[mussoy]

What if the analyte is too dark and we can't tell if the indicator is changing colour or not in an acid-base titration reaction

Hi,

I was working on a school project and needed to titrate coffee brewed at 5 different temperatures. I tried doing so, but everything went horribly, and I ended up wasting a couple of hours and about 30 grams of precious Brazilian coffee. The coffee I brewed turned out to be too dark, and I couldn't observe its colour changing after I started titrating it with 0.1 M sodium hydroxide. My supervisor told me to improve my method and then come back, so I scanned the internet to see if any source mentioned my problem, but I couldn't find anything and ended up here. I'm not very good at chemistry, and I don't have much experience with the lab. I would be more than happy if you could help me

P.S. I used phenolphthalein as the indicator

[chenbeier]

No way to use Indicator in dark solutions. Why Not use a pH Probe.

[mussoy]

No way to use Indicator in dark solutions. Why Not use a pH Probe.

My school sadly doesn't offer one

[Borek]

pH meter would be the preferred way to go, so if you can't use one I am afraid there is not much that can be done.

Trick is, most of the ways of remove the coloration require adding something, and most of substances that could remove color (like bleach) will change the titration result (bleach is basically a NaOH solution saturated with Cl₂, it easily destroys most colored compounds, but you will be titrating the NaOH added, not the original sample).

Perhaps - but that's just a wild guess - adding activated charcoal would remove the coloration without changing solution acidity.

That being said - what do you hope to measure with the acid/base titration? Perhaps it is just that the titration is a wrong method selection?

[mussoy]

pH meter would be the preferred way to go, so if you can't use one I am afraid there is not much that can be done.

Trick is, most of the ways of remove the coloration require adding something, and most of substances that could remove color (like bleach) will change the titration result (bleach is basically a NaOH solution saturated with Cl₂, it easily destroys most colored compounds, but you will be titrating the NaOH added, not the original sample).

Perhaps - but that's just a wild guess - adding activated charcoal would remove the coloration without changing solution acidity.

That being said - what do you hope to measure with the acid/base titration? Perhaps it is just that the titration is a wrong method selection?

My intention was to look at the acidity content of coffee brewed at different temperatures and see if there is any relationship between temp. and acidity content (which I know there is). I didn't have a ph meter in hand, so I thought titration would be the way to go (couldn't think of anything else, really), and my supervisor didn't seem to object, so I gave it a shot.

[Borek]

Beware: pH is one thing, acidity is another.

In general, you can't determine pH of the solution by titration, you can determine amount of acids present (that's what I would call acidity, by analogy to alkalinity). These are two different things. pH meter will let you both measure the pH and detect end point point of the titration.

[chenbeier]

@ mussoy
What you can try  is to use the coffee as titrant. You use a  very low NaOH concentration  like 0.01 M or lower and add the indicator. Then you titrate with the coffee in the burette. You need some trials to figure out that the coffee colour don't disturbs the indicator color.