Demonstration of the use of a universal indicator
THE use of a mixed indicator for the determination of hydrogen ion concentration was demonstrated. I t was explained that the idea of an indicator showing changes of colour corresponding to the spectrum was originally suggested by Dr. J. Moir ( J . Chem. Met. and Min. SOC. S. Africa, February, 1917, p. 129), who used a mixed solution of methyl red, naphtholphthalein and phenolphthalein for determining the neutrality of waters. This mixture has been improved by the addition of bromothymol-blue
... bromothymol-blue or thymol-phthalein and cresol-phthalein or cresol red. Mr. Cocking and Mr. Chappel have thus obtained a mixture which exhibits a series of colour changes over a range of P, from 3 to 11, combining the advantages of the several indicators. A solution containing this mixture affords a convenient and rapid means of approximately determining the P, value of a solution. By adding the indicator to a series of prepared buffer solutions, Mr. Carr showed that the colours produced were as follows:-P, 3, pale red; P, 4, red; P, 5, orange red; P, 6, orange; P, 6.5, yellow; P, 7, greenish yellow; P, 8, green; P, 8.5, bluish green; P, 9, greenish blue; P, 9.5, blue; P, 10, violet; P, 11, reddish violet. DISCUSSION. Mr. HINKS enquired why the colours in Mr. Carr's exhibit went in the spectrum order, as he could see no connection between the spectrum and hydrogen ion concent ration.