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Measuring pH

Middle schoolChemistryTypical pH for household liquids, exact equilibrium for lab solutions

Lemon juice

pH ≈ 2

pH scale

01234567891011121314
AcidicNeutralAlkaline

The outlined band is all this method can tell you.

Results table

Press Record result to add a row.

About this experiment

Test lemon juice, milk, soap and other everyday liquids with four different methods: universal indicator, litmus paper, phenolphthalein and a pH meter. Each method tells you something different. Litmus only says acid or alkali, phenolphthalein only reacts above pH 8.2, universal indicator gives a whole-number estimate and the meter gives a precise value. Switch to lab solutions to dilute strong and weak acids and bases and see how the pH changes.

Learning goals

  • Classify solutions as acidic, neutral or alkaline using the pH scale
  • Compare what litmus, phenolphthalein, universal indicator and a pH meter can each tell you
  • Explain that each step of one pH unit is a tenfold change in hydrogen ion concentration
  • Show that diluting a strong acid ten times raises its pH by one unit, while a weak acid changes less

How to use it

  1. 1Choose universal indicator and test five household liquids. Record each result in the table.
  2. 2Test the same liquids with litmus paper and phenolphthalein. Which liquids can these indicators not tell apart?
  3. 3Switch to the pH meter and check your universal indicator estimates.
  4. 4Under Lab solutions, test HCl at 1, 0.1, 0.01 and 0.001 mol/L, then repeat with ethanoic acid (acetic acid, CH₃COOH) and compare.

The model

Definition of pH, with concentration in mol/L
Ionic product of water at 25 °C
Ethanoic acid only partly ionizes, so its pH is higher than HCl at the same concentration

Assumptions

  • Household liquids are mixtures, so their pH values are typical values, not calculations. Real samples vary.
  • Lab solution pH comes from the full charge balance at 25 °C with ideal behavior. Activity effects are ignored, which matters most at 1 mol/L.
  • Indicator colors show the indicator only. In a real test, colored drinks such as cola or coffee can hide the color change.
  • Litmus changes color around pH 4.5 to 8.3 and phenolphthalein turns pink from about pH 8.2.

Try this in class

  1. Q1Lemon juice has a pH of about 2 and black coffee about 5. How many times more H⁺ ions does lemon juice contain?
  2. Q2Why does phenolphthalein give the same result for lemon juice and pure water?
  3. Q30.1 mol/L HCl and 0.1 mol/L ethanoic acid have the same concentration. Why are their pH values different?
  4. Q4Dilute 0.1 mol/L NaOH a thousand times. Predict the new pH before you test it.

Frequently asked questions

What does pH 7 mean?

At 25 °C, pH 7 means the concentrations of H⁺ and OH⁻ ions are equal, so the solution is neutral. Below 7 is acidic and above 7 is alkaline.

Can the pH be below 0 or above 14?

Yes. Very concentrated strong acids can have a negative pH. The 0 to 14 scale covers the solutions met in school and everyday life.

Why does diluting an acid never make it alkaline?

As the acid gets very dilute, the H⁺ from water itself takes over and the pH approaches 7 from below. Adding water cannot create more OH⁻ than H⁺.