This page contains some theory on acid and base strength, a procedure for estimating pH with indicator paper, a procedure for measuring and calculating the pH of HCl solutions with a meter and a simple experiment to show the difference between a strong (HCL) and weak (HAc) acid. You can also go to my home page, or back to the lab schedule.



The definition of pH is: pH =  log [H^{+}] In words, the pH is 1 times the (almost always) negative exponent of ten that equals the hydrogen ion concentration in moles per liter. Thus a pH of 7 means that there are 10^{7} moles of H^{+} ions in a liter of solution (0.0000001 M = [H^{+}]). You should know how to calculate pH and [H^{+}] (see the appendix).
Would you like to go to the top of this page, to my home page, or back to the lab schedule?
Procedures: pH estimates with indicator papers
Numerous dyes are available which ionize and therefore change their color in solutions of different pH. This property can be used to estimate the pH of a solution, or to determine the amount of an ionizable compound in a solution by titration. The first procedure is used to estimate the pH of a fluid with pH paper.
Would you like to go to the top of this page, to my home page, or back to the lab schedule?
Procedures: Weak vs. Strong Acids
A more accurate determination of the [H^{+}] can usually be made with the help of a pH meter which measures the potential difference between two electrodes caused by the hydrogen ions in the solution, and therefore the pH. You can use this instrument (see below) to discover the differences between strong and weak acids. Ask your instructor to demonstrate the use and standardization of the pH meter, if necessary. DO NOT touch the tip of the electrode to any hard surface. They are easily broken and very expensive.
Would you like to go to the top of this page, to my home page, or back to the lab schedule?
Sample calculation:
From the equation pH =  log [H^{+}], you can see you need
the [H^{+}] to calculate pH. Remember that HCl is a strong acid,
which means it almost totally dissociates into H^{+} ions and Cl^{}
ions in dilute water solutions.
Thus, the [H^{+}] is the same as the HCl concentration, which
you can calculate.
[HCl] = [H^{+}] = [Cl^{}]
Use M_{1}V_{1} = M_{2}V_{2} to calculate [HCl], or realize that the [HCl] is diluted, for example, by 0.50 mL /50.0 mL = 1/100. If the starting [HCl] = 0.10 M, then 0.10 M x 1/100 = 0.001 M = 10^{3} M, which = [H^{+}]. Thus: pH =  log [H^{+}] =  log 10^{3} = (3) = 3.0. See?
Would you expect the same pH values if you added the same amount of a weak acid like acetic acid (HAc) to the water, instead of HCl? To test your hypothesis, repeat the steps above adding aliquots of HAc instead of HCl to dH_{2}O, mixing and testing the pH. Calculate the [H^{+}] from each pH measured with the meter.
You can also calculate the % of each acid that actually ionized: % Ionized = [H^{+}] / Total [acid] x 100
Would you like to go to the top of this page, to my home page, or back to the lab schedule?
If you have questions or comments, write the:
Author of this page: Terry
Helser  helsertl@oneonta.edu
Web Coordinator: Steve
Maniscalco  maniscsj@oneonta.edu
Or return to the SUNY @ Oneonta Home
Page to see where we live and work.
Last Modified on 7/2/07