How to make a 500 mM EDTA solution

Getting EDTA into solution is easier said than done. I just needed 50 ml of a 500 mM EDTA stock, but it really does not dissolve well. Buying it was not an option since I couldn't find any solution that was not buffered, and I wanted it as pure as possible. So I prepared it myself.

First Try
In the beginning I made the mistake of starting with the pure acid which has to be completely deprotonated before it goes into solution, and that is a long way to go. A nasty surprise was that such a solution goes up to a pH of 8 when a NaOH solution is added, but then decreases to 7.8 within a few minutes without any visible effect of dissolving the salt. I had to discard this failure because I exceeded the 50 ml maximum volume.

Second Try
Next time I tried to use water and pure NaOH tablets, again starting with the EDTA acid. This resulted in a white sludge (I did not keep a proper record on the number of NaOH tablets added after various waiting times) which spilled out of a not properly closed Falcon tube. In the end I got it into solution, but because of the leak I did not trust the concentrations anymore and dumped it.
I finally got the idea that starting with an acid and taking it's pH up is not the best thing to do if there is already a disodium salt available.

Third Try
Eventually I managed to do it like this (for 50 ml of 500 mM EDTA with a pH of ~9):

• 9.3 g EDTA disodium salt dihydrate (the stuff we had on the shelf, 372.24 g/mol) in 50-ml-Falcon tube. Watch out, that stuff is charged and reacts to static electricity!
• Add 40 ml water. Mix and watch the precipitating EDTA settling down.
• Add 10 tablets of pure NaOH. Mix and wait for 1 hour. Feel the warmth spreading through the tube.
• Add another 10 tablets of pure NaOH. Mix and wait for 1 hour.
• The solution was now clear. A pH check gave me 9.71, which was good enough since this was a stock solution to be added to some buffer. Next time I will be more careful about the second lot of NaOH tablets.
• Fill up to 50 ml and filter with 0.2 µm syringe filter (just to have done it).

This is probably more than I will ever need.