Practical Asessment - Acids & Bases

Aim:

To measure the pH of several solids and liquids using universal indicator solution and a pH pen

Equipment:

Spotting tile

Universal indicator solution

Samples of several solids and liquids

pH pen

Method:

1.       Take two samples of each solid/liquid to be tested and arrange on your spotting tile. (If you are testing a solid add a drop of water and mix it before adding indicator)

2.       Place one drop of universal indicator on ONE of each sample.

3.       Use the pH pen to measure the pH of the other sample of each substance

Results:

Substance	Colour with pH	Description	pH Meter reading	Description
Eg. Substance A	Pale blue	Weak base	8.3	Weak base
Sugar	Blue/Green	Weak Base	6.2	Weak Acid
Lux flakes	Light Green	Weak Acid	9.4	Weak Base
Detergent	Pale Yellow/Green	Neutral	6.9	Very Weak Acid
Toothpaste	Blue	Strong Base	5.7	Weak Acid
Fruit juice	Pale Red	Weak Acid	1.9	Strong Acid
Window cleaner	Blue	Base	6.7	Weak acid
Citric acid	Yellow/Orange	Weak Acid	6.3	Weak Acid
Cheesels	Blue	Base	12.3	Base

Conclusion:

The results from using Universal Indicator and the pH Pen were varied depending on the substance. Sometimes it gave you an accurate answer but sometimes you were left confused. For example when testing the Citric Acid with Universal Indicator it went Yellow/Orange and then when tested with the pH pen it tested to be 6.3, this shows that both tests result in Citric Acid being a weak acid. But when testing sugar with Universal Indicator it went Blue/Green and then when tested with the pH pen it tested to be 6.2. This shows that the Universal Indicator indicates the sugar to be a weak base but the pH Pen result showed the Sugar to be a weak acid.

The pH pen is more useful when testing a coloured solution because you might not be able to see the change in colour all the time. For example is orange juice is a orange solution, if Orange Juice is tested with the Universal Indicator and the solution is orange you may not be able to see it accurately, or the colour may mix with the solution causing a wrong colour to form.