Metal finishing Q&As since 1989

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Improvement of colour change of bromophenol blue indicator in glycerine

I am doing a project to determine the soap content in distilled and bleached glycerine. I'm using bromophenol blue [on eBay or Amazon] indicator solution in the glycerine mixed with water and titrated it with 0.01N HCl. the problem is I cannot determine the end point, that means I can't see clearly when the colour changes from green to yellow, it's very confusing using only naked eyes as different people will see different colours. Is there any suggestion to improve my experiment?

Thank you,

Kimberley Walsh
- Georgetown, Penang, Malaysia
2003

What I used to do with colorometric indicators was to back titrate with a few drops of equal strength titrant, NaOH in your case and then add a couple more drops of indicator and forward titrate again. After several tries, I would find the minimum amount of indicator that would work for me. It also lets you know what the color change will look like when you use too much indicator and with too little. When using a strong acid titrant, the amount that the indicator enters the reaction is irrevelant. It can be calculated, but is a very tiny amount unless you are using massively too much. Another way would be to use a pH meter, or to use a pH meter with the indicator until you determined the color change.

James Watts
- Navarre, Florida
2003