Measuring the rate of release of metal ions

A method of measuring the relative rates of release of a target metal ion from two or more samples, each sample comprising at least one source of the target metal ion, which method comprises:

• (i) providing at least two aliquots of a solution including an indicator, the indicator having a colour;
• (ii) dispersing each sample into one of the aliquots;
• and
• (iii) during step (ii), monitoring the colour of the indicator in situ to determine the rates of release of the target metal ion from the samples relative to one another;
• wherein the indicator comprises zincon;
• wherein each sample is an oral care composition selected from a toothpaste, a tooth gel, and a tooth powder;
• and
• wherein the target metal ion is selected from stannous or stannous and zinc (II) in combination.