Biphthalate-hydrochloric acid buffer

The mixtures of Clark and Lubs are very easy to prepare. The starting materials are readily obtained in the pure state, and the arrangement of solutions at pH intervals of 0.2 unit is very convenient. The biphthalate-hydrochloric acid buffers, however, have two disadvantages which detract somewhat from its usefulness. 

SvERRE Stene has measured the pH of a number of phosphate buffer mixtures, biphthalate solutions, and borate buffers with the hydrogen electrode at 150 . He found that the pH of biphthalate-hydrochloric acid solutions at 150° was about 0.2 unit greater than at 20°, the pH of biphthalate-sodium hydroxide mixtures was 0.7 greater than at 20°, while that of boric acid-borate buffers diminished with increasing temperature. Solutions of the latter system with pH s up to 9.0 were 0.5 unit less at 150°, 0.6 unit less for pH 9.2, 0.8 unit less for 9.6, and a whole unit for pH 10.0. Because certain assumptions introduced in his calculations were not entirely justified, these data must be accepted with reserve. Thus the boric acid-borate solutions behave differently from other buffers consisting of a weak acid and one of its salts. Walbum (table, page 250) also has found this diminution of pH with temperature. 

Fig. 9. Polarographic curves of 10" mole/liter solution of phenolphthalein in 25 o aqueous ethanolic solution [46]. 1) 3.8 N hydrochloric acid + 0.001% fuchsin 2) biphthalate buffer at pH 4.13 3) phosphate buffer at pH 7.40,...

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