The hygroscopic nature of NaOH requires its solution to be standardized with a stable primary standard such as potassium hydrogen phthalate. The exact molar concentration of NaOH solution can then be accurately determined.
1.1) Prepare the stock NaOH solution by dissolving about 4 g of NaOH pellet in 100 mL of deionized water. Note that NaOH is a hazardous chemical which is corrosive to skin and irritant to eye, be cautious and wear proper personal protection equipment (PPE) to avoid skin or eye contact.
1.2) Prepare the diluted NaOH solution by adding 25 mL of the stock NaOH solution prepared in part 1.1 to a 500 mL bottle and top it up to 250 mL with boiled deionized water. Cap the bottle to prevent the absorption of CO2. Calculate the approximate molar concentration of this diluted NaOH solution. …show more content…
1.3) Dry 4-5 g of the primary standard acid, KHC8H404 at 110oC for several hours in a drying oven and then cool the sample in a dessicator.
Record the mass accurately.
1.4) Measure and dissolve about 4 g dried KHC8H404 in 250 mL of deionized water and. Calculate the molar concentration of the KHC8H404 solution.
1.5) Pipette 25 mL of KHC8H404 into a clean and dry conical flask and add 2 drops of phenolphthalein.
1.6) Clean a 50 mL burette and a funnel thoroughly with detergent and water using a long burette brush. Flush the buret with tap water and rinse at least 3 times with deionized water. Rinse the burette with the diluted NaOH solution 3 times, making sure the NaOH wets the entire inner surface and drain the waste through the tip.
1.7) Fill the clean burette with the diluted NaOH solution and read the volume by viewing the bottom of the meniscus after 10 seconds. Record this initial
volume.
1.8) Titrate the KHC8H404 solution by slowly adding NaOH solution to the acid. Swirl the flask after each addition. Initially add the NaOH solution in 1-2 mL increments. As the endpoint approaches, the color of the indicator fades more slowly. Add the titrant drop by drop. The endpoint should be within half a drop of a slight pink color. Read and record the final volume of the NaOH in the burette.
1.9) Repeat the titration at least twice more to obtain consistent data. Calculate the molar concentration of the diluted NaOH solution. The reaction between the primary acid standard and sodium hydroxide is shown in Equation 2:
KC8H504 + NaOH H2O + NaKC8H404 (2)