LABORATORY WORK No 8

Performed by:

Kristyna Rihova, Gabriela Hradova, Jana Parizkova

Conditions:

t = 18.5 °C; p = 1008.2 hPa; φ = 40.9 %

Topic:

pH of Foods

Tasks:

1.

Establish the pH of liquid foods in cooking.

2.

Make up the bar chart and determine which of the foods are acid and which are alkaline.

Theory:

pH is a quantity that indicates acidity or alkalinity of a solution. pH of substances dissolved in water makes the quantities 0 to 14. The solution is neutral (e. g. distilled water) if pH = 7. The solution is acid if its pH is lower than 7. The solution is alkaline if its pH is higher than 7.

If you add any acid into water, for instance vinegar CH₃COOH, then the molar concentration of cations H⁺ will rise and pH will sink. If you add stronger nitric acid HNO₃ to the water then the pH of more acid solution is for example pH = 2. The more acid the solution, the higher concentration of H⁺ ions i.e. lower pH.

If you dissolve lime CaO or baking soda NaHCO₃ with alkaline chemical reaction  in water, then the concentration of H⁺ ions in water solution will sink, i.e. the pH will rise. For example, the pH of caustic soda NaOH solution can be pH = 13.

Aids:

Indicator papers with a scale, liquid foods according to the  table, Petri dish.

Procedure:

1.

Dip the indicator paper into the water solution of cooking substance for a few seconds.

2.

Compare the paper colour change with the pH scale.

3.

Make up the bar chart in which the length of the column is directly proportional to the quantity of pH.

4.

Determine from the chart which foods´ nature is acid (0 < pH < 7) and which is alkaline (7 < pH < 14). Find the most acid and the most alkaline foods.

Picture:

Table:

No

Foods

Colour of pH paper

pH [1]

1.

lemon juice

orange

3

2.

tea

yellow

6

3.

milk

dark yellow

7

4.

vinegar

light orange

5

5.

flavoured mineral water

orange

3

6.

beer

light orange

5

7.

oil

light orange

5

8.

orange juice

light pink

2

9.

rum

yellow

6

10.

salt and water

light orange

5

11.

water

yellow

6

12.

water and detergent

yellow

6

Bar chart:

Conclusion:

The chemical reactin of nearly all used foods was acid. The most acid were orange juice, flavoured mineral water and lemon juice (pH = 2 – 3). For example: pH of milk was the highest i.e. pH = 7. You can recognize the acidity of vinegar also by smelling it.