Solution for Melamine: Why is My Solution Solid?

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SUMMARY

The discussion centers on the challenges of dissolving melamine in an acidic solution, specifically targeting a pH of 4. The solubility data from SciFinder indicates that melamine's solubility is 262g/L at pH 4, yet the user encountered solid formation despite using hydrochloric acid (HCl) to adjust pH. Key calculations revealed that the protonation of melamine at this pH requires a concentration of 2.04M, but the user failed to achieve a clear solution even after adding HCl and stirring. The presence of cyanuric acid as a contaminant may also be contributing to the solubility issues.

PREREQUISITES
  • Understanding of acid-base chemistry, particularly pKa values.
  • Familiarity with solubility concepts and calculations.
  • Experience with pH measurement techniques, such as using a pH meter.
  • Knowledge of melamine's chemical properties and its behavior in acidic solutions.
NEXT STEPS
  • Research the solubility of melamine in various pH conditions using reliable chemical databases.
  • Learn about the role of cyanuric acid as a contaminant and its effects on melamine solubility.
  • Investigate methods for accurately measuring and adjusting pH in chemical solutions.
  • Explore alternative solvents or conditions that may enhance the solubility of melamine.
USEFUL FOR

Chemists, chemical engineers, and researchers involved in formulation chemistry or those working with melamine solutions will benefit from this discussion.

a227589
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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.
 
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I think you may need more hydrochloric acid. How many "protonable" nitrogens in melamine?
 
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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.
 
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.
 
Cyanuric acid, the hydrolyzate of melamine, is likely a contaminant. It forms a salt with melamine and is not very soluble.
 

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