determination of citric acid

DETERMINATION OF FRUIT ACIDS BY
TITRATION AND CALCULATION OF THE SUGAR/ACID RATIO
It is the sugar/acid ratio which contributes towards giving many fruits their characteristic flavour and so is an indicator of commercial and organoleptic ripeness. At the beginning of the ripening process the sugar/acid ratio is low, because of low sugar content and high fruit acid content, this makes the fruit taste sour. During the ripening process the fruit acids are degraded, the sugar content increases and the sugar/acid ratio achieves a higher value. Overripe fruits have very low levels of fruit acid and therefore lack characteristic flavour.
Titration is a chemical process used in ascertaining the amount of constituent substance in a sample, e.g. acids, by using a standard counter-active reagent, e.g. an alkali (NaOH).
Once the acid level in a sample has been determined it can be used to find the ratio of sugar to acid. There are two methods specified for the determination of the titratable acidity of fruits:
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Method using a coloured indicator;

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Potentiometric method, using a pH meter, which should be used for very coloured juices.

Material:
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A laboratory burette of 25 or 50ml capacity or an automatic burette is used. A 10ml pipette, beaker
(250ml), a filter (muslin cloth or fine filter) and an extractor or homogeniser.

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A bottle of distilled water.

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Sodium Hydroxide (NaOH): The Standard Laboratory solution of 0.1M which is used in the actual titration is considered to be dilute, and can readily be purchased in this form.

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Phenolphthalein:

This is a 1% w/v solution of phenolphthalein in 95% v/v ethanol which is flammable and toxic if ingested. This is only required for the method using a coloured indicator.

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Indicator stripes

To check the exact point of neutrality an indicator stripe should be used. Not necessary if pH Meter is used.

Sampling
To evaluate the lot selected for inspection, take a sample