Application of Neutralization Reactions PDF

Application of Neutralization Reactions Applications of Neutralization Reactions Direct Titration of Acids and Bases: In this method, the sample to be analyzed is titrated with a standard acid or base. A suitable indicator has to be chosen: Strong acids are titrated against NaOH using ph.ph...

Application of Neutralization Reactions Applications of Neutralization Reactions Direct Titration of Acids and Bases: In this method, the sample to be analyzed is titrated with a standard acid or base. A suitable indicator has to be chosen: Strong acids are titrated against NaOH using ph.ph or M.O. Weak acids are titrated against NaOH using ph.ph. Weak bases are titrated against HCl using M.O or M.R. N.B: If the dissociation constant, Ka or Kb is less than 10-7, the acid or the base is said to be weak and can't be determined directly. Determination of Boric acid: Boric acid is a weak acid; (Ka = 5.8 X 10-10), therefore it can't be determined directly against standard alkali. Addition of glycerol enhances the acidity of boric acid as follows: The liberated protons can be directly titrated against standard NaOH using ph.ph. Double Indicator Titration It is a direct titration using two indicators; it is used for determination of: Two mono-protic acids. Di-protic acids e.g. H2SO4 , H2S. Poly-protic acids e.g. H3PO4. Requirements: Ka ≥ 10-7 The ratio of Ka of the strong and weak acid ≥ 104 or pKa1 – pKa2 ≥ 4 Determination of Di-Protic Acids: The ionization of the dibasic acid takes place into two steps each has its own ionization: Thus for a dibasic acid H2A: H2A ⇋ H+ + HA- K1 = [H+][HA-] / [H2A] HA- ⇋ H+ + A 2- K2 = [H+][A2-] / [HA-] [I] If the ratio of K1/K2 < 104, i.e.: pK2 - pK1 < 4: All the protons will be neutralized together, one inflection only will be observed. e.g. H2SO4 (K1 is strong, K2 = 1.15X 10-2). [II] If the ratio of K1/K2 ≥ 104, i.e.: pK2 - pK1 ≥ 4: Each proton can be determined alone, two inflections could be observed. The pH corresponds to each proton can be determined: pH1 = pK1 + pK2 / 2 pH2 = 1/2pKw + 1/2pKa- 1/2 pCs [C] Determination of Poly-Protic Acid (H3PO4): The dissociation of phosphoric acid proceeds in three steps: H3PO4 ⇋ H+ + H2PO4- pK1 = 2.1 H2PO4- ⇋ H+ + HPO42- pK2 = 7.2 HPO42- ⇋ H+ + PO43- pK3 = 12.7 The pH of the three equivalence points can be determined as follows: First Equivalence Point: pH1 = pk1 + pK2 / 2 = 2.1 + 7.2 / 2 = 4.6 "M.O" Second Equivalence Point: pH2 = pk2 + pK3 / 2 = 7.2 + 12.7 / 2 = 9.9 "ph.ph" Third Equivalence Point: pH3 = 1/2 pkw + 1/2 pK3 – 1/2 pCs = 12.8 "No Suitable Indicator!!" Therefore the pH break is sufficient to determine the first proton by M.O and the second by ph.ph, while the third is too weak to be determined (pK3 = 12.7). Such proton can be determined by replacement with neutral CaCl2 producing equivalent amount of HCl which is titrated with standard NaOH: 2Na2HPO4 + 3CaCl2 → Ca3(PO4)2 + 4NaCl + 2HCl Titration Curve of Phosphoric acid showing three inflections considering the addition of CaCl2 in the last ionization step N.B: Phosphoric acid can be titrated after adding neutral CaCl2 from the start of the titration; one inflection is only observed which corresponds to the three protons: 2H3PO4 + 3CaCl2 → Ca3(PO4)2 + 6HCl Determination of Easily Hydrolysable Salts (Displacement Titrations): [I] Determination of Borax: If a pure sample of borax is dissolved in water, it readily hydrolyses into: Na2B4O7 + 7H2O → 2NaOH + 4H3BO3 As boric acid is a very weak acid, the produced NaOH can be directly titrated with standard HCl using M.O. The neutralized solution can then be used for determination of boric acid after addition of equivalent amount of glycerol, and then titrate with standard NaOH using ph.ph end point. Therefore, when borax is titrated, the volume of standard alkali used would be exactly double the volume of the standard acid of the same normality. [II] Determination of Borax/Boric acid Mixture: Direct titration with standard acid using methyl orange will correspond to the borax. Adding glycerol to the neutralized solution, then titrate against standard alkali will determine the boric acid resulted from hydrolysis of borax and boric acid originally present. 3- Biphasic Titration: This method is applied for the water soluble salts, the acid of which is insoluble in water but soluble in organic solvent, e.g. sodium salicylate: The liberated salicylic acid is so strong to affect the indicator. It must, therefore, be removed as soon as it is produced. Being soluble in ether 250 times greater than in water, three volumes of ether as the aqueous layer are added to increase the solubility to salicylic acid. Titrate with standard HCl to bromophenol blue end point. 4- Indirect – Residual or Back Titration: Back titration is suitable in the following cases: Volatile substances such as ammonia and formic acid where loss of the sample due to volatility is expected. Insoluble substances such as calcium oxide, zinc oxide and calcium carbonate where heat is required for the reaction to take place. Substances which require the presence of excess standard for the reaction to be complete. In such cases, a known excess of the standard solution is added and allowed to react with the sample. The residual or the un-reacted quantity of the added standard is then determined. [I] Determination of CaCO3: A known weight of CaCO3 is treated with known excess of standard HCl and heated gently to dissolve the precipitate. The solution is cooled, diluted with water. The residual HCl is back titrated with standard NaOH using M.O or ph.ph. CaCO3 + 2HCl → CaCl2 + CO2 + H2O [II] Determination Soluble Salts (CaCl2, BaCl2 and SrCl2): Known excess of standard Na2CO3 is added, where the metal carbonates are precipitated. Residual Na2CO3 is titrated with standard HCl using ph.ph and follow the precautions of the half carbonate step. N.B: Methyl orange is not used to prevent the HCl from attacking the precipitates formed. [III] Determination of Inorganic Ammonium Salts: The determination depends on the following reaction: NH4Cl + NaOH, boil → NaCl + NH3↑ + H2O The solution of ammonium salt is boiled with known excess of NaOH till no NH3 odor. The residual NaOH is back titrated with standard HCl using M.O indicator. Other Indirect Titrations: These titrations are used for determination of chemical compounds which are neither acids nor bases, but they can be converted to acidic or basic substances by suitable treatment. [I] Determination of Aldehydes and Ketones: Such determination can be achieved by the reaction with hydroxyl amine hydrochloride: RCH= O + H2N-OH. HCl ⇋ RCH=N-OH + H2O + HCl RR'C= O + H2N-OH. HCl ⇋ RR'C=N-OH + H2O + HCl The liberated HCl equivalent to the aldehyde or ketone is titrated against standard NaOH using M.O indicator. The reaction is strongly reversible, and the end point, therefore, is difficult to be detected, so titration is continued to the full yellow color of the indicator. To avoid error due to hydrolysis of hydroxyl amine HCl, its solution has to be first neutralized to the full yellow color of he indicator. [II] Determination of Mercury Oxide: When HgO is dissolved in KI solution, an equivalent amount of alkali is produced: HgO + 4KI + H2O → K2HgI4 + 2KOH The liberated KOH is titrated with standard HCl using ph.ph or M.O. [III] Determination of Aspirin: Sodium hydroxide is used for the hydrolysis of esters containing esterified hydroxyl group, e.g. Aspirin: Therefore: A known weight of aspirin is refluxed with a known excess of standard NaOH. Residual NaOH is back titrated with standard HCl using ph.ph or M.O. [IV] Formol Titration for Determination of Amino Acids and Ammonium Salts:: 1- Amino Acids: Amino acids are neutral to common indicators and may be regarded as neutral salts; Zwitter ion: + H N- CHR – COO - 3 The blocking of the basic group by this method, however, allows the determination of the acidic – COOH group, by titration against a standard base using ph.ph: H2N-CH2-COOH + O = CH2 → CH2= N-CH2 - COOH + H2O "Glycine" "Shiff's base" 2- Ammonium Salts: When neutral formaldehyde is added to a solution of ammonium salt, hexa- methylene tetra amine is formed with an equivalent amount of acid: 4NH4Cl + 6HCHO → (CH2)6N4 + 6H2O + 4HCl The liberated acid is titrated with standard NaOH using ph.ph because hexamine is basic.