Solution for Melamine: Why is My Solution Solid?

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Discussion Overview

The discussion revolves around the challenges faced in creating a melamine solution, specifically why the solution remains solid despite following calculated steps to achieve a desired pH. The scope includes experimental procedures and solubility considerations related to melamine in acidic conditions.

Discussion Character

  • Experimental/applied
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant outlines their method for preparing a melamine solution at pH=4, including calculations for protonation and expected concentrations.
  • Another participant suggests that more hydrochloric acid may be needed, questioning the number of protonable nitrogens in melamine.
  • A different viewpoint indicates that melamine, being a base, will neutralize the acid, recommending adding melamine to water first before adjusting pH with HCl.
  • One participant requests verification of their concentration calculations and expresses concern over the solubility data from SciFinder, noting that their attempts to dissolve melamine in HCl did not yield results.
  • Another participant introduces the idea that cyanuric acid, a hydrolyzate of melamine, could be a contaminant affecting solubility.

Areas of Agreement / Disagreement

Participants express differing opinions on the correct method for preparing the melamine solution and the implications of the solubility data. There is no consensus on the best approach or the accuracy of the solubility information.

Contextual Notes

Participants note potential issues with the solubility data and the presence of contaminants, which may affect the outcomes of their experiments. There are also unresolved questions regarding the correct concentration calculations and the appropriate method for achieving the desired pH.

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?
 
Last edited by a moderator:
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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