Standardization of NaOH with a KHP solution: Acid-Base Titration
Background: NaOH is not an expensive, is highly soluble and easy to use base for simple titrations; however, solid NaOH absorbs water from the air so accurately weighing a sample during the preparation of a solution is not possible. Solution of NaOH also absorbs atmospheric CO2, which is a weak acid. The reaction between the CO2 and NaOH partially neutralizes the NaOH solution, which means that the 0.100 M solution of NaOH is prepared might change over time. For this reason it is standardize NaOH solutions to be used in titrations.
Standardization is typically carried out by titrating the NaOH solution with a primary standard, and ultra-pure soluble acid such as potassium hydrogen phthalate (KHP). KHP can be used to prepare a solution whose concentration is known with a high degree of precision. The only way to evaluate the molar concentration of a previously prepared NaOH solution is to titrate it with a solution such as KHP. Once the NaOH solution has been standardized and its concertation is well-known and can be used to titrate other solutions.
In this week’s experiment, the concentration sodium hydroxide will accurately be determined by titrating it against a known concertation of a monochromatic acid, potassium hydrogen phthalate (KHP). The equation for this reaction is as follows:
KHP (aq) + NaOH (aq) H2O (aq) + KNAP (aq)
Each mole of KHP yields 1 mole of hydronium ion (this is a monoprotic acid). From the molarity of KHP (0.500 M) used for titration, you can determine the number of moles of hydronium ion produced and thus the number of moles of NaOH required for neutralization. The concentration of base can be determined.
Instructions:
Retrieve the KHP, ~1 M NaOH solution and the phenolphthalein from the stock room. Note the volume of KHP, which can be found in the solution info window, record volume of KHP used in the experiment.
Add a small amount (0.3 mL) of indicator to the KHP solution.
Fill the burette (buret) with NaOH solution (you can add 50 mL NaOH solution).
Your Experimental Set up:
Titrate small volumes (0.5 mL or less) of NaOH from the burette into the KHP until a color change occurs (end point). Every addition record your pH, and total amount of NaOH (mL) added, and color of the solution; record total amount NaOH needed to see the color change, you will need total volume of the NaOH to calculate exact concentration of NaOH.
Continue adding 0.5 mL of NaOH (4-5 times) to see if there is an additional color change. Record the added volume of NaOH, the color and pH for each addition, but do not use additional amount of NaOH to calculate concentration, you only need extra data points for plotting the graph.