Acid-Base Applications Acid-Base Applications
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This document describes several indirect titration methods, including: - Determining insoluble substances like ZnO or CaO by dissolving them in excess acid and back-titrating the residual acid. - Determining volatile substances like ammonia by boiling them to drive off the substance and back-titrating the excess base. - Determining substances that require a reaction with excess reagent, like lactic acid which requires excess base to fully react. - Other examples given include determining formaldehyde, alcohols, aldehydes, ketones, esters, and amino acids using various indirect reactions and back-titration methods. Proper calculations and a blank titration are emphasized.
Acid-Base Applications Acid-Base Applications
- 1. By Dr. Dalia Nagy
- 2. The method is applied for a water soluble salts. Ex: of this method is ammonium and sodium salicylate and alkali metal benzoate. The liberated salicylic acid is so strong to give an acidic pH and thus affect the indicator (bromophenol blue indicator), therefore be removed as soon as it is produced. The idea depends on the greater solubility of salicylic acid in ether (organic immiscible solvent) than in water (250 : 1). To minimize this solubility use 3 volumes of ether as the aqueous layer(25 ml water + 75 ml ether). Using separating funnel , the colour change from bluish green to permanent green colour. IV- Biphasic Titrations:
- 3. In these methods, a known excess of the standard solution is first added and allowed to react completely with the sample, the residual quantity of the added standard is then determined. V- Indirect Titration Methods (Residual or Back): In some cases the direct determinations are not suitable enough as in: 1. Volatile substances, e.g., ammonia or formic acid HCOOH, which may be lost during titration. 2. Insoluble substances, e.g., some oxides “CaO or ZnO” or carbonates “CaCO3 and BaCO3”. 3. Substances which required heating with standard reagent during the determination. 4. Substances for which a quantitative reaction needs excess reagents.
- 4. Determination of ZnO: 1. ZnO is water insoluble but dissolved in HCl. 2. Aknown weight of ZnO is treated with a known excess of standard HCl, heated gently to dissolve the oxide. ZnO + 2HCl ZnCl2 + H2O 3. Ammonium chloride (double the amount of ZnO)is then added to avoid the precipitation of Zn(OH)2 (why). 4. The solution is cooled, diluted with water, then treated with standard NaOH using methyl red as indicator(pH = 4.4 – 6.2). Determination of Insoluble Oxides: Determination of CaO: CaO & CaCO3 are determined as ZnO but the addition of amm. chloride is not necessary and M.O. is used as indicator. CaO can be determined directly by suspending in few mls of alcohol, then add 10% sucrose solution, forming calcium saccharate complex which is water soluble and have the same alkalinity of CaO. Titrate with standard hydrochloric acid using ph.ph. As indicator. Alcohol to prevent lumpiness of CaO on addition of sucrose solution.
- 5. Determination of substances need excess reagent: Determination of lactic acid: 1. Lactic acid consists of a mixture of lactic acid and lactic anhydride (lactide), the reaction is too slow for direct titration. 2. So, to increase the reaction for direct titration we need excess reagent (NaOH) to react with both of them. 3. Heat to hydrolyze lactide to lactic acid, then excess NaOH is back titrated with standard acid ph.ph. As indicator. Determination of substances require heating: Determination of formaldehyde: Formaldehyde is oxidized to formic acid with oxidizing agent (H2O2), add excess NaOH to prevent loss of the volatile formic acid, by the warming on water bath till oxygen ceases to evolve HCHO + H2O2 HCOOH + H2O HCOOH + NaOH HCOONa + H2O The excess of standard alkali is back titrated with standard HCl acid using ph.ph. as indicator.
- 6. Determination of volatile substances: Determination of inorganic ammonium salts: The determination depends upon the following reaction: 1. Indirect method: where the ammonium salt is boiled with a known excess of standard sodium hydroxide solution. The boiling is continued until no more amm.. escapes, the excess of sodium hydroxide is titrated with a standard acid using methyl red indicator. NH4Cl + NaOH NaCl + NH3 + H2O 2. Direct method: where solution of ammonium salt is titrated with a solution of strong base, e.g., sodium hydroxide, and the mixture is distilled, ammonium is quantitatively collected in an excess of standard acid, the excess acid is back titrated in the presence of methyl red. Distillation of the resulting ammonia could be done in 4% aqueous boric acid solution, where it is fixed as ammonium salt 2NH3 + 4H3BO3 (NH4)2B4O7 + 5H2O Ammonia polyborate, being strong hydrolysable salt, can be titrated directly with standard acid using methyl red indicator, as in the case of borax.
- 7. Other indirect titrations: These determine some chemical compounds which are themselves neither acid nor bases, but they can be converted to acidic or basic substances by suitable treatment. Determination of Alcohol: 1. Alcohol is reacted with acetic anhydride in pyridine or ethyl acetate solution. (CH3CO)2O + ROH CH3COOR + CH3COOH 2. When the reaction is complete, excess anhydride is reacted with water. (CH3CO)2O + H2O 2CH3COOH 3. The total produced is titrated with NaOH. CH3COOH + NaOH CH3COONa + H2O 4. The same quantity of acetic anhydride is now reacted with H2O and the resulting CH3COOH is titrated with NaOH. 5. The difference between the second and the first titration volumes represents the quantity of ester produced in the alcohol reaction.
- 8. Determination of Aldehyde & ketones: 1. By the reaction with hydroxylamine hydrochloride: RCHO + H2N.OH.HCl RCH=N.OH + HCl + H2O RRCO + H2N.OH.HCl RRC=N.OH + HCl + H2O 2. The liberated HCl, equivalent to the aldehyde or ketone, can be titrated with standard alkali using methyl orange indicator. 3. The reaction is reversible, for this titration is continued to the full yellow color of the indicator. 4. To avoid error due to hydrolysis of hydroxylamine hydrochloride, its solution has to be first neutralized to the full yellow color of M.O.
- 9. Determination of Esters: “Aspirin” 1. The reaction of ester with alkali is slow and need excess of the reagent. Esters can be, therefore, hydrolyzed by reflex with sodium hydroxide solution into an alcohol and acid “sodium salt”. RCOOR’+ NaOH RCOONa + R’OH 2. Aqueous sodium hydroxide is sufficient for the hydrolysis of esters containing esterified hydroxyl groups, e.g. Aspirin. 3. The determination is carried out by reflex a known weight of aspirin with a known excess of standard alkali. 4. The residual alkali is then back titrated with standard acid using ph.ph. As indicator.
- 10. Determination of Ammonium salts “Formol Titration” When formaldehyde is added to a solution of ammonium salt, hexamethylene tetramine (hexamine) is formed together with an equivalent amount of acid. 4NH4Cl + 6HCHO (CH2)6N4 + 4HCl + 6H2O 2(NH4)2SO4 + 6CH2O (CH2)6N4 + 2H2SO4 + 6H2O 2. The produced hexamethylene tetramine (hexamine) does not affect the indicator as it is very weak base. The used formaldehyde must be neutralized for any formic acid, HCOOH, which might be formed by oxidation. 3. The same reaction is applied to determination of amino acids where the free amino group is converted into methylimino derivative and reduces the basicity of amino group so that the free carboxyl group can be titrated against standard alkali using ph.ph. H2N-CRR’-COOH + HCHO H2C=N-CRR’-COOH + H2O Amino acid are neutral to common indicators and may be regarded as neutral salts, Zwitter ion +NH3-CH(R)-COO – The blocking of the basic group by this method, however, allows the determination of the acidic-COOH group, by titration against a standard base ph.ph. As indicator, e.g. the simplest amino acid, glycine (H2N-CH2-
- 11. Blank determination: A blank determination is defined as a separate determination in which all conditions are identical with those employed in the analysis except that the sample is omitted. Calculation: The concentration of a sample is expressed in: 1. Grams per liter. Or 2. Grams percent. Two essential steps has to be done: 1. Writing a balanced quantitative equation. 2. Calculate the equivalence of the unknown corresponding to each milliliter of the standard used in the titration, so called millequivalence factor.
- 13. Thank You
Editor's Notes
- #5: Determination of ZnO: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.
- #6: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.
- #7: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator. Distillation is the technique of heating a liquid to create vapor which is collected when cooled separate from the original liquid. It's based on the different boiling point or volatility values of the components. The technique may be used to separate components of a mixture or to aid in purification.
- #8: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.
- #9: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.
- #10: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.
- #11: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.
- #12: Sodium hydroxide is react with HCl then at end point react with NH4Cl and give NH3 which is give pH less than NaOH so do not precipitate Zn as Zn(OH)2 but enough to give pH change the indicator.