Eriochrome black-T indicator: dissolve 300 mg eriochrome black-T in a mixture of 25 ml propan-l-01 and 15 ml triethanolamine. Use three drops of this solution. This indicator turns from red to blue and titration must be carried out until a blue colour is obtained which is free from red tinges. It changes colour only when magnesium is present. If necessary add 0.1 ml of standard solution 4.1.