Question

(Achem Lab)

I did the experiment that making soda's pH with citric acid and NaOH citrate and compared with original soda, but

They had similar initial pH but had different concentrations and equivalent point.

Why does the recreated buffer behave differently from the soda sample?

Thank you

Answer 1

Citric acid is a polyprotic acid (can release three H⁺s) that is a bit on the weak side (i.e., tends not to ionize completely). In solution in fruit juices, it lets a small portion of the H⁺ go, however this small amount of acid is enough to create a pH = ~3 solution and a sharp taste on the palate. If strong base is added to citric acid it will sequentially lose its three protons in the following manner:

in this experiment we are going to determine the amount of citric acid in a fruit juice by using a base-acid reaction. If we know exactly how much base we add to completely remove all the H⁺ ions (called “deprotonating”) from the citric acid in the juice, we can calculate how much citric acid is in the solution. This process of employing one reagent of known concentration to determine a compound of unknown concentration in solution is termed titration. It usually involves slowly adding small amounts of the titrant to the analyte until a reaction is just barely complete. The apparatus typically used is a buret. The equivalence point (or end point) is the exact point where all the analyte in solution has reacted. Since the equivalence point of many titrations do not result in observable changes, end point indicators are added to indicate the end point.

In this experiment we will use a solution of NaOH to titrate the acid in a fruit juice. To be accurate, the exact concentration of the NaOH solution you prepare must be known. Powdered NaOH (from which you will make your solution) is known to slowly decompose upon reaction with CO₂ in the air to generate NaHCO₃. Thus you will need to standardize, or precisely determine the concentration, the NaOH solution using a stable primary standard. Only after you have satisfactorily determined the exact concentration of your NaOH solution can you accurately determine the citric acid concentration in the fruit juice.

There are several good primary standards for standardizing base solutions, but one of the best and cheapest is the compound oxalic acid dihydrate, H₂C₂O₄ ● 2 H₂O. (Oxalic acid is a natural acid found in rhubarb leaves; it is toxic so don’t eat it.) It is important to note that the chemical equation (shown below) shows a stoichiometry of one moles of oxalic acid to every two mole of NaOH in this reaction.

H₂C­₂O_4(aq) + 2 NaOH_(aq) → C₂O₄^2-_(aq) + 2 Na⁺_(aq) + 2 H₂O_(l)

The indicator we will use in both is phenolphthalein, a common indicator of acid-base titration. Phenolphthalein was the active ingredient in Ex-lax until recently when it was phased out due to its carcinogenicity.