Positive and Negative Controls

In chemistry, controls are a way to validate the results of your experiment.

A negative control contains all of the reagents used in the experiment, except for the material that is being detected. Therefore, the negative control should give a negative result.

A positive control contains the material that you are detecting, so the positive control should give a positive result.

A negative control validates the positive results, and a positive control validates the negative results. Without comparing our unknown samples to controls, we can’t be sure if our results are caused by the presence or absence of the compound we are testing for, or caused by an error in the procedure.

Example: When testing for reducing sugars, the negative control is water and the positive control is glucose. Water does not contain any reducing sugars so will give a negative result. Glucose is a reducing sugar so should give a positive result. If the controls do not give the expected results, we can conclude that there has been an error in the procedure.

Figure 1 - Results of the negative and positive controls for Fehling’s test for reducing sugars.