POLYPROTIC ACID – STRONG BASE TITRATIONS PHOSPHORIC ACID DETERMINATION

Phosporic Acid – Sodium Hydroxide Titrations Phosporic acid is a polyprotic acid since it can donate 3 protons: pH at equivalence point Ka H3PO4 H2PO4- + H+ 4.6 7.5x10-3 H2PO4 HPO42- + H+ 9.6 6.2x10-8 HPO4- PO43- + H+ 12.3 4.8x10-13 Suitable indicators for equivalence points should be selected. 1st equivalence point: bromocresol green (pH range 4.0-5.6) 2nd equivalence point: phenolphthalein (pH range 8.0-10) It is difficult to tirate 3rd proton of phosphoric acid as it is very weak. It requires special pre-treatment and in this lab we will titrate first two proton of phosphoric acid. Dilute the given sample to 100 mL with distilled water. Transfer 20-25 mL of sample to an erlenmeyer flask. Add 1-2 drops of bromocresol green and phenolphthalein. Titrate with 0.1 N NaOH until blue-green color and note the volume of titrant used (V1) Continue titration until violet color and note the volume of tirant used for second part. (V2) CALCULATIONS 1st proton (1st acidity) V1 x N1 = Vsample x Nsample calculate Nsample C (g/L) = Nsample x gram equivalent of H3PO4 C (g/L) = NH3PO4 x 98/1

V2 x N1 = Vsample x Nsample calculate Nsample 2nd proton (2nd acidity) V2 x N1 = Vsample x Nsample calculate Nsample C (g/L) = Nsample x gram equivalent of H3PO4 C (g/L) = NH3PO4 x 98/1 DETERMINATION OF TOTAL ACIDITY 20-25 mL of diluted sample is transferred to an erlenmeyer flask. 1-2 drops of phenphthalein is added and it is titrated until pink color. VNaOH x NNaOH = Vsample x Nsample calculate Nnumune C (g/L) = Nsample x Gram equivalent of H3PO4 C (g/L) = NH3PO4 x 98/2 REFERENCE: Analitik Kimya Pratikleri – Kantitatif Analiz (Ed. Feyyaz ONUR), 2014