Monday, January 9, 2012

Standardization of HCl and NaOH using a primary standard solution


The net result of the displacement titration between the tetraborate ion with hydrochloric acid is

 The pH at the equivalence point in the titration of 0.2M Sodium tetraborate with 0.2M HCl is 5.6.

The boric acid so formed is a weak monoprotic acid (Ka = 6.4 x 10^ -10). Therefore it cannot be directly titrated with standard alkali. However by the addition of certain organic polyhydroxy compounds it is converted to a much stronger acid which can be titrated using phenolphthalein. This is due to the complex formation between hydrated borate ion and 1,2 or 1,3 diols.

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Prepare a 0.05M solution of borax in a 250mL volumetric flask by weighing required amount accurately. Pipette out 25.00 mL of the borax solution in to a titration flask and add few drops of methyl red indicator. Titrate this solution with the HCl solution. Repeat the titration with two other portions.

I. Calculate the mean and the standard deviation of your reading.
II. Using the mean value calculate the concentration of the HCl acid.
III. Draw the titration curve.
IV. What are the advantages of using borax as a primary standard to standardize strong acids?

Pipette out 25.00 mL aliquots of the borax solution into two conical flasks to perform a duplicate determination. Add the volume of standard HCl determined by the above titration. Cover the flask with a watch glass, heat to simmering temperature for 7-8 minutes to expel carbon dioxide and then cool the solution to room temperature. Introduce one drop of methyl red indicator and if necessary add just sufficient NaOH to restore the basic transition color of the indicator.

Introduce 2gof Glucose, swirling gently to dissolve. Add 2 drops of phenolphthalein and titrate with the NaOH provided. When the first permanent pink color is produced add further 0.5g glucose. If the pink color disappears, titrate with more NaOH until the pink color reappears.

Standardize the NaOH against the HCl using phenolphthalein indicator.

I. Calculate the concentration of NaOH,
a) From the titrations against HCl and
b) From the titrations in which glucose was added to the borax

II. Explain the important stages of the above procedure.
III. Write down the titration methods to determine concentrations of each component in a mixture of,
a) Boric acid and strong acid
b) Sodium tetraborate and boric acid


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