Analytical Chemistry

If n = 1, the range of values of E over which a colour change is observed is 0.118 volt and is shifted by

0.059 volt for each unit change in pH. is often known as the transition potential for the indicator. A
representative list of redox indicators is given in Table 5.5.

Table 5.5 Some common redox indicators

Indicator Oxidized colour Reduced colour Transition potential

(volts)

Solution conditions

5 - nitro-1, 10-phenanthroline iron

(II) complex

pale blue red-violet +1.25 H 2 SO 4

(1 mol dm–^3 )

2,3'-diphenylamine dicarboxylic

acid

blue-violet colourless +1.12 H 2 SO 4

(7–10 mol dm–^3 )

1,10-phenanthroline iron(II)

complex

pale blue red +1.11 H 2 SO 4

(1 mol dm–^3 )

erioglaucin A bluish red yellow-green +0.98 H 2 SO 4

(0.5 mol dm–^3 )

diphenylamine sulphonic acid red-violet colourless +0.85 dilute acid

diphenylamine violet colourless +0.76 dilute acid

p-ethoxychrysodine yellow red +0.76 dilute acid

methylene blue blue colourless +0.53 acid

(1 mol dm–^3 )

indigo tetra-sulphonate blue colourless +0.36 acid

(1 mol dm–^3 )

phenosafranine red colourless +0.28 acid

(1 mol dm–^3 )

One important group of colour indicators is derived from 1:10 phenantholine (ortho-phenanthroline)
which forms a 3:1 complex with iron(II). The complex known as 'ferroin' undergoes a reversible redox
reaction accompanied by a distinct colour change