Acids and Bases
Determination of the pH Scale by the Method of Successive Dilutions

Objective: It is common knowledge that neutral water has a pH of 7, acids have a pH less than 7 and bases have a pH greater than 7, but few people understand this in terms of the actual hydronium ion concentration. Our objective is to develop an understanding of logarithmic scales by developing a pH scale. Background: The pH scale describes the hydronium ion concentration in aqueous systems: pH = -log[H3O+] [H3O+] = 10-pH = 1/10pH The Method of Successive Dilutions is an experimental technique for preparing a series of solutions of different concentrations from one volume of stock solution. Procedure: Lets look at a series of half dilutions. With the virtual lab, fill 5 flasks with a constant amount of water (less than half the volume of the flask). For simplicity, we will use 20 ml, but any amount will do. Virtual Lab Tip:You can right click on each flask and label it using the Rename menu item. Now add the same amount of stock 1M HCl to the first flask (20 mL), and note that the concentration of [H3O+] has been diluted in half with a value of 0.500M or 1/2 (1/21) the original molarity. From this flask transfer 20 mL to the second flask and note the it has been diluted in half again; [H3O+] = 0.250M or one fourth (1/22) the concentration of the original stock solution. Repeating this procedure with the remaining 3 flasks gives: 3rd dilution: [H3O+] = 0.12500 or 1/8 (1/23) the original stock molarity. 4th dilution: [H3O+] = 0.06250 or 1/16 (1/24) the original stock molarity. 5th dilution: [H3O+] = 0.03125 or 1/32 (1/25) the original stock solution. In more detail: [H3O+] = 2-n = 1/2n where n is the number of successive dilutions and by using a dilution factor of one to two, you have come up with a log base 2 scale. Question: Would changing the volume of the original stock solution and the incremental dilution volumes to a new constant value effect the successive concentrations? (For example, starting with 10 mL and transferring in 10 mL increments?) If you say yes, repeat the experiment with 10 mL increments and explain.