Hi I am looking for a simple standardization procedure in standardizing 0.25M Phosphoric acid, perhaps with NaOH, please if you know a procedure. do tell.
Thank you!
You can use methyl orange or thymolphthalein.
Please visit this web page:
http://www.titrations.info/acid-base-titration-phosphoric-acid
Let us refer to the titration curve for H3PO4 with NaOH, presented in some chemistry textbooks. The acid dissociation constants for phosphoric acid correspond to pKa's of 2.15, 7.20, and 12.15. The first equivalent point of phosphoric acid can be detected by about pH = (2.15 + 7.20)/2 = 4.7, depending somewhat on acid concentration, where 1 mol of H3PO4 is neutralized, to its first equivalent point, by 1 mole of NaOH. The second equivalence point of phosphoric acid occurs by about pH = (7.20 + 12.15)/2 = 9.7. From the first equivalent point of phosphoric acid to the second, just he second proton of H3PO4 is titrated.
About speciation of the phosphate species ― Generically speaking, we expect aq. solutions where both NaH2PO4 and Na2HPO4 are present to have pH between approx. 4.7 and 9.7. The contributions of Na3PO4 and H3PO4 can in principle be neglected within this pH range. Below approx. pH 4.7 the contribution of Na2HPO4 can be neglected, but that of H3PO4 should be considered. Above approx. pH 9.7 the contribution of NaH2PO4 can be neglected, but that of Na3PO4 should be considered.
About pH of a NaH2PO4 aq. sol. ― By approx. pH 4.7 the contributions of H3PO4, Na2HPO4 and Na3PO4 can in principle be neglected, and the solution is essentially of just NaH2PO4, hence weakly acidic. The exact pH depends somewhat on concentration.
About pH of a Na2HPO4 aq. sol. ― By approx. pH 9.7 the contributions of H3PO4, NaH2PO4 and Na3PO4 can in principle be neglected, and the solution is essentially of just Na2HPO4, thus moderately basic. The actual pH depends somewhat on concentration.
From approx. pH 4.7 to approx. 9.7 conversion between NaH2PO4 and Na2HPO4 can be quantitatively completed by titration with NaOH (aq. sol.): NaH2PO4 + NaOH → Na2HPO4 + H2O. The reverse conversion would be also possible, within this pH range, by back-titration of Na2HPO4 with H3PO4.
The midpoint between the first equivalent point of phosphoric acid, by about pH = 4.7, and the second equivalence point of phosphoric acid, around pH = 9.7, is found by pH = 7.2 ≈ pKa2. We then expect maximum buffering capacity for NaH2PO4 / Na2HPO4 and [NaH2PO4] ≈ [Na2HPO4]. This corresponds to half titration/conversion from a sol. of one of this salts to a sol. of the other, upon the addition of either H3PO4 or NaOH. An equimolar sol. of both salts also have pH around 7.2; the exact pH depends weakly on concentration. It is quite reasonable to take, as effective buffer range, pH = pKa2 ± 1 (6.2―8.2).
We can predict the pH within the above mentioned buffer range after an 'adapted' Henderson–Hasselbalch equation ― even if not strictly intended for mixtures of weakly acidic/basic salts: pH = pKa2 + log10(CNa2HPO4/CNaH2PO4), where C refers to nominal (formal) concentrations of both salts in the mixed solution.
Strictly; the Henderson–Hasselbalch equation is intended for pH buffer solutions composed from either a weak acid with its conjugate (strong) base, or from a weak base with its conjugate (strong) acid. This corresponds to a strict concept of pH buffer.
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