EDTA refuses to dissolve!!! - argh!! (Jul/22/2009 )

Hello peeps,

I am using the disodium version of EDTA (that is the only one available in my lab right now) and the molecular weight is 372.24g/mol. I tried to make a 1M stock by dissolving 186.1g of EDTA in 500mL of water, but it REFUSES to dissolve. I went on to check that it is necessary to change the pH of the solution to 8.0 before the powder will go into the solution, but still, it DOESNT dissolve.

Next, I had a look at sambrook and russell's molecular cloning book appendix and it says that I should make 186.1g of EDTA in 800mL of water and adjust the pH to 8.0. In the book, it says that will give me 0.5M, but this isn't true! To get 0.5M, I have to dissolve 186.1g of EDTA in 1L of water, isn't it? I left my 186.1g of EDTA and 800mL of water to stir overnight with a magnetic stirrer and I am hoping to see it dissolved tomorrow but I dun have much hopes. I can't afford to make another solution because the EDTA is running out. Any help? I should add 200mL water to increase the volume to 1L, right? That SHOULD give me 0.5M, right? I don't get why the book says it will give 0.5M

ANy help will be appreciated!!

Thanks in advance.

Sorry to add on to this.

I realized from one website that NaOH PELLETS are used to adjust the pH to 8.0. The thing I did was to make 5M NaOH stock and added it to the solution to adjust the pH... does this make a difference? ALmost all the websites I went to says that I add NaOH to adjust the pH... I didnt know that it should be pellets...

nope you dont really need it to be in pellet.

by the way you need to add really really a lot of NaOH. I too have just tried to make EDTA just last week but on a smaller scale of only 100ml (cause i only needed to make TBE with them). I was following the Sambrook's formula which stated for 1L i needed to add abount 20g NaOH pellet. So I thought..ok...so for 100ml I should only need to add 2g....big mistake.

Not sure how many g i added but it was way above the calculated 2g. Just add them slowly and you should be fine.


MAD

To prepare EDTA, I take time, reading papers in the same time.
(I prepare only 100 mL of 100 mM).
I put the solution on a magnetic stirrer, and add NaOH from time to time until it dissolves. If I'm hurry the pH will be to high. That's why I take time and I don't need to check the pH, I add NaOH, and wait, and add again and wait, when it is dissolved, the pH is close to 8.0.

DiSodium EDTA has a soloublity limit of about 10% (MSDS at: http://www.jtbaker.com/msds/englishhtml/e0170.htm )

hey peeps,

thanks for replying fast.

I went back to the lab just a while ago to follow your suggestions of adding the NaOH slowly. I also increased the volume to a little less than 1L with dH20 so I can still add NaOH to adjust the pH at the same time. It is horrifying how much NaOH I had to add to adjust the pH. It took forever but at least it is dissolving now. I am letting it stir overnight.

By the way.. when you guys made NaOH, ever noticed how the solution seem to feel very very very warm? I felt uncomfortable handling it.

mad on Jul 22 2009, 04:39 PM said:

nope you dont really need it to be in pellet.

by the way you need to add really really a lot of NaOH. I too have just tried to make EDTA just last week but on a smaller scale of only 100ml (cause i only needed to make TBE with them). I was following the Sambrook's formula which stated for 1L i needed to add abount 20g NaOH pellet. So I thought..ok...so for 100ml I should only need to add 2g....big mistake.

Not sure how many g i added but it was way above the calculated 2g. Just add them slowly and you should be fine.


MAD

jiajia1987 on Jul 22 2009, 11:28 AM said:

hmmmm okay. I hope I don't have to handle so much of hydrochloric acid and sodium hydroxide next time round. Im always getting into some sorta minor spills.

Anyway, thank you all!!!

Hi If i am right, it is not possible to prepare 1M EDTA,ie what even sambrook and russell's molecular cloning book suggest to prepare 0.5M EDTA. And regarding calculation when the molecular weight is 372.24g/mol,then to prepare 1M solution you have to take 372.24g in 1litre and to prepare 0.5M you have to take 186.1g ie what sambrook and russell's molecular cloning book suggesting you.

But if you prepare it by adding excess of NaOH or HCL by any means it is wrong...and it has to go directly to dust bin

So point is you cannot or should not prepare 1M EDTA and there is nothing wrong in the book...