Mixed indicator is a Color-change indicator for acid-base titration end points in which a mixture of two indicator substances is used to give sharper end-point color changes. For example: A mixture of 3parts of phenolphthalein and 1 part of naphtholphthalein shows a change in color from pale rose to violet at pH = 8.9. The mixed indicator is useful for the titration of phosphoric acid to diproti stage.
使用
Mixed indicator solutions of bromocresol green (pk 4.9) and methyl red (pk 5) give a red color in acidic conditions and green color in basic conditions with a sharp transition through grey color at pH 5.1 as the two colors are complementary.
製造方法
Preparation of Methyl Red-Methyl Blue Indicator (Mixed Indicator) 1. - Weigh out 0.1 g of methyl red (as sodium salt) and dissolve completely in 50 ml of ethanol (ethyl alcohol). 2. - Weigh out 0.05 g of methylene blue and dissolve completely in 50 ml of water. 3. - When both are thoroughly dissolved, mix them together and store a portion in a dropping bottle for use.
定義
A mixture of two or
more indicators so as to decrease the pH
range or heighten the color change, etc.
