Abstract
Previous studies 1 led us to expect pH changes on nitration of buffers through cellophane membranes. We present a preliminary report of data which confirm our expectations.
We used phosphate buffers of pH 7.0 and 7.2, having phosphate concentrations of 0.005 M and 0.01 M. They were prepared by the addition of CO2-free NaOH to twice recrystallized NaH2PO4:H2O. The 0.5 M stock solution was diluted with conductance water. Prior to filtration the cellophane 600 plain membranes were evacuated at 20 mm. Hg. and the buffer solution added. After repeated washing and soaking for several days the unsupported membrane was placed between pyrex cylinders of 2 cm. diameter and filtration carried out at 38 cm. Hg. The upper cylinder, into which the filtrate passed, communicated with a 125 cc. Erlenmeyer flask containing some of the buffer. Concentration of the filtrate by evaporation was thus prevented. The pH measurements were carried out according to the micro method of Bodine and Fink, 2 quinhydrone being used. A 25°C. ± 0.01 thermostat and type K Leeds & Northrup potentiometer were used. pH determinations of the original buffer and of the filtrate were carried out simultaneously. As we are here interested only in pH differences, the quinhydrone salt error was not evaluated. About 30 filtrates were examined with 2 or more pH determinations on each filtrate. The maximum fluctuation on duplicate determinations was 0.015 pH. The filtrate of the 0.005 M pH 7.2 phosphate buffer showed a maximum change toward the acid side after 3 hours of 0.46 pH and an average of 0.24 after from 2 to 15 hours.
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