Introductory Biochemistry (Chemistry 330)

pH and Acid Strength Measurement

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.

Any substance which releases protons (hydrogen ions) when dissolved in water is called an acid. The general equation for this is:

HA <-----> H⁺ + A^-

(Actually the H⁺ is not free in solution but is bonded to one or more molecules of water to form hydronium or more complex ions: HA + H₂O <-----> H₃O⁺ + A^-, but we will use the "hydrogen ion" term here.) Acids and bases are classified as strong or weak, depending on the degree to which they dissociate in aqueous solution. A strong acid dissociates completely in dilute solutions:

HCl (Hydrochloric Acid) + H₂O -----> H₃O⁺ + Cl^-

A weak acid ionizes only to a limited degree:

CH₃COOH (Acetic Acid, HAc) + H₂O <-----_-> H₃O⁺ + CH₃COO^- (Acetate ion, Ac^-)

A base is a substance which releases hydroxide ions (OH^-) in water or removes protons (H⁺). An example would be ammonia:

NH₃ + H₂O <-----> NH₄⁺ (Ammonium ion) + OH^-

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Acids and bases are involved in many important processes in living systems, including many metabolic reactions on which life itself depends. The acid-base balance, in other words the hydrogen ion concentration ([H⁺]) of solutions, must be maintained at an equilibrium if the organism is to survive, since slight deviations may cause critical changes in the rate of reactions. Thus the [H⁺] of solutions is an important concept in biochemistry, which can be measured and symbolized by the logarithmic scale termed the "power of the hydrogen ion concentration" or pH. The meaningful values of the pH scale are:

0 <----------------->7 <----------------->14

Acidic

Neutral

Basic

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).

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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.

Obtain a small sample of a fluid in a cup. Dilute concentrates or dissolve dry samples in H₂O as directed on the containers, if necessary.
Dip a clean glass rod into this solution and touch the drop to a small piece of pH paper on a clean filter paper or towel. Never stick pH paper directly into the sample! Why not?
Compare the color while wet to the pH charts in the dispenser. Record your best estimate of pH to the nearest 0.5 units.

The slippery feel and bitter taste of bases, and the sour taste of acids can also be diagnostic, but must be done only if the samples are known to be safe.

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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.

Standardize the meter to read pH 7.0 and 4.0 accurately (see pH meter).
Measure the pH of distilled or deionized water (dH₂O) and Record it.
Calculate the expected pH if you add _.__ mL of _.___ M HCl to the water.
Test your prediction. Add the HCl, mix, and read the pH from the meter. Is it close to the prediction?
Test the predicted pH for a second and third addition of _.__ mL of HCl.

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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.

HCl (Hydrochloric Acid) + H₂O -----> H₃O⁺ + Cl^-

Thus, the [H⁺] is the same as the HCl concentration, which you can calculate.
[HCl] = [H⁺] = [Cl^-]

Use M₁V₁ = M₂V₂ 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 dH₂O, 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

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If you have questions or comments, write the:

Author of this page: Terry Helser - helsertl@oneonta.edu
Web Coordinator: Steve Maniscalco - maniscsj@oneonta.edu
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Last Modified on 7/2/07