5
$\begingroup$

I need to prepare the $\pu{1M}$ solution of sodium crotonate, however, I don't have one in lab, but I do have sodium hydroxide and crotonic acid.

Thus, I calculated that for $\pu{100 mL}$ of $\pu{1M}$ solution I need: ~$\pu{4g}$ of $\ce{NaOH}$ and ~$\pu{8.6g}$ of crotonic acid. After preparation of solution (approx $\pu{80 mL}$ of DI water to dissolve the crotonic acid in a beaker, add $\ce{NaOH}$, mix until dissolving, let it cool down, quantitatively transfer a solution into a $\pu{100.0 mL}$ volumetric flask, adjust the volume), the $\mathrm{pH}$ of the final solution is 6.1 (which also stays the same after purging with $N_2$ gas and autoclaving).

To my understanding, the solution of sodium crotonate supposed to have a basic $\mathrm{pH} \ (\gt 7)$ since it is formed by weak crotonic acid (with a $K_\mathrm{a} = 1.97 \times 10^{-5} $, reference) and a strong base. Thus, the rough estimation of $\mathrm{pH}$ should be done according to formula:

$$ \mathrm{pH} = 14 + \lg \left(\sqrt {\frac{K_\mathrm{W} \times C_\text{sodium crotonate}}{K_\mathrm{a}}} \right)\\= 7 + \lg \left(\sqrt {\frac{C_\text{sodium crotonate}}{K_\mathrm{a}}} \right) \\= 7 + \lg \left (\sqrt\frac{1}{1.97 \times 10^{-5}} \right) = 9.35 $$

Which results in a far higher value.

On the other hand, if I suspect the formation of buffer solutions as sodium crotonate + crotonic acid (if there was not enough hydroxide), then the final $pH$ should be estimated according to:

$$ pH = pK_a + \lg \frac{C_{salt}}{C_{acid}} $$ Which roughly estimates 3.8% of unreacted acid in solution. However, even if I arbitrally assume only 0.1% of unreacted acid in solution, the formula results in $pH$ of 7.7, which is anyway higher than I obtained, but still not a 9.35 as should be.

So, my questions are:

  • What is the real value of $\pu{1M}$ solution of sodium crotonate supposed to be (like if prepared from salt reagent)?
  • I think I should adjust the $\mathrm{pH}$ with $NaOH$, but for what final value? 9.35 or 7.7?
  • Are there any verified protocols for preparation such solution (USP / EuP / etc.)?

P.S. I also found paper where sodium crotonate solution was prepared by reacting sodium bicarbonate and crotonic acid followed by adjusting $\mathrm{pH}$ to a final range of 6.7 ... 7.0, but I still don't get why they stopped at $\mathrm{pH}$ of 7, not higher.

P.P.S. As also stated here, the pH "without neglecting the water autoionization" can be estimated by solving this equation: $$ \left\{\begin{matrix} \left [ H^+ \right ] ^3 +\left ( K_a + C_{salt} \right ) \times \left [ H^+ \right ] ^2 - K_w \times \left [ H^+ \right ] - K_w \times K_a = 0 \\ \left [ H^ + \right ] > 0 \end{matrix}\right. $$

However, it results into $ \left [ H^ + \right ] = 4.43847 \times 10 ^{-10} $ and $pH = 9.35$ which is not much different from the previous estimation.

Improve this question
$\endgroup$
8
  • 1
    $\begingroup$ Recall buffers. What do you think aqueous crotonic acid/sodium crotonate soulution would be? $\endgroup$
    – Mathew Mahindaratne
    Mar 4, 2021 at 22:07
  • 1
    $\begingroup$ Solutions of salts of weak acids are always mildly alkaline. pH = 14 - 0.5 ( pKb - log ) = 7 + 0.5 ( pKa + log c ). Acidic pH in the case using bicarbonate was due dissolved CO2. Similar thing in lesser extend can happen, if hydroxide contains carbonate. $\endgroup$
    – Poutnik
    Mar 4, 2021 at 23:35
  • $\begingroup$ @Poutnik I wasn't considering the influence of $CO_2$ due to: 1) the solution is autoclaving, which should stimulate the release of $CO_2$ from solution, but pH still remains about 6.1 after autoclaving; 2) the crotonic acid is stronger than carbonic acid, and no carbonates should be in solution. Do you think it is still a factor after autoclaving? $\endgroup$
    – Araneus0390
    Mar 5, 2021 at 7:44
  • $\begingroup$ @MathewMahindaratne I was considering the buffer system (see updates to the post), but even those calculations are higher than 6.1. Which still remains the question, what the final pH should be? $\endgroup$
    – Araneus0390
    Mar 5, 2021 at 7:46
  • $\begingroup$ 1/ Autoclaving is essential info, that should be provided in the question. How should we know ? But even at high temperature, with aventual reached CO2 partial pressure, there would be still significant CO2 dissolved, affectingn pH. 2/ The stronger the acid is, the weaker base is its salt. And it is weak acid ( CO2 ) versus weaker base ( NaA ) pH clash. I have said carbonate in hydroxide. Reaction of the acid with such hydroxide releses CO2. Another important factor is the accuracy of reached stoichiometry ratio, due calculation and purity deviation errors. $\endgroup$
    – Poutnik
    Mar 5, 2021 at 8:07

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.