The Standardization of NaOH and KHP
By: Juno Kim
In this experiment the concentration of potassium hydrogen phthalate (KHP) in an unknown sample was determined through volumetric analysis. The volumetric analysis allows for the calculation of the unknown amount or concentration by a reaction with a known amount of reagent. For this lab, the unknown potassium hydrogen phthalate sample was titrated with a standardized sodium hydroxide solution. A standard solution has an accurately known concentration and for this experiment sodium hydroxide was standardized with a primary standard acid, potassium hydrogen phthalate. A primary standard could be any pure chemical that can be used as the initiation of a quantitative analysis. Sodium hydroxide, however, is not an ideal primary standard because it is hydroscopic, meaning it reacts and absorbs components from air such as moisture and carbon dioxide. In general, a primary standard holds these qualities:
- A standard solution is easily prepared, purified and dried.
- A standard solution is stable and easy to store.
- A standard solution must be non-hydroscopic.
- A standard solution must be pure to a measureable degree.
- The standard solution must react in a reasonable amount of time in a quantitative reaction setting.
Potassium hydrogen phthalate makes for a good primary standard because it is stable, high in purity, non-hydroscopic, highly soluble, non-toxic, high in molecular weight, cheap, and available.
Needless to say in this quantitative experiment extra care was given in taking and recording measurements. The volume of the 50 mL buret used in titration was recorded to the 0.01 mL and the masses were recorded to the nearest 0.1 mg to obtain the best results. All the glassware were scrupulously cleaned and dried before use and the buret was rinsed down with its reagent before the titration. It was vital to check the buret for any air bubbles present before performing the titrations.
At the beginning of the experiment, about 3 g of the stock potassium hydrogen phthalate and the unknown sample were put in a 110 ‘C oven to dry. To prepare the sodium hydroxide solution a liter of distilled water was boiled for 10 minutes and cooled to remove the carbon dioxide. That step is necessary because Sodium hydroxide and carbon dioxide react in a solution to form an unwanted carbonate ion. Since carbonate ions are insoluble in concentrated sodium hydroxide so a 50% stock solution of sodium hydroxide was used.1 After cooling the boiled water about 3 mL of 50% sodium hydroxide was added to form a 0.1 M sodium hydroxide solution. Note that to cool the boiled liter of water in shorter time an ice bath was used. Crushed ice and cold water mixture serves well for the purpose because crushed ice alone is not efficient for cooling due to the air in spaces which is a poor conductor of heat2.
In the standardization process, three samples of known potassium hydrogen phthalate underwent titrations by the prepared 0.1 M sodium hydroxide solution. Each sample consisted of 0.7 ~ 0.8 g of potassium hydrogen phthalate (measured to the nearest 0.1 mg) in a 100 mL of water and two drops of phenolphthalein indicator that would turn the solution lightly pink to mark the end of the titration. The phenolphthalein is an indicator that reacts to the addition of hydrogen or hydroxide ions, turning colorless with an addition of hydrogen ions and turning pink when hydroxide ions are added.3 The masses of each potassium hydrogen phthalate samples were converted to mols which would also reveal the mols of sodium hydroxide since the mol ratio between potassium hydrogen phthalate and sodium hydroxide is one-to-one for this experiment; one mol of potassium hydrogen phthalate reacts with one mol of hydroxide ion. The mols of sodium hydroxide were divided by the volume of 0.1 M sodium hydroxide solution used in titration to find its molarity.
Similar step was taken to find the concentrations of unknown potassium hydrogen phthalate. The average molarity of sodium hydroxide calculated from step before was multiplied by the volume of the titrating solution added and converted into mols of potassium hydrogen phthalate in the sample. Then using the molecular weight 204.233 g/mol for potassium hydrogen phthalate, the mols were converted to grams which would then be a proportion of the mass of its sample, yielding a potassium hydrogen phthalate percentage or concentration in the unknown sample. The unknown KHP sample used was #52 LO so about 3.0 g samples were used in titrations.
The results and calculations are as follows in the next pages.
The indicator used, phenolphthalein is a weak acid that responds to additions or subtractions of the hydrogen ion so it was proper that the unknown potassium hydrogen phthalate was diluted with a neutral diluents. The titrations had to be done with much care and precision because of its sensitivity. A stir bar was used during titration which could hamper with the endpoint noticed by a slight pink indication that lasts about thirty seconds so during the experiment the titration was put to an end if the pink lasted about 10 seconds. It is important to note that the pink color in three titrations were all slightly different, some being darker than others. It was crucial to make use of the half drop technique as well because the endpoint was very sensitive to even 0.05 mL. The average molarity of the sodium hydroxide solution was found to be 0.155 M after standardization. Using that solution, the average concentration of potassium hydrogen phthalate was found to be 27.99% in the #52 LO unknown solid. The standard deviation was calculated as 2.411 which is acceptable yet a small cause of concern since the second experimental trial yielded a KHP concentration of 25.31% which lies outside one standard deviation away from the mean. Perhaps a better result could have been attained if equal amounts of unknown mass were used for each trial.
The purpose of the experiment was to standardize a sodium hydroxide solution to be used in titration in the presence of an indicator, phenolphthalein, to determine the concentration of an unknown sample consisting of potassium hydrogen phthalate and neutral diluents. Due to the simple nature of the experiment it was crucial to have accurate measurements and clean equipments. The titrations yielded an average concentration of 27.99% KHP in unknown sample. With a standard deviation being 2.41 the experiment concluded with 95% confidence that the KHP concentration in the unknown sample measures between 25.58% and 30.4%.
1 Laverman, L.E. Experiments in Analytical, Physical and Inorganic Chemistry, 3rd Edition; p. 16.
2Mohrig, J.M., Hammond, C.N., Schatz, P.F. Techniques in Organic Chemistry, 3rd Edition; W. H.
Freeman and Company: New York, 2010, p. 57.
3 Bruice, P. Y. Organic Chemistry, 6th Edition; Prentice Hall: Illinois, 2011, p.552.