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DENNIS S~EVERS Central Community Htgh Schwl Route 50 West Breese. IL 62230
A Device for Direct Potentiometric Measurements
of Samples with Very Large Electrical Resistance S. Oman University of E.K. of Ljubljana, 61001 Ljubljana, Yugoslavia According to the accepted definition of pH, thermodynamically exact pH values cannot be determined with conventional methods. Most pH determinations are made using the established activity scale of hydrogen ions ( 1 ) .Since the pH values measured by the common procedure are often the only reproducible quantitative characteristicsof samples, they are of critical importance in technologicalprocesses or quality control (2). ~ e a s k n e n t and s interpretations of pH values of water solutions usually are not difficult. Prohlems arise with partly aqueous media and with samples containing small amounts of water and having a large specific electrical resistance like cosmetic and pharmaceutical emulsions (especially those of the water-in-oil type) and cream-. The chemistry of cosmetics (3)and a laboratory course in cosmetic science ( 4 ) have been recentlv described in THISJOURNAI..TOinsure compliance with the health and sanitary regulations, the pH of cosmetic products must be monitored. Because the direct pH measurement of creams and emulsions, which have very high electrical resistance, has been either not nossible or not nrecise enoueh. their DH values traditional~; have been esgmated indiGctly by measuring the DH of their water extracts (5).Recentlv. a method was ~ r o posed for direct pH measurement of &ch samples using a specially designed and conditioned, combined glass electrode (6).Direct pH measurement of samples with electrical resistance nearly equal to the input resistance of pH meters is impossible using separated glass and reference electrodes; however. such measurements are ~ossiblewhen a combined glass electrode is used. Electrical iesistance of the hydrated elass surface between the membrane and the liquid iunction of the combined glass electrode shunts the high;esi&ance of the sample, thus making the measurement possible with commercially available pH meters. If the specially designed, combined glass electrode is not available, similar results and reproducibility (within 0.14.3 pH units) can be achieved with the improvised assembly shown in the figure. A glass electrode with low alkaline error should be used. The bulb and 2 to 3 cm of the electrode body next to the bulb are slightly etched in a dilute H F solution and thoroughly rinsed with water. Two or three layers of filter paper are wrapped on the etched part of the body and a funnel-shaped rubber or plastic tuhe is pushed tightly over this filter paper, so that a few millimeters of paper extend out of the tube. A saturated solution of KC1 is poured into the funnel, and the tip of the saturated calomel reference electrode is placed in it. The glass electrode of this assembly is then put in the sample (e.g., cream) so that the filter paper is immersed. The electrodes are connected to the pH meter, screened, if necessary, by a Far-
An iwovised Eombinati of single elecbodes for pH measurememof sarrples having very high electrical resistance. -lass electrcde. R-reference electrode. FP-filter paper. S-sample, KCI-saturated KCi solution. F-funnel-shaped rubber or plastic tube.
aday cage. Before each measurement the bulb of the glass electrode must be thoroughly cleaned, calibrated using pH standards and immersed in water for 30 min. Analogous assemblies can he made for pNa or pK measurements. Because the combined glass electrodes for pNa or pK determinations are seldom available, the Na or K glass electrode can be combined with the reference electrode in the same way as has been described for the pH glass electrode, thus makine-. Dossible the direct determinations of uNa and pK values as well. Literature Clted (1) Barn, R G., "Determination ofpH."kdd., John Wileyand Sona,New Ymk,l9l.2. (2) Mclnnee, D.A, "The Principles of Eleetroehemiatry," Reinhold, New York, 1939. (3) J. CWeM. EDuc.,SS.802 (1978). (41 Kayc, S. R.,J. CHEM. Eouc.. 57.M (l98o). (51 Sum, J.M., and lbaanez. S.;'EatUdio Experimental del pH en Sitemaa Dispnoade Inter& Farmsc6utieo:11. Enaavos Definitivos oara la Selffcih de Thicas."A m
Phorrn.. 11.215 (1970). (6)Oman, S.,"ElecirometricpH Determinationof EmulaiansHaving Very HighElectrlc Resistersee? Vpstn. Slou.Kern. Drw. 31,195 (1984).
Volume 61 Number 11 November 1984
1029
