (Model 90-02) with the outer barrel filled with 10% potassium nitrate solution completed the cell. An Orion combined glass electrode was used for pH adjustment. The electrochemical cell used for potential measurements was: Ag-AgCl/lO-*M aqueous NH4ReQ4 + 10m2M NaCI/10-2M nitron perrhenate in nitrobenxene// porous membrane//ReQ; test solution/Ag-AgCl reference electrode. Reagents

All reagents were of analytical arade unless otherwise stated and doubly distilled water w& used throughout. A 10e2M nitron solution was nrenared in 10% v/v acetic acid. A lo-*M aqueous perrhenate stock solution &as prepared from ammonium perrhenate of purity not less than 99.8%. Dilute perrhenate solutions (10-6-10-3M) were freshly prepared by serial dilution. Nitron perrhenate complex was prepared by mixing about 20 ml of lo-*M ammonium nerrhenate and 25 ml 10-2M nitron acetate. The grey precipitate was collected in a G4 sintered-glass crucible, washed with cold water, dried at 80 for 1 hr and ground to a fine powder. Elemental analysis of the precipitate gave C 42.4%, H 2.9% and N 9.7%; C,H,,N,.HReQ,; C 42.6%, H 2.9% and N 9.7%. The most significant absorption bands in the infrared spectrum of the precipitate were those at 3240-3260 cm-’ and 920-940 cm-‘, assigned to stretching vibrations of the secondary amine and perrhenate groups, respectively. Elecirode preparation and calibration

An Orion electrode barrel (Model 92) was used with an Orion microporous membrane (92-06-04) to separate the organic and aqueous phases. The organic ion-exchanger liquid membrane was a IO-*&f solution of the nitron perrhenate complex in nitrobenxene, and the internal reference solution was a mixture of equal volumes of aqueous 10-*&f ammonium perrhenate and sodium chloride solutions. The electrode was conditioned after preparation, by being soaked in 10-3M ammonium perrhenate for at least 24 hr and was stored in the same solution when not in use. 361

SAADS. M. HA&UN and M. A. HAMADA

362

The perrhenate electrode and the reference electrode were immersed in approximately 20 ml of 10-6-10-2M standard perrhenate solution in a 50-ml beaker. The emf readings were recorded when stable to f 1 mV and plotted as a function of pRe0,. Measurement of solubility products About 0.5 g of a freshly prepared sparingly soluble perrhenate was suspended in 20 ml of doubly distilled water and kept in an airtight Erlenmeyer flask. The mixture was shaken vigorously for 3 hr in a thermostat adjusted to 25 f 1” and allowed to stand for 1 hr before the ootential was measured. The concentration of perrhenate in solution and hence the solubility product was calculated. RESULTS AND DISCUSSION

Electrode response and stability

The performance of the electrode system was characterized by measuring the useful linear response range, slope, effect of pH, response time, membrane selectivity and electrode stability. The IUPAC recommendationszs were used fey evaluating these parameters. In pure aqueous ammonium perrhenate solutions, the electrode displayed near-Nemstian response for 3 x 10-s-10-2M perrhenate with a slope of a 56 mV/pReO,. The limit of detection was 4.5 x lo-‘M. Figure 1 shows the dependence of the electrode potentials on pRe0,. Similar results were obtained for perrhenate solutions in O.lM sodium chloride. Least-squares analysis of data collected for 3 different electrodes over 3 months gave the relationship: E (mv) = (-56 + 0.6) log C - (12 f 0.Q the standard deviation being 1.2 mV. The reproducibility of the potential readings from day-today measurements for the same solutions was within f 2 mV and the variation in the slope. did not exceed 2 mV/pReO, over a period of 6 weeks. The useful lifetime of the electrode is 5-6 weeks, after which the membrane of the electrode should be renewed. The

220

-

\\ I\ t

-Log

CRetOJ (MI

Fig. 1. Potential-response curve for the nitron perrhenate liquid membrane electrode.

Em

lO-2 M

12

e0OO

PH Fig. 2. Effect of pH on the potential response of the nitron perrhenate liquid membrane electrode.

electrode was stored in doubly distilled water between measurements and in 10m3M ammonium perrhenate when not in use.. The stability of the nitron perrhenate membrane is attributed to the stable aromatic structure of the triaxolium cation of nitron, which is closely related to the well known stable tetraxolium cation.26 The static response time of the electrode was evaluated by exposing the electrode to a rapid change in perrhenate concentration and recording the resulting emf as a function of time. Steady potentials were established after 30 set for concentrations 2 lo-‘M, 50 set between 10m3and IO-‘M, and 80 set for concentrations Q lo-‘M. The potential readings remained stable within +2 mV for at least 5 min. EfSect of pH and foreign ions

The effect of pH on the electrode potential readings was studied for 10-2-10-4M perrhenate adjusted to different pH values with sodium hydroxide solution or hydrochloric acid. The potential-pH plots (Fig. 2) indicate that the potentials were almost independent of pH in the range 3-8. At low pH there is hydrogenion interference, and at high pH the emf decreases, probably owing to dissociation of the membrane material. The effect of some common anions on the potential response of the electrode was tested by determining the selectivity coefficients of the membrane by the separate solutions method, as described previously.z’-2s The results (Table 1) show good selectivity relative to 14 common anions at the 10e2M level. The electrode shows poor selectivity, however, with respect to periodate and perchlorate, both of which also interfere with some previously described perrhenate electrodes.‘5*‘9 Rhenium is commonly associated with molybdenum and tungsten in its ores and alloys. Prior extraction2’+r* or solvent separation by

Liquid membrane electrode for perrhenate Table 1. Potentiometric selectivity coe&ients for some anions, obtained by the separate solution method Interferent (B)*

Q&s

Br-

2.1 x 1o-2 0.91 4.2 x IO-’ 4.6 x lo-’ 4.0 x 10-r 5.6 x lo-’ 3.1 x lo-’ 3.8 x 10-S 4.8 x lo-’

I-

IO, NO, NO, F$&Mk-

1.5 x 10-J

3.8 x lo-” 1.1 x 10-j

Ace&e

Oxalate

*All tested at W2M level.

chromatography29sW is thus an unavoidable step in almost all previous procedures for analysis of rhenium-molybdenum and rhenium-tungsten mixtures. Molybdate and tungstate seriously interfere with some perrhenate membrane electrodes,19 but have negligible effect on the response of the proposed nitron perrhenate electrode. Up to a 200-fold molar ratio of either ammonium molybdate or ammonium tungstate to 10-5-10-3M ammonium perrhenate has no significant effect on the electrode potential. Determination

of perrhenates

Analysis of lo-2000~pg/ml rhenium solutions prepared from standard sodium perrhenate solutions, in triplicate, by direct potentiometry with the membrane electrode and a calibration graph prepared with ammonium perrhenate solutions gave the results shown in Table 2. The average recovery was 98.7% and mean relative standard deviation 2.1%. The solubility products of some sparingly soluble perrhenates were also determined as described, and the results (given in Table 3) agreed fairly well with the reported values. 3’,32For silver perrhenate, howTable 2. Direct potentiometric determination of perrhenate Rhenium added, pgglml

Recovery,*

Standard deviation,

%

%

10.0 30.0 70.0 100 150 200 400 800 1200 1800

98.8 99.1 98.9 99.5 98.1 99.5 98.1 98.7 98.2 98.0

2.1 2.0 1.9 2.2 2.1 1.9 1.8 2.0 2.2 2.3

*Mean of 3 measurements.

Table 3. Direct potentiometric determination of the solubility products of some sparingly soluble perrhenates

Compound

5.1 x lo-’ 6.7 x 1O-3 0.76 3.5 x 10-3

ClClO,ClO,

363

KReG, CsReG, TlReG, AgReO, Nitron-HReOd

k;, Electrode method (?I = 4) 2.6 f 4.0 f 1.2 f 6.8 f 6.3 +

0.1 0.3 0.2 0.2 0.3

x x x x x

lo-’ 10-4 lo-s lO-4 lo-*

Literature values 1.9 x 4.0 x 1.2 x 7.9 x 9.0 x

10-j 1o-4 10-s lo-’ lo-*

Reference 31 31,32 31 31 2

ever, the value found was 6.8 x 10m4(literature value 7.9 x 10e5). The value found by measuring the silver ion concentration with a silver sulphide ion-selective electrode (Orion 94-16) was 6.3 x 10e4, close to the value obtained with the perrhenate electrode. The results show that direct potentiometric measurement of perrhenate with the nitron perrhenate electrode gives a simple and rapid determination of this ion. The electrode system proposed shows useful sensitivity, reasonable selectivity, fast response times and adequate stability. The method involves minimal sample pretreatment, compared with many of the previously described procedures.2-”

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SAADS. M. HK+UN and M. A. HAMADA

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