This experimental method was obtained from the official website of the Fourth Military Medical University
Operation method
Experiments on preparation of buffer solutions and pH determination
Principle
Determination of solution pH, which is widely used in all fields of natural science, is of great importance in the field of medicine. For example, the control of synthetic or analytical reaction conditions, the preparation of injections or other pharmaceuticals, etc., often need to accurately determine the pH value. So the determination of solution pH is a very high degree of popularity in instrumental analysis of a method. 1. basic concept to a certain extent can resist the addition of a small amount of acid, alkali or dilution, and keep the solution pH value of the basic unchanged role is called buffer effect. A solution with buffering action is called a buffer solution.2. Buffer Solution Composition and Calculation Formulas Buffer solutions are generally composed of conjugate acid-base pairs, such as weak acids and weak acid salts, or weak bases and weak base salts. If a buffer solution consists of a weak acid and a weak acid salt (e.g., HAc-NaAc) 3. Buffer Solution Properties a. Acid/base resistance, anti-dilution effects Because buffer solutions have acid-resistant and base-resistant components, their pH is essentially unchanged by the addition of small amounts of strong acids or bases. Dilute the buffer solution, acid and base concentration ratio does not change, appropriate dilution does not affect its pH value. b. Buffer capacity buffer capacity is a measure of the size of the buffer capacity of the buffer solution. The size of the buffer capacity is related to the concentration of buffer components and the ratio of buffer components. The greater the concentration of buffer components, the greater the buffer capacity; buffer component ratio of 1, the buffer capacity is the largest.
Materials and Instruments
HAc NaAc NaH2PO4 Na2HPO4 NH3-H2O NH4Cl HCl NaOH Acid standard buffer solution pH=9.18 standard buffer solution Methyl red solution Extensive pH test paper Precision pH test paper Move 1. Preparation of buffer solutions and pH determination For more product details, please visit Aladdin Scientific website.
Acidity meter Test tube Measuring cylinder Suction tube Suction paper
Through the calculation of the preparation of three different pH buffer solution, and then use precision pH test paper and pH meter to determine their pH values, pH meter should be used in strict accordance with the instructions for the operation of the instrument, calibrated with the appropriate pH standard material in advance. After the measurement, the results will be compared to see whether the theoretically calculated value and the experimental value of the two measurement methods are consistent (the solution is reserved for later experiments).
2. The nature of the buffer solution
(1) Take 3 test tubes, add distilled water, pH = 4 HCl solution, pH = 10 NaOH solution 3mL each, use pH paper to measure its pH value, and then add 5 drops of 0.1 mol / L HCl to each tube, and then measure its pH value. Using the same method, the effect of 5 drops of 0.1 mol/L NaOH on the pH of the above three solutions was tested. The results were recorded in Table 2.
(2) Take three test tubes and add 3 mL of each of the buffer solutions of pH=4.0, pH=7.0, and pH=10.0 prepared by yourself in turn. then add 5 drops of 0.1 mol/L HCl to each tube and measure its pH value with precision pH test paper. Using the same method, the effect of 5 drops of 0.1 mol/L NaOH on the pH of the above three buffer solutions was tested. The results were recorded in Table 2.
(3) Five test tubes were taken and 1 ml each of HCl solution at pH=4, NaOH solution at pH=10 and buffer solution at pH=4.0, buffer solution at pH=7.0 and buffer solution at pH=10.0 were added to the tubes in turn, and the pH of the solution in each tube was measured by using precision pH paper. Then 10 ml of water was added to each tube, mixed well and then its pH value was measured with precision pH paper to examine the effect of diluting the pH value of the above five solutions. The results of the experiment are recorded in Table 2, by the results of the above experiment, what properties of the buffer solution.
3. buffer capacity of the buffer solution
(1) Relationship between buffer capacity and concentration of buffer components : Take two large test tubes, add 3 ml each of 0.1 mol/L HAc and 0.1 mol/L NaAc to one test tube, and add 3 ml each of 1 mol/L HAc and 1 mol/L NaAc to the other test tube, and then mix well to determine the pH of the solutions in the two tubes (are they the same) by using a piece of precision pH test paper? What color does the solution appear when 2 drops of methyl red indicator are added to each of the two test tubes? (Methyl red is red at pH < 4.2 and yellow at pH > 6.3). Then add 1 mol/L NaOH solution drop by drop to each of the two test tubes (shake well for each drop of NaOH added) until the color of the solution turns yellow. Record the number of drops of NaOH added to each test tube to show which test tube has a large buffer capacity of the buffer solution.
(2) Relationship between buffer capacity and buffer component ratio: Take two large test tubes, use a pipette to add 10 ml each of NaH2PO4 and Na2HPO4 to one test tube, and add 2 ml of 0.1 mol/L NaH2PO4 and 18 ml of 0.1 mol/L Na2HPO4 to the other test tube, and then mix well to measure the pH of the solution in the two test tubes with a piece of precision pH test paper. Then 1.8 ml of mol/L NaOH was added to each test tube, mixed well and then the pH of the solution in the two test tubes was measured with precision pH paper.