Standardization of Hydrochloric Acid by Sodium Carbonate
Concentrated hydrochloric acid is roughly 11 M. Pour out into a measuring cylinder about 2 cm3 of concentrated hydrochloric acid. Transfer it to a 250 cm3 flask and make up to the mark with water. Shake well. Put some pure sodium hydrogen carbonate or anhydrous sodium carbonate into an evaporating dish and heat gently over a low flame for about fifteen minutes, stirring continuously.
Take care not to heat the mass too strongly or fusion may take place which will seriously retard solution and also cause slight decomposition. It will be obvious when carbon dioxide is begin evolved as the mass appears usually light as it is being stirred. Allow the dish to cool in a desiccator because anhydrous sodium carbonate absorbs moisture to form the monohydrate. Weigh, then heat again for five minutes, cool and reweigh. Repeat this process until the mass is constant.
Na2CO3 + 2HCl 2NaCl + H2O + CO2
Relative molecular mass of anhydrous sodium carbonate is 106. Mass of sodium carbonate for 250cm3 0.05 M solution is
0.25 x 0.05 x 106 = 1.325g
Weigh a clean, dry weighing bottle and weigh our exactly 1.325g of the pure sodium carbonate into it. Transfer the carbonate to a beaker containing a little hot water (shaking gently as the carbonate comes in contact with the water) and wash the weighing bottle carefully by means of the wash-bottle, allowing the washings to drop into the beaker. Stir gently to ensure dissolution. Then cool to room temperature before proceeding. Smear an almost imperceptible amount of vaseline on the lip of the beaker and pour the solution down a glass rod into a funnel resting in the neck of a clean (but not necessarily dry) 250 cm3 flask.
Wash the beaker out with further small quantities of water, pouring all washings down the rod and funnel into the 250 cm3 flask to ensure that no solution is left on the walls of the beaker. It is as well to remember that once you have weighted out the sodium carbonate into the weighing bottle, every particle of it, whether as solid or as a solution, must be transferred into the 250 cm3 flask. Make up to the mark with water, adding the last few drops from a pipette, and shake well or pour into a large dry beaker in order that the solution may become homogeneous.
By pipette take 25 cm3 of 0.05 M sodium carbonate solution, run it into a conical flask, add two or three drops of screened methyl orange solution, make a note of the burette reading and run in the acid from a burette. The first 15 to 20cm3 of the acid may be run in 5 cm3 at a time without fear of overshooting the end-point. Then run in 1 cm3 at a time, shaking after each addition until the colour changes form yellow to orange. This is a trial titration and its accuracy may not warrant its use in the calculation. The titration should now be repeated with further portions of 25 cm3 of the carbonate solution until two readings are obtained which agree to 0.1 cm3 or three readings which don not show any trend in value, up or down.
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