Experiment No. 1 Preparation and Standardization of 0.5N Sodium Hydroxide Volumetric Solution I
PRINCIPLE INVOLVED Sodium Hydroxide is one of the standard alkalis because it is a strong base as it offers ease of computations in titration with acids. Half-normal Sodium Hydroxide Volumetric Solution is standardized using a standard solution of hydrochloric acid as a secondary standard or potassium bipthalate as a primary standard. The standardization involves direct titration of the bipthalate with the sodium hydroxide thus it is considered as a neutralization reaction or more specifically, alkalimetry. The solution of potassium bipthalate is titrated with the sodium hydroxide until the solution, previously dropped with phenolphthalein, turned light pink (Knevel, 1977). Carbon dioxide-free water is also advised as the solvent for the reaction as carbon dioxide, an acidic oxide, will react with the sodium hydroxide to produce carbonates thus altering the desired concentration. (Jeffery, 1989)
PROCEDURE (USP 35, 1083) Dissolve 162 (81) g of sodium hydroxide in 150 mL of carbon dioxide-free water, cool the solution to room temperature, and filter through hardened filter paper. Transfer 54.5 mL of the clear filtrate to a tight, polyolefin container, and dilute with carbon dioxide-free water to 1000 mL. Accurately weigh about 5 (2.5) g of potassium biphthalate, previously crushed lightly and dried at 120º for 2 hours, and dissolve in 75 mL of carbon dioxide-free water. Add 2 drops of phenolphthalein TS, and titrate with the sodium hydroxide solution to the production of a permanent pink color. Each 204.23 mg of potassium biphthalate is equivalent to 1 mL of 1 N sodium hydroxide.
III OFFICIAL REQUIREMENT Sodium Hydroxide contains not less than 95.0 percent and not more than 100.5 percent of total alkali, calculated as NaOH, including not more than 3.0 percent of Na2CO3. IV CLEAN-UP PROCEDURE Neutralize the excess titrant with sodium bisulfate then flush down the sink. The V
REASONS FOR IMPORTANT STEPS 1 The use of CO2-free water – prevents the formation of undesired sodium carbonate which alters the concentration of the desired solution. NaOH + CO2 → Na2CO3 + H2O (Jeffery, 1989 2 Sodium Hydroxide pellets must be weighed in weighing bottles because it is hygroscopic. 3 Potassium Biphthlate must be dried at 120º for 2 hours if the purity is not indicated p.293 4 Strong Alkalis must be kept at polyethylene bottles as it could break glass. P291 5 Limit exposure of the solution (sodium hydroxide) with air as it could also introduce carbon dioxide into it. P291 6
VI CHEMICAL EQUATIONS KHC8H4O4 + NaOH → KNaC8H4O4 + H20