Solution for Melamine: Why is My Solution Solid?

[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.

 

[Borek]

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

 

[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.

 

[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.

 

[chemisttree]

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