1.0 Acids, Bases and Indicators Acid Base Indicator – a substance (usually a pigment solution or dye) that changes colour in presence of acids and bases. E.g: 1. phenolphthalein is clear in acid but turns pink in base. 2. Litmus is red in acid and blue in base. 2. Bromothymol blue is green in water, blue in base and yellow in acid.

Everyday uses of indicators In Chemical Research – Determine if a solution is acidic, basic or neutral. – Monitor the changes in acidity during volumetric analysis when an acid is being used to neutralise a base. – Detect the end-point of an acid-base titration e.g. use of phenolphthalein to indicate the endpoint when acetic acid id being titrated against a standard solution of sodium hydroxide

Everyday uses of indicators -continued Testing Water Acidity – Monitor acidity levels of swimming pools – Sodium hypochlorite (NaOCl) is added to swimming pools to kill microbes. – The hypochlorite reacts with water producing unstable hypochlorous acid (HOCl) and OH - ions. – HOCl is the active form that kills microbes but it is relatively unstable: – OCl - + H 2 O HOCl + OH - – The OH- ions raises the pH of water making it more basic. HCl can be added to make this water near neutral (pH 7.2 – 7.6). This must be done to prevent the eye and throat irritation of swimmers – If pH drops below 6.5 (acidic) then metal fittings of pool circulation system will be attacked.

Everyday uses of indicators -continued Testing soil acidity – Some plants grow best in slightly acidic soils (pH 7). – Acidic soils may be neutralised by adding bases like crushed calcium carbonate or dolomite – Basic soils can be neutralised with fertilisers like sulfate of ammonia or compost and manures containing natural acids. – Soil pH can be measured using electronic instruments or universal indicator – A test tube is 1 / 3 filed with soil, water is added to within 2com of the top. Tube is stoppered and shaken, small amount of BaSO 4 is added to aid the settling process. A Pasteur pipette is used to withdraw the supernatant water which is then tested with universal indictor.

Acids and Bases Acids Tastes sour Attack metals and limestone Does not react fats Releases H + ions in water Examples; hydrochloric acid, sulfuric acid, nitric acid, vinegar, soda water, lemon juice Have a pH between 0 and 7 Bases Tastes bitter Does not attack metals Reacts slowly with fats Release OH - ions in water Feels slippery between fingers Examples; ammonia solution, washing soda solution, baking soda solution, lime water, sodium hydroxide etc.. Have a pH between 7 and 14

Strong Acids and Bases A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. Strong acids and bases are corrosive The strength of an acid is directly proportional to the number of H + or H3O + ions released in the solution. The strength of a base is directly proportional to the number of OH - ions released in the solution.

Experiments Prepare and test a natural indicator (red cabbage) Gather information about the colour changes of a range of indicators Use indicators to classify some household substances as acidic, neutral or basic