- #1
rwooduk
- 762
- 59
I was trying to use the ethylenediaminetetraacetic acid (EDTA) method using this simple guide to measure the calcium ions in my CaCl2 (2.4 mM) + NaHCO3 (6.4 mM) solution. But the solution immediately turns dark blue (like really dark) when I add the indicator to the calcium solution.
From the guide; excess calcium ions form a complex with the PR indicator (pink), but then as the EDTA is added they start to form a complex with EDTA and it turns blue.
## [Ca-PR] + EDTA^{4-}\rightarrow PR + [Ca-EDTA]^{2-}##
The background to this comes from some papers that say the EDTA method is via magnesium precipitatation as magnesium hydroxide by addition of potassium hydroxide. Then, calcium forms a stronger complex with EDTA than magnesium. Where does the magnesium fit into the above reaction? Where's the potassium?
Now, there is no magnesium or potassium in my calcium solution, so I believe this may be the problem.
My PR indicator immediately turns dark blue when added to my calcium solution. Do I need to add magnesium to my solution and use potassium hydroxide? However, nowhere in the "Solutions Needed" part do they mention magnesium. The guide is quite confusing.
Also, in order to make the calcium solution, NaOH (0.1 N) is added to the ##NaHCO_{3}##, to get the pH to 9.1. Then it is mixed with the ##CaCl_{2}## resulting in a final pH of 8.8. In my experiment the pH was measured at 12.78 and it stayed at that value when the ##CaCl_{2}## was added (no idea why, will try using less NaOH). Could the pH have been to high?
Please could someone give me a point in the right direction? What do I need to do to my calcium solution in order to use the EDTA method?
From the guide; excess calcium ions form a complex with the PR indicator (pink), but then as the EDTA is added they start to form a complex with EDTA and it turns blue.
## [Ca-PR] + EDTA^{4-}\rightarrow PR + [Ca-EDTA]^{2-}##
The background to this comes from some papers that say the EDTA method is via magnesium precipitatation as magnesium hydroxide by addition of potassium hydroxide. Then, calcium forms a stronger complex with EDTA than magnesium. Where does the magnesium fit into the above reaction? Where's the potassium?
Now, there is no magnesium or potassium in my calcium solution, so I believe this may be the problem.
My PR indicator immediately turns dark blue when added to my calcium solution. Do I need to add magnesium to my solution and use potassium hydroxide? However, nowhere in the "Solutions Needed" part do they mention magnesium. The guide is quite confusing.
Also, in order to make the calcium solution, NaOH (0.1 N) is added to the ##NaHCO_{3}##, to get the pH to 9.1. Then it is mixed with the ##CaCl_{2}## resulting in a final pH of 8.8. In my experiment the pH was measured at 12.78 and it stayed at that value when the ##CaCl_{2}## was added (no idea why, will try using less NaOH). Could the pH have been to high?
Please could someone give me a point in the right direction? What do I need to do to my calcium solution in order to use the EDTA method?