Experiment 2 Preparation of Primary Standard solutions and Standardizing Acid and Base solutions
Objectives: The objective of this experiment is: 1- To prepare two primary standard solutions, KHP and Na2CO3 2- To standardize a sodium hydroxide solution using the prepared primary standard KHP. 3- To standardize a hydrochloric acid solution using the prepared primary standard Na2CO3. 4- To calculate the concentration of an unknown acid or base. Introduction A primary standard is a standard that is accurate enough that it is not calibrated. For a compound to be considered as a primary standard it should have several important characteristics, the most important of which are high purity, stability, low hygroscopicity, high solubility, and high molar mass. A primary standard solution is a solution of known concentration made from a primary standard. Primary standard solutions are used in determining the concentrations of other solutions to an extremely high accuracy. They are typically used in titrations and other analysis techniques as standardization solutions. A secondary standard solution, such as HCl solution, is a solution which must be standardized first against a primary standard, but afterwards, it will be stable enough for titrimetric work (Titration). Titration involves the gradual addition of a solution of accurately known concentration (standard solution) to another solution of unknown concentration (or vice versa), until the chemical reaction is complete. Titrations are based on reactions which go to completion rapidly. A reaction is complete when stoichiometric amounts of the reacting substances are combined. This is the stoichiometric point (equivalence point) in the titration. The equivalence point is detected visually using an indicator. An indicator is a substance (added at the beginning of the titration to the flask) that changes color at (or very near) the equivalence point. The point where the indicator actually changes color is called the end point of the titration. In this experiment, two primary standards will be used. The first is potassium hydrogen phthalate (KHC8H4O4, abbreviated as KHP, molar mass = 204.23 g/mol), an acid primary standard which will be used to standardize a sodium hydroxide solution. The structure of KHP is shown below:
O COH CO K O
The chemical equation of the reaction can be written as: KHP(aq) + NaOH(aq) Or, expressed as a net ionic equation, HP-(aq) + OH-(aq) P2-(aq) + H2O(l) The second primary standard to be used is sodium carbonate, Na2CO3 (molar mass = 105.99), a base, by which a hydrochloric acid solution will be standardized. The chemical equation of the reaction is: 2HCl(aq) + Na2CO3(aq) → CO2 (g) + 2NaCl(aq) + H2O(aq) KNaP(aq) + H2O(l)
The reaction above generates CO2, which dissolves into the solution to generate an acid. The presence of dissolved CO2 thus interferes with the pH and the detection of the end point of the titration. However, the CO2 can be driven off by boiling the solution, enabling an accurate titration. Procedure I. Standardization of NaOH a. Preparation of the acid primary standard 1. Obtain a bottle containing ~2g of KHP and weigh it with the cap on the analytical balance. Record the mass in Table 2.I. 2. Transfer the solid KHP to a 100.0 mL volumetric flask using a funnel, re-stopper the bottle and weigh it. Record the mass in Table 2.I. 3. Rinse the funnel to wash any sticking solid using a washing bottle and add more distilled water into the volumetric flask to dissolve the KHP (1/2 its capacity). Swirl the flask; make sure to dissolve the solid completely. Add more water (2/3) and swirl again. Dilute to the mark carefully, stopper or cover with a parafilm paper and invert several times with swirling to homogenize the KHP solution. b. Preparation of an approximately 0.1 M NaOH solution 1. Obtain about 6 mL of a 50 % (w/v) NaOH solution in a clean and dry graduated cylinder from the stockroom. Transfer the NaOH to a clean 1L...
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