Narration : 4. Chelete Titration
- In chelate titration, you use an indicator to make a chelate compound with metal ion. The color of the indicator differs from that of the chelate compound.
- Next, you add an EDTA titrant to the reaction solution and observe the color change. Because EDTA can chelate metal ion more tightly than the indicator does, you can observe the color change during the titration.
- First, add a prescribed amount of buffer solution of pH 10 into the solution containing magnesium ion in order to produce the chelate compound completely.
- Next, add the BT indicator drop by drop to the reaction solution till it turns slightly red.
- This color change is ascribed by the formation of magnesium-indicator complex.
- Next, add the EDTA solution using a burette.
- Around the end point, EDTA takes away the magnesium ion from the magnesium-indicator complex to chelate magnesium ion.
- The reaction solution will gradually change from red through purple to blue during the process.
- When the color turns purple near the end point, read and note the scale after addition of each drop.
- The color turns completely blue at the end point. Further addition of the EDTA solution makes no color change.
- Here the amounts of magnesium ion in the solution equal to the quantity of added EDTA.
- After the end point, the color of the solution will not change if you keep adding. Hence it is important to carefully observe the color change towards the end point.
- It is difficult to observe the color change if the quantity of the indicator added is more or less. So it is important to add an appropriate amount.
- Before titration, you may judge the solution with less amount of the indicator to be appropriate at first glance. But, the solution will become almost colorless around the end point which makes observation impossible.
- In such a case, an little extra indicator may be added.
- If the color of the solution is too strong, it is difficult to ascertain the color change, and you need to start all over again.
- Since the solid indicator is difficult to dissolve, it is important not to add excess of it.
- The concentrations of the magnesium ion and calcium ion in tap water can be determined by setting the pH properly and also employing appropriate indicators.