DETERMINATION OF PERCENTAGE ACIDITY IN MILK

DETERMINATION OF PERCENTAGE ACIDITY IN MILK

The acidity in milk can be determined by the process called titration and is based upon the chemical principle that equal volumes of acids and alkalies of the same strength will exactly neutralize each other. The point of neutralization is determined by means of an ‘indicator’ which gives one definite colour in an alkaline medium and another colour in an acid medium. The indicator generally used in this connection is phenolphthalein which is colourless in acids and pink in alkalies.

In finding the percentage of acid in milk, it is necessary to know the weight of the milk taken and the strength of alkali solution used. The weight of milk can be found by ordinary weighing or by multiplying the specific gravity of the sample by the number of ml of milk taken, and the strength of alkali used is tenth normal or decinormal (expressed as N/10). One ml of N/10 lactic acid contains 0.009 g of lactic acid by weight. Since equal volumes of acids and alkalies of the same strength exactly neutralize one another, 1 ml of N/10 alkali solution would just neutralize 0.009 g of lactic acid. Therefore, in order to find out the percentage of acidity in milk when N/10 alkali is used, the number of ml of N/10 alkali necessary to neutralize the acid in the amount of milk taken is multiplied by 0.009 and the product is divided by the weight of milk taken in grams and the quotient multiplied by 100. The following formula represents the calculations:

No. of ml of N/10 alkali used x 0.009

X 100=Percentage of acidity

Weight of milk in grams

Now, supposing 1.7 ml of N/10 alkali solution were used to neutralize the acid in 10g of milk, the percentage of acidity in the sample of milk will be as follows:

1.7 x 0.009

X 100 = 0.015 percent

10

Apparatus and reagents

Burette with a clamp and stand

Pipette

Alkali solution (N/10 sodium hydroxide)

Phenolphthalein solution (as indicator)

Procedure

The N/10 alkali solution is put in a burette provided with a glass stop cock so that the alkali can be allowed to flow into the milk drop by drop. The milk is carefully measured or weighed in a glass beaker or white cup. Before opening the stop cock two to three drops of phenolphthalein are added to milk and then the alkali is made to run slowly until the milk shows a faint but stable pink colour for at least one minute. While the alkali is being added, the beaker is gently stirred by a glass rod in order to mix thoroughly the alkali with milk. The number of ml of N/10 alkali used is noted and the calculation is made.

Precautions

●             Care should be taken not to run in excess of alkali.

●             Pink colour must remain stable for at least one minute.

●             Try to obtain the same degree of pink colour each time.

●             Stirr the milk in the beaker gently and carefully so as not to splash the milk out.

●             Record observations in duplicate.

EXERCISES

1.                  Find the percentage of acidity in the given sample of milk.

2.        Measure out 30 ml of above sample of milk, add 10 ml of water to it and then find the   percentage of acidity in the sample of watered milk and compare the percentage obtained in 1  above.

3.          It required 2.2. ml of N/10 alkali solution to neutralize the acid in 10 ml of milk of 1.031   specific gravity. Determine the percentage of acid in this sample of milk.

4.                  Make a sketch of the apparatus used in this experiment.

5.                  In the formula used in this experiment what does the factor 0.009 represent?

6.                  How is lactic acid produced in milk?

7.                  Would the temperature of milk have any effect on the acidity tests?

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