Question

1. What effect does the presence of metal cations have on the color of a solution containing EBT at pH 10

2. What effect does the presence of metal cations have on the color of solution containing EDTA at pH 10?

3. Which ligand, EBT or EDTA, should be used as an indicator for a metal complexation reaction? What metals could this indicator be used for? Why?

4. Which ligand, EBT or EDTA, has a greater affinity for metal cations? What observations support your answer?

5. The water hardness titration is performed by first adding EBT and pH 10 buffer to the water sample, and then titrating with EDTA solution. Assuming that the water sample has calcium cations, what will be the color of the solution before and after the endpoint is reached? What do you tihnk the color at exactly the endpoint volume will be?

Answer 1

1. The colour turns from blue to wine red due to the complexation of metal with EBT. The EBT indicator is blue in its protanated form.

2. EDTA -meal complexes are not coloured. So, there won't be any colour change due to the complexation with meal ion. Thats the reason we need to use indicaors such as EBT in the titration.

3. EBT should be used as inidicator. only EBT changes colour upon complexation from blue to red. On the other hand, EDTA is only chelating agent and does not give any colur changes though it has has greater affinity towards metal ions. EDTA can form complexes with Ca²⁺, Mg²⁺ , K⁺,etc.

4. EDTA has greater affinity towards metal ions compared to EBT. The EBT ligand in metal-EBT complex can be substituted with EDTA quite easily. This is evident during complexometric titrations.

5.Intially, Ca2+-EBT complex forms winered colour. At the end point all the EBT ligands are replaced with EDTA. And at the end point blue colour comes from the free EBT indicator.