I have a solution of citric acid, tartaric acid, water and glucose. How do I find the concentration of the citric acid present in the solution?
Citric acid has $\mathrm{p}K_\mathrm{a}$-values of 3.1, 4.7, and 6.4, while those of tartaric acid are 3.0 and 4.3. If you adjust the $\mathrm{pH}$ to 6.4, tartaric acid would be roughly 100% deprotonated, while 50% of citric acid still has one proton to give off.

If you titrate this solution with $\ce{NaOH},$ you could estimate the buffer capacity, giving you an estimate of the concentration. The biggest systematic error is that the apparent $\mathrm{p}K_\mathrm{a}$ is dependent on ionic strength, which you don't know unless you know the concentrations in the first place.