# When mixing lye and citric acid, which citrate will form?

I'm trying to combine precise amounts of citric acid and lye to get sodium citrate with minimal amount of reacted acid/base. I'm not so sure how stochiometry works, but I could sure learn that from some web article.

What's the real problem is, that I don't know which of three sodium citrates will form. Depending on the number of $\ce{Na}$ molecules, the amounts of $\ce{NaOH}$ greatly differs.

Which forms and why?

What is the pH? You can find the pKa's of the three acidic protons in citric acid and use the Hendersson Hasselbalch equation to find what percent of each proton is ionized at that pH.

http://www.chemteam.info/AcidBase/HH-Equation.html

• Depending on the citrate form, there will also be some ammount of unreacted acid. – Tomáš Zato Oct 18 '14 at 14:00
• Yes, the concentration of all the possible citrates will be determined by the pH. – Brinn Belyea Oct 18 '14 at 14:01
• Too bad I'm using Earl Grey tea to determine pH. – Tomáš Zato Oct 18 '14 at 14:09
• Make an Arduino pH meter. You have to buy the electrode. Also, universal paper is cheap. – Brinn Belyea Oct 18 '14 at 14:12
• I really wish to make arduino pH meter. I can't afford arduino however, and I have horrible problems when programing Atmel chips - both Atmega 328 and AtTiny. – Tomáš Zato Oct 18 '14 at 14:14

Citric acid has three pKs: 3.1, 4.8 and 5.4. If you add 3 equivalents of lye, you will get essentially only trisodium citrate. If you add a lesser amount, you will have a mixture of the various citrates (at least from microscopic point of view), as well as some remaining citric acid if you add less than one equivalent.

One "equivalent" being one mole of lye per mole of citric acid.