2. Determination of Carbonate and Bicarbonate in Mixture The analysis of such mixture requires two titrations, one with an alkaline-range indicator, such as ph.ph. and the other with an acid-range indicator, such as methyl-orange. Na2CO3 + HCl NaHCO3 + NaCl ......... pH 8.3 → MaHCO3 + HCl CO2 + H2O + NaCl ...... pH 3.8 → A portion of cold solution is slowly titrated with standard hydrochloric acid using ph.ph. as indicator. This volume of acid (V1) corresponds to half the carbonate: CO32- + H+ HCO3- → Another sample of volume is then titrated with the same standard acid using methyl-orange, as indicator. The volume of acid (V2) corresponds to carbonate + bicarbonate; hence 2 V4 = carbonate and V2 - 2Vi = bicarbonate. 3. Determination of a Mixture of Carbonate and Hydroxide (analysis of commercial caustic soda) (Must be in cold (as near 0°C), and loss of carbon dioxide must be prevented as far as possible by keeping the tip of the burette immersed in) Method 1: First portion: Total alkali (carbonate + hydroxide) is determined by titration with standard acid, using methyl orange as indicator. E.p. 1 Second portion: For hydroxide content only, carbonate is precipitated with a slight excess of barium chloride solution, and, without filtering, the solution is titrated with standard acid using phenolphthalein as indicator. E.p. 2 Na2CO3+ BaCl2 = BaCO3 (insoluble) + 2 NaCl – e.p. 1 e.p 2 = carbonate Method 2: First portion : For OH- and ½ CO32-(to HCO2-) (two The solution titrated with acid using a mixed indicator (composed indicators of thymol blue and cresol red) till rose color = e.p 1 method) OH− +H+ = H2O CO32- + H+ = HCO3− Second portion For total alkalinity (all Carbonate and Hydroxide) titration with acid using methyl orange as indicator = e.p. 2
2. Determination of Carbonate and Bicarbonate in Mixture The analysis of such mixture requires two titrations, one with an alkaline-range indicator, such as ph.ph. and the other with an acid-range indicator, such as methyl-orange. Na2CO3 + HCl → NaHCO3 + NaCl ......... pH 8.3 MaHCO3 + HCl → CO2 + H2O + NaCl ...... pH 3.8 A portion of cold solution is slowly titrated with standard hydrochloric acid using ph.ph. as indicator. This volume of acid (V1) corresponds to half the carbonate: CO32- + H+ → HCO3- Another sample of volume is then titrated with the same standard acid using methyl-orange, as indicator. The volume of acid (V2) corresponds to carbonate + bicarbonate; hence 2 V4 = carbonate and V2 - 2Vi = bicarbonate. 3. Determination of a Mixture of Carbonate and Hydroxide (analysis of commercial caustic soda) (Must be in cold (as near 0°C), and loss of carbon dioxide must be prevented as far as possible by keeping the tip of the burette immersed in) Method 1: First portion: Total alkali (carbonate + hydroxide) is determined by titration with standard acid, using methyl orange as indicator. E.p. 1 Second portion: For hydroxide content only, carbonate is precipitated with a slight excess of barium chloride solution, and, without filtering, the solution is titrated with standard acid using phenolphthalein as indicator. E.p. 2 Na2CO3+ BaCl2 = BaCO3 (insoluble) + 2 NaCl e.p. 1 – e.p 2 = carbonate Method 2: (two indicators method) First portion : For OH- and ½ CO32-(to HCO2-) The solution titrated with acid using a mixed indicator (composed of thymol blue and cresol red) till rose color = e.p 1 OH− +H+ = H2O CO32- + H+ = HCO3− Second portion For total alkalinity (all Carbonate and Hydroxide) titration with acid using methyl orange as indicator = e.p. 2
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