The aromatic, naphthene, and paraffin separations obtained are illustrated in Figure 3, which shows a chromatogram that is typical of a temperature-programmed run on a wide boiling range feedstock. A comparison between the results of capillary column GLC analysis of a number of distillates and the totals obtained for the aromatics, CS, CS, C7, and Cg naphthene and paraffin isomers on a quantitative basis gave excellent agreement. In addition to the above use the separation of aromatics on a short column of Type 1OX sieve is useful for the rapid detection and estimation of the total concentration of any aromatics present as impurities in saturated petroleum gases and solvents. Moreover, because the individual aromatics are backflushed together, there is a significant improvement in the lower limit of detection. Also, the ability to collect the total aromatic fraction is useful, if a further more detailed examination is required, be-
cause it enables trace quantities of individual aromatics to be estimated which would otherwise be undetectable in the direct analysis of a feedstock of wide boiling range. In the absence of benzene and toluene, the retention times of Cs and higher aromatics on Type 1OX sieve are so high: that by a suitable choice of column length the total aromatic content of distillates of much higher boiling range may be measured rapidly. Conversely if Cg and higher aromatics are absent, a suitable column containing Type 1OX molecular sieve permits the direct determination of benzene and toluene together with ethane and the other higher saturated hydrocarbons present in natural gas.
RECEIVED for review December 16, 1968. Accepted March 25, 1969. Permission to publish this paper has been given by the British Petroleum Company Ltd.
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Liquid-Liquid Membrane Electrodes Based on Ion Association Extraction Systems C. J. Coetzee' and Henry Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 85721 IN A previous communication ( I ) the preparation of anion sensitive electrodes based on ion association extraction systems has been reported. In the present study additional information on these electrodes is being reported regarding pH-dependency and the effect of interfering anions on electrode response. EXPERIMENTAL All chemicals used were of reagent grade quality. Solutions of sodium salts of organic acids, were prepared by neutralization of a known weight of the acid with standard NaOH. Electrode assemblies consisted of Orion calcium electrode barrels (Model 92-20) kindly supplied by James Ross of Orion in which the appropriate organic and aqueous reference phases were placed using Orion (92-20) membranes as separators. The organic phase consisted of a 10 vjv of solution of Aliquat 3368 (found to be 0.21N) in 1-decanol which had been converted to the proper quaternary ammonium salt by repeated shaking with an aqueous O.1Msolution of the potassium or sodium salt of the appropriate anion in order to effect the exchange. The aqueous reference phase consisted of a 0.100M NaCl solution that was also 0.100M in the appropriate sodium or potassium salt. After preparation, the electrode was conditioned by immersion in 0.1M solution of the salt to be measured for several hours and was stored in this manner when not in use. A Beckman Research Model pH meter equipped with a Beckman saturated calomel electrode was used for all potentiometric measurements. 1
On leave from University of Orange Free State, Bloemfontein,
So. Africa. (1) C. J. Coetzee and Henry Freiser, ANAL.CHEM., 40,2071 (1968). 1128
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION Each of the sixteen electrodes prepared was tested in pure solutions of the appropriate salts over a concentration range of 10-1 to lO-5M. Equilibrium potentials were achieved quickly (20 sec to 1 min) and values were reproducible to ~ t O . 2mV. In every case the potential response gave a strictly linear change with logarithm of anion activity from 10-1 to at least 10-SM although for some anions this extended to lO-4M. In every case except salicylate, the variation of electrode response with activity was large enough to be useful at concentrations down to 10-4 or lO-SM. These results are summarized in Table I. A series of iodide-responsive electrodes were prepared in which the Aliquat concentration in the decanol phase was varied. These were found to have the same general response characteristics and gave very nearly the same potential readings in solutions of the same concentrations (Table 11). The small variation observed is of the same order as that between electrodes prepared as duplicates. The electrode response can be said to be essentially independent of the decanol concentration of the quaternary ammonium salt in accord with Eisenman's expectation (2). Of course, if the Aliquat 336 concentration is further decreased and the membrane resistance increases, a lower limit would probably be reached below which the membrane would not respond. The effect of temperature on these electrodes was examined by measuring the slope of the concentration response curve of the perchlorate electrode at 15 "C and 40 "C. At both temperatures observed values close to the Nernstian (57.0 and 62.0 mV, respectively) indicated that the electrodes were behaving reversibly in this temperature range. Small changes in Eo values on standing were observed
(2) G. Eisenman, ANAL.CHEM.,40,310 (1968).
Table I. Summary of Behavior of Anion-Responsive Electrodes at 25 "C Concentration Slope range of Useful Ion (mV/log a ) linear response concentration range 10-1-10-3 10-1-10-4 Perchlorate 59.2 10-1-10-3 10-~10-5 Chloride 56.0 10-1-10-3 10-1-10-4 Bromide 59.0 10-3-10-3 10-1-10-4 Iodide 59.0 10-~10-3 10-1-10-4 Nitrate 57.0 10-2-10-4 10-l-10-6 Sulfate 36.0 10-1-10-3 10-l-10-6 Thiocyanate 58 .O 10-2-10-4 10-1-10-6 Oxalate 40.05 Formate 53.Oa 10-L 10-3 10-1-10-6 10-1-10-3 10-l-10-5 Acetate 53.0a Propionate 57.55 10-1-10-3 10-1-10-5 Benzoate 58.6= 10-1-10-3 10-1-10-4 Salicylate 56.Oa 10-l-10-3 10-1-10-3 10-~10-3 10-1-10-4 rn-Toluate 58,Oa 10-l-10-3 10-1-10-5 p-Toluate 57.Oa mV against log C taken.
Table 11. Effect of Organic Phase Concentration of Electrode Response Aliquat 336 concentration, vlv
Potential us. SCE in 0.100MKI solution, mV
1
-328.4 -326.7 -329.2 -326.0
5
10 25
Table 111. Evaluation of Acid Dissociation of Acetic Acid at 25 "C Using the Acetate Electrode Acetate molarity
PK,
0.05
4.61 4.74 4.71 4.58
0.10 0.25 0.50
electrodes, therefore, can be applied to the determination of acetate, benzoate, and similar anions in various acid-base mixtures. A systematic examination of the extent of interference from foreign anions on electrode response was undertaken. Electrode responses in solutions containing 0.01M of the appropriate anion and amounts of the interfering anion varying from 10-1 to lO-5M were measured and the results used to evaluate selectivity ratios K, as defined by
requiring almost daily restandardization of all electrodes. This has been also observed with similar commercial electrodes (3). The useful lifespan of these electrodes was found to be from one to two months with failure always arising from mechanical problems. Although the electrode response for anions of negligible proton affinity--e.g., C1O4--benzenesulfonate was pH independent in the range studied (pH 2-10), the response of anions having basic characteristics, such as acetate and benzoate, changed when the pH became low enough to transform a significant fraction of the electrolyte into the corresponding acid. If such changes can be attributed to the assumption that the electrodes respond to changes in anion activity but are independent of neutral acid activity, then the pH profile of the responses of these electrodes should permit the calculation of the acid dissociation constants of the acids. The results of such calculations shown in Table 111, unquestionably validate the assumption. The
where aA is the activity of the anion An- and that of the interfering anion of charge z- (4). These values, shown in Table IV, are in most cases essentially constant but some show a dependence on the concentration of the interfering ion. In such cases, although there is a systematic concentration dependence which could be expressed as log
(3) G.A. Rechnitz and Z. F. Liu, ANALCHEM., 40, 696 (1968).
(4) J. Ross, Science, 156, 1378 (1967).
Electrode Perchlorate Thiocyanate Nitrate Iodide Bromide Chloride Sulfate Oxalate Formate Acetate Propionate Benzoate
Table IV. Selectivity Ratios for Various Interfering Anions Expressed as log K c1Noss04'OBzMiscellaneous anions -2.1 -0. 5= -2.0 For c103-, -0.8; Po43-, -2.3; p-Tosate, -0.25
