Indicator

Keep the volumes and concentrations of milk, sodium carbonate and lipase constant. Also control the amount of indicator, the endpoint used, the type of milk and the stirring method. Only the temperature of the reaction mixture should be deliberately changed.

OCR’s PAG 4 techniques include observing and measuring biological changes, using heating equipment safely, and measuring rates through gas production, water uptake or an indicator’s colour change. These are examples of measurement techniques, not a requirement to fit every method into one experiment. The PAG table also calls for accurate measurements of length, area, mass, time, temperature, volumes of liquids and gases, and pH.