Reference no: EM133142275

Experiment - pH titrations, acid base titrations and the determination of acid dissociation constants

A successful titration depends on:

-availability of a suitable titrant

-a fast and quantitative reaction

-a means of estimating the equivalence point or end point

Analysis (after the lab) - A - Strong Acid

(a) Using the volume of base needed to neutralise the acid calculate the exact concentration of the hydrochloric acid. Quote the answer obtained to four significant figures and repeat the calculation using the volume i) from the pH titration and ii) from the mean titre.

(b) Using the standard deviations of your titres calculate the 95% confidence interval for the acid concentration determined by the titraton with indicator.

B - Weak Acid

(c) What range of pH is spanned by the steepest part of your pH curve? Find point X and record the volume of titrant at X in cm³.

(c) Plot the derivative graph for the above weak acid data ΔpH/Δv (ordinates) against v, volume of added sodium hydroxide and also determine X in cm³ from this graph.

(d) Find from your pH vs volume graph the point Y which is "half way to X" and record the volume of titrant at Y in cm³. (Use whichever X value you believe is most accurate to find the volume at Y)

(e) Deduce from your graph the pH of the solution when the weak acid has been half neutralised, i.e. where the change in pH per cm³ NaOH solution added is a minimum. This is equal to the pK_a of ethanoic acid.

(f) Compare your pKa values with a literature value, quoting the reference.

(g) Using the volume of base needed to neutralise the acid calculate the exact concentration of the ethanoic acid. Quote the answer obtained to four significant figures and repeat the calculation using the volume i) from the pH titration and ii) from the mean titre.

(h) Using the standard deviations of your titres calculate the 95% confidence interval for the acid concentration determined by the titraton with indicator.

(i) Once the acid concentration is known we can use the Henderson-Hasselbalch equation to calculate the pK_a of the weak acid using the measured pH value for any known volume of added base (or to calculate the pH for each volume). Using a pH value recorded for ~10 cm³ added NaOH recalculate pKa and compare this to your previous value.

(j) Use your pKa value to calculate K_a and combining this with the acid concentration calculate the expected pH of the weak acid with no added base and compare to the measured value.

Questions -

Q1. What is a pH electrode? How is this device able to detect changes in acidity?

Q2. Why is it that the solvent interferes with the determination of the equilibrium constant for strong acids and indeed strong bases?

Q3. Which graphs best identify the endpoint of the titration (pH against v) or (ΔpH/Δv against v). Why is this the case?

Conclusions (How could the method or your results be improved?)

Attachment:- Experiment File - pH titrations.rar