Bedlasky
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Registered: 15-4-2019
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Indophenol derivatives - interesting pH and redox indicators
Hi.
I few days ago spectrophotometrically meassure NH4+ concentration in samples in my job. I used set intended for this determination. It essentially
contain three bottles:
1. NaOH solution
2. Some solid chlorine source (NaDCCA or TCCA, I don't know which one of these)
3. Sodium nitroprusside + thymol solution in i-PrOH
Reaction proceeds in two steps:
1. 5ml of sample + 0,6ml NaOH solution + small spoon NaDCCA are mixed together and let stand for 5 minutes. Ammonia reacts with NaDCCA to form
chloramine.
2. Addition of four drops of sodium nitroprusside/thymol solution --> Thymol reacts with chloramine to form indophenol derivative, sodium
nitroprusside is catalyst. Another 5 minutes standing. Green solution is slowly formed.
3. Meassuring
This reaction can be done with any source of chlorine (hypochlorite, NaDCCA, TCCA etc.) and any phenol (phenol, salicylic acid, thymol, hydroquinone
etc.). I some time ago read that reaction proceeds at pH 10-11, but solution that I made in work have pH 12 and reaction still proceeds.
I observed interesting phenomena - I poured this solution in to beaker and continue in other analysis. Than I poured in to the same beaker solution
from determination of silica (which is acidic). Intense green colour turns in to light (but intense) blue colour. So I wondered if this solution is pH
sensitive or there is some reaction between two solutions. So I added acid in to another indophenol solution - and it changed colour in to the blue!
So when I had all the analysis done, I prepare 20ml of this indophenol solution and did titration with meassuring pH. And I observed this:
1. At pH 7,6-8 solution change colour from green to blue
2. At pH 3,7-4,2 solution change colour from blue to colourless
These values are just indicative, I didn't do an exact titration. Transition between blue and green isn't much sharp, colours are too similar
(espacially in dilute solution). In short summary:
1. Basic solution - green form
2. Neutral to slightly acidic solution - blue form
3. Strongly acidic solution - colourless form
I also read that indophenol is redox indicator. So I tried if this derivative also have this property. Green form after addition of hydroxylamine
decolorized after few seconds, blue form maybe little change in colour, but nothing distinct. After addition of some KMnO4 colourless solution turned
back to green.
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