Question

Using a pH meter, the pH of a lemon was found to be 2.24. Determine the [H+]. Titration of a 1.0 ml sample of the above juice to a pH of approximately 7 required 02.34 ml of 0.10M NaOH. Determine the total [H+] in the juice Explain why the above two numbers are not the same.

Answer 1

We know,

pH= -log(H⁺) …………………………………….1

Given,

pH=2.24

Therefore from equation 1,

2.24= -log(H⁺)

Or, log (H⁺) = -2.24

Or, [H⁺] = 10^-2.24

[H⁺] = 0.00575

Now in the second case,

V₁ = 1 ml          V₂ =2.34 ml                        (S= concentration of solution)

S₁ =? (M)          S₂ =0.10 (M)

Now the required formulae,

V₁S₁ = V₂S₂

Or, 1 x S₁ = 2.34 x 0.10 (putting all the values)

Or, S₁ =0.234 (M)

Citric acid is present in lemon. Citric acid has 3 ionizable COOH groups. In presence of NaOH (strong base), it completely ionizes and produces 4 protons which react with NaOH and produces water.

C(OH)(COOH)₃ = C(O^-1)(COO)₃^3- + 4 H⁺

Therefore, [H⁺] = 4 x 0.234

                           = 0.936

The two values are different is due to in presence of strong base (NaOH), OH protons also ionizes and produces protons.