Melamine Solution

1. Jan 8, 2010

a227589

Hi,

I need to make a melamine solution for my project right now. From SciFinder, I found the solubility of melamine in water is, 35g/L at pH=5, 262g/L at pH=4, and 1000g/L at pH=1~3. Therefore, I made the solution based on the following calculations and steps. However, what I got is totally solid. Could anyone help me find out what went wrong? Thanks!!!

1. To get a pH=4 solution. I used HCl to adjust the pH. Since the pKa of melamine is 5.66, at pH=4, most of melamine molecules are protonated. If the solution is saturated, [melamine]=262/126.2=2.08. (the molecular weight of melamine=126.2) And, based on its pKa, I got [protonated melamine]=2.04M, [unprotonated melamine]=0.4M. And, from this, the acid needed for this protonation is 2.04M (free protons=10^-4 M, which could be ignored, compared to 2.04M).

2. Then I made a solution based on this concentration. I saved some spaces for melamine since it's not dense and occupies some volume in the final solution.

3. I added melamine powder into the acidic solution and tried to use stir bar to mix.

4. And here comes the white solid. When I just added around 1/5 of the saturated level of melamine, it still didn't form a clear solution. You could see the powder in the beaker.

5. I tried to shake it under 37 degrees overnight, but it didn't work.

2. Jan 9, 2010

Staff: Mentor

I think you may need more hydrochloric acid. How many "protonable" nitrogens in melamine?

Last edited by a moderator: Aug 13, 2013
3. Jan 11, 2010

chemisttree

Melamine will neutralize the acid since it is a base. You need to add the melamine powder to water and then drip in HCl until the pH goes to 4. You will need a pH meter or some way to measure the pH during the addition.

4. Jan 11, 2010

a227589

Can someone check if the concentration that I calculated is correct for me please? I added like, 10 ml of 9N HCl on top of the white solid that I got, but didn't see any changes even to the top part of the solid. And when I add just small amount of melamine (like, 1/5 of saturated amount) to ~4N HCl solution, it still remained insoluble. Is it possible that the solubility data from SciFinder is wrong? Thanks.

5. Jan 11, 2010

chemisttree

Cyanuric acid, the hydrolyzate of melamine, is likely a contaminant. It forms a salt with melamine and is not very soluble.