148 



LAURENCE F. FOSTER AND SAMUEL B. RANDALL 



had been recrystallized three to five times. Triple distilled water 

 served as solvent. The stock solutions, as well as the standard 

 buffer mixtures, were kept in heavily paraffined, glass-stoppered 

 bottles. Check determinations on the mixtures at the outset 

 and after a period of seven months showed that the standard buf- 

 fers, from bottles in which the paraffin was not broken, had re- 

 mained constant in Ph in spite of the fact that molds had devel- 

 oped in some of the Hquids. Sorensen (1909a) reported a similar 

 observation on solutions after nine months standing. The de- 

 sired Ph ranges and the solutions used in their preparation are 

 given below: 



Solutions Pg 



m/5 Potassium acid phthalate, m/5 NaOH 4.0-5.8 



m/5 KH2PO4, m/5 NaOH 5.8-7.6 



m/5 H3BO3, m/5 KCl, m/5 NaOH 7.8-9.0 



Indicators 

 The indicator solutions were the following: 



CHEMICAL N.\ME 



Ortho carboxy benzene azo di-methyl 



aniline 



Di brom ortho cresol sulphon phthalein 



Phenol sulphon phthalein 



Thymol sulphon phthalein (alkaline 



range) 



COMMON NAME 



Methyl red 



Brom cresol purple 



Phenol red 



Thymol blue 



CONCEN- 

 TK.ITION 

 IN 50 PER 



CENT 

 C2H6OH 



per cent 



0.02 

 0.04 

 0.02 



0.04 



RANGE 



Ph 



4.4-6.0 

 5.2-6.8 

 6.8-8.4 



8.0-9.6 



Color standards 



Color standards were prepared by adding 0.3 cc. of the required 

 indicator solution to 5 cc. of the buffer mixture. Tubes of color- 

 less glass and uniform bore, 4 by f inches were used for the color 

 standards as well as for the test Hquids. Fresh standards were 

 made up each week, as fading is apt to occur if the solutions are 

 allowed to stand for a longer period. This is most pronounced 

 in the methyl red series and least noticeable in the brom cresol 

 purple series. 



