Polyprotic acid - Lab Report Example

An acid is defined, according to Brønsted and Lowry, as a compound which donates one or more protons [hydrogen ions, (H+)] (Chemistry - Dictionary.Com). The acid that donates one proton (e.g., acetic acid) is called a monoprotic acid while that which donates more than one proton is termed polyprotic. Phosphoric acid (H3PO4, also known as orthophosphoric acid) is a polyprotic acid whose three hydrogen atoms dissociate at different pH values. Therefore, H3PO4 has three pKa values (Give Ref of the text book you follow).

H3PO4 forms an important functional group in biomolecules because of its ability to donate H+ under physiological conditions (www.sciencedaily.com). H3PO4 undergoes 3 ionization steps in aqueous condition as follows: An acid can be neutralized by the addition of an appropriate amount of a base by the technique known as titration, widely used in chemistry. When H3PO4 is titrated against a base such as sodium hydroxide (NaOH) solution, the pH of H3PO4 solution will stay fairly constant until the moles of NaOH i.e., the titrant added equal the moles of H3PO4 present.

At this stage, further additions of titrant will cause an abrupt change in pH before it eventually stabilizes. A graph of pH versus added titrant is called a titration curve, and the point at which the pH changes significantly is called the equivalence point. The titration curve of H3PO4 will have more than one equivalence point as it is a polyprotic acid. The successive ionization steps taking place can be observed by constantly monitoring the pH during the titration, and plotting a graph of the pH versus volume of NaOH added. (i) 0.1M phosphoric acid solution: Using an autopipet, 3.

38mL of concentrated H3PO4 solution (molarity = 14.8) were pipeted out into a clean and dry 500-mL volumetric flask. The acid was diluted up to the mark with distilled water, the flask was stoppered and the solution was mixed well. The dropping buret was filled to just above the top mark