Question

Determining the ethanoic acid content of a commercial brand of vinegar?

It's for planning and designing a lab. I've done it several times, failed each time.
Problem statement: **Jason had to determine the ethanoic acid content of a commercial brand of vinegar by titration. The following indicators were available:

Bromothymol Blue
Phenolphthalein
Methyl Orange

When he used metyl orange with a standard sodium hydroxide solution, he did not observe an end point. Propose a hypothesis for this observation and design an experiment to test it.**

I've tried using hypotheses revolving around 'Indicators having specific pH ranges in which they can be used', and 'methyl orange indicator not showing an end point due to ethanoic acid being a weak acid ...'

I don't know what else to do. I've failed the lab several times, and I'm totally lost. Please, help!

Answer 1

Warning! Long Answer. Here's what I get.

Explanation:

If you are titrating acetic acid with sodium hydroxide, you have heard of concepts like acidity, pH, and titration curves.

pH = 7 is neutral. pH < 7 is acidic. pH > 7 is basic.

The pH of the acetic acid is probably close to 3 or 4.

As you add `"NaOH"`, the pH increases slowly until you get close to the equivalence point, when it rises quite steeply. Then it again increases slowly to a pH of 11 or 12.

The equivalence point occurs when you have added a volume of `"NaOH"` corresponding to the steepest part of the curve (it is also the point at which the curvature of the graph changes from concave up to concave down).

How do we detect the equivalence point?

(a) Use a pH meter

We could use a pH meter to follow the pH as we add the `"NaOH"`. Then we could plot the titration curve and locate the equivalence point.

(b) Use an indicator

An indicator is a substance that changes colour at a given pH.

We could add an indicator and then titrate with `"NaOH"` until the colour changes.

Why doesn't methyl orange give an endpoint?

Methyl orange changes colour from red (in acid) through orange to yellow (in basic solution).

(a) Hypothesis

Perhaps the acetic acid has a pH at which methyl orange is already yellow. Then, no matter how base we add, the solution will remain yellow.

(b) Test the hypothesis

Try other indicators that change colour at different pH values.

Bromothymol blue changes colour from yellow (in acid) through green to blue (in basic solution).

Phenolphthalein changes from colourless (in acid) through pale pink to red (in basic solution).

(c) Observe the results

Both bromothymol blue and phenolphthalein give colour changes at the same volume of `"NaOH"`.

(d) Tentative conclusion

Even though the acetic acid is acidic, its pH is high enough that methyl orange is already in its yellow (basic) form.

(e) Further research

Look up the pH at which these indicators change colour.

• Methyl orange: pH 3 to pH 4

• Bromothymol blue: pH 6 to pH 8

• Phenolphthalein: pH 9 to pH 10

The titration curve shows that both bromophenol blue and phenolphthalein have colour changes close to the equivalence point, but methyl orange is yellow long before we have reached the equivalence point.

Hypothesis confirmed!