Tris-Borate EDTA?

Question

I have a question when making the .5 M EDTA solution to mix with the boric acid and tris base. My lab manual says to bring the pH up to 8.0 with sodium hydroxide to get it to dissolve, but after mixing the EDTA with some distilled water I checked the initial pH and it was already above 10! I tried this several times and got the same result. do you think the EDTA might be contaminated?

Answer 1

Good question.

When I was doing research work with Acid Digested Fibers, we used an EDTA solution as a detergent. I always saw a variance of pH whenever I put the EDTA into DI (Deionized) Water. The key to remember with EDTA (Ethylenediaminetetraacetic acid) is that it will show a variance of pH early in the reaction with water. Let it sit for 4 or 5 hours and you'll see the pH stabilize. If it doesn't stabilize, let it sit overnight. Check it in the morning, possibly re- measure and try again. If it still doesn't stabilize, then the EDTA that you are using could be contaminated and should be replaced.

Now, if the EDTA does stabilize and the pH is high, try adding some acetic acid. We tried this and it seemed to work well.
Also, don't forget to agitate the solution as the agitation with a glass rod will help to get the EDTA into solution.

Take a look at Bellerophon's explanation on tetrasodium. He gives a great explanation on the salt forms.

Good Luck!

Answer 2

EDTA is not very soluble in water. The solubility is higher for its salts. Depending on which compound you have (acid, mono, di, tetra-salt) you will have different pH values. For tetra-sodium salt the pH should be high but higher than 10 sounds a bit weird.

So if you are using disodium EDTA then there is something wrong with the EDTA;you should be needing some NaOH to get it dissolved anyway. When I was preparing 100 ml of 0.5M EDTA, using the disodium salt, I needed something like 9-10 NaOH pellets to get the EDTA in solution and then adjusted the pH with a freshly prepared concentrated NaOH solution.

If it gets easily dissolved and has a quite alkaline pH it could be tetrasodium EDTA. Then you would need to adjust the pH down to 8.0 with acid.

Check the label carefully.

Answer 3

Yes, there is definately something wrong. EDTA is a weakly acidic (4 carboxylic acid groups). Most likely, you are using something like tetra sodium EDTA, which will be basic when you put it into solution, since the carboxylic groups are deprotanated and paired with sodium ions.

You can do a UV-VIS or H NMR to test the purity of the EDTA. As long as the only organic is EDTA, you can just use HCl to get the pH down to 8.

Answer 4

maybe it needs more acid (like boric acid)?