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