The pH Scale and Acid-Base Indicators

• The pH, short for potential of hydrogen, is a measure used to determine the acidity, neutrality or basicity (alkalinity) of a solution. The pH has no unit of measurement.

• The pH scale is used to compare the acidity, neutrality or basicity (alkalinity) of solutions. The pH scale generally ranges from 0 to 14.

• An acid is a substance that allows the release of |\text{H}^+| ions in an aqueous solution.
• A base is a substance that allows the release of |\text{OH}^-| ions in an aqueous solution. Basic substances are also often called alkaline.
• The pH, short for potential of hydrogen, is a measure used to determine the acidity, neutrality or basicity (alkalinity) of a substance. The pH has no unit of measurement.
• The pH scale is used to compare the acidity, neutrality or basicity (alkalinity) of substances. The pH scale generally ranges from 0 to 14.

A difference of one unit on the pH scale causes the acidity or basicity (alkalinity) of a solution to change by a factor of 10.

• A decrease by one unit on the pH scale means that the acidity is increased by a factor of 10.

• An increase of one unit on the pH scale means that the basicity (alkalinity) is increased by a factor of 10.

Tomato juice (pH 4) is 1 unit below coffee (pH 5) on the pH scale. Therefore, tomato juice is 10 times more acidic than coffee.

Tomato juice (pH 4) is 2 units below milk (pH 6) on the pH scale. Therefore, tomato juice is 100 times more acidic than milk |(10\ \times 10=100).|

Answer the following questions to compare laundry detergent (pH 10) and bleach (pH 13).

1. Which of the two substances is more basic?

2. What is the variation in basicity between the two substances?

|[\text{H}^+]=10^\text{-pH}|
where
|[\text{H}^+]:| concentration of |\text{H}^+| ions in moles per litre |(\text{mol/L})|
|\text{pH}:| pH value

An aqueous solution of hydrochloric acid |(\text{HCl})| has a pH value of |2.5.| Here is the dissociation equation of hydrochloric acid in water.

||\text{HCl}_\text{(aq)}\rightarrow\text{H}^+_\text{(aq)}+\text{Cl}^-_\text{(aq)}||

What is the molar concentration of |\text{H}^+| ions in this solution?

Acid-base indicators, commonly referred to as pH indicators, are substances that change colours depending on the pH.

Phenolphthalein is an indicator that can be used to determine if the solution is basic. When a few drops of phenolphthalein are added to the solution in question, a colour change may occur.

• If phenolphthalein remains colourless, the solution is either acidic (pH below 7), neutral (pH 7), or basic (pH 7-8.2).

• If the indicator turns pink, the solution is basic with a pH between 8.2 and 10.

• If the indicator turns fuchsia, the solution is basic with a pH above 10.

In short, if phenolphthalein turns pink or fuchsia when mixed with a solution, the solution is definitely basic.

A few drops of an acid-base indicator thymol blue are added to a test tube filled with lemon juice (pH 2). Thymol blue is thoroughly mixed with lemon juice and a colour change occurs.

Referring to the image The colour range of some acid-base indicators depending on the pH, determine the colour of thymol blue after it is mixed with lemon juice.

A few drops of methyl yellow are added to an unidentified substance and the solution turns orange. In a second test tube, bromocresol green is added to the same unidentified substance and it turns green.

Referring to the image The colour range of some acid-base indicators depending on the pH, determine the pH interval of the unidentified substance.