I plan on creating 100 mL of 100mg/L aspirin solution by dissolving 10 mg of pure aspirin powder in 100 mL of buffer solution (of different pHs). Problematically, aspirin doesn’t tend towards dissolving in polar solution easily, so I thought about dissolving the 10 mg of aspirin in 1 mL of ethanol first, and then taking that ethanol-aspirin solution and adding it to 99 mL of the buffer solution.

I was wondering if this method seems reasonable/feasible. If not, are there any better suggestions?

  • $\begingroup$ The salts of aspirin are much more water soluble $\endgroup$
    – Waylander
    Aug 8 '20 at 6:37

According to Wikipedia, solubility of aspirin in water is $\pu{3 g/L}$ or $\pu{3 mg/mL}$ at $\pu{25 ^\circ C}$. Thus, it is not hard to make a $0.3\% (w/v)$ aquious aspirin solution (maximum concentration, which equals to $\pu{300 mg}/\pu{100 mL}$ at $\pu{25 ^\circ C}$). Therefore, your target, $\pu{10 mg}/\pu{100 mL}$ at $\pu{25 ^\circ C}$ can be easily achieved without any modifications. You have not mentioned the what different $\mathrm{pH}$ values of buffers you are planning to use. Since $\mathrm{p}K_\mathrm{a}$ of aspirin is about $3.5$ and the amount of it above your target value would dissolve in water (so final $\mathrm{pH}$ would be around $2.12$ for $\pu{300 mg}/\pu{100 mL}$ soution), it is guaranteed to dissolve your desired amount in any buffer with $\mathrm{pH}$ above $2.12$.

None the less, dissolve aspirin first in small amount of ethanol and diluted it with desired buffer is not bad idea. As a matter of fact, in aspirin preparation lab, we use ethanol to recrystallize the final product, therefore, would not arise any problem on dissolution. Thus, this would be an ideal alternative procedure.


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