Weak acid-strong base titration

Strong Acid-Strong Base Titrations Weak Acid-Strong Base Titrations Strong Acid-Weak Base Titrations Acid-Base Indicators... 

STRONG ACID-WEAK BASE AND WEAK ACID-STRONG BASE TITRATIONS... 

When an acid in solution is exactly neutralized with a base the resulting solution corresponds to a solution of the salt of the acid-base pair. This is a situation which frequently arises in analytical procedures and the calculation of the exact pH of such a solution may be of considerable importance. The neutralization point or end point in an acid-base titration is a particular example (Chapter 5). Salts may in all cases be regarded as strong electrolytes so that a salt AB derived from acid AH and base B will dissociate completely in solution. If the acid and base are strong, no further reaction is likely and the solution pH remains unaffected by the salt. However if either or both acid and base are weak a more complex situation will develop. It is convenient to consider three separate cases, (a) weak acid-strong base, (b) strong acid-weak base and (c) weak acid-weak base. 

O <2ZD In a titration, a basic solution is added to an acidic solution, and measurements of pH are taken. Compare a strong acid-strong base titration and a strong acid-weak base titration in terms of... 

In textbooks of computational chemistry you will invariably find examples calculating the pH = - lg [H+]/(mol/l)> in weak acid - strong base or strong acid - weak base solutions. Indeed, these examples are important in the study of acids, bases and of complex formation, as well as for calculating titration curves. Following (ref. 24) we consider here the aquous solution that contains a weak tribasic acid H A and its sodium salts NaH, Na HA and Na A in known initial concentrations. The dissociation reactions and equilibrium relations are given as follows. 

Acid-base Various questions about strong acid/weak base titration... 

TLie pH at tlie equivaLence point of an acid-base titration depends on liydroiysis of the salt formed in the neutraiization reaction. For strong acid-strong base titrations, the pH at the equivaLence point is 7 for weak acid-strong base titrations, the pH at the equivalence point is greater than 7 for strong acid-weak base titrations, the pH at the equivalence point is less than 7. 

Understand how an acid-base indicator works, how the equivalence point and end point in an acid-base titration differ, and how strong acid-strong base, weak acid-strong base, and strong acid-weak base titration curves differ explain the significance of the... 

Sketch the titration curve for a weak base titrated by a strong acid. Weak base-strong acid titration problems also follow a two-step procedure. What reaction takes place in the stoichiometry part of the problem What is assumed about this reaction At the various points in your titration curve, list the major species present after the strong acid (HNO3, for example) reacts to completion with the weak base, B. What equilibrium problem would you solve at the various points in your titration curve to calculate the pH Why is pH < 7.0 at the equivalence point of a weak base-strong acid titration If pH = 6.0 at the halfway point to... 

For a strong acid/weak base titration, such as that between 0.1 mol dm" aqueous ammonia and 0.1 moldm hydrochloric acid, the indicator needs to change between pH values 4 and 7. Methyl orange is a suitable indicator, but phenolphthalein is not. 

Indicators that change color at pH lower than 7 are useful in determining the equivalence point of strong-acid/weak-base titrations. Methyl orange is an example of this type. The equivalence point of a strong-acid/ weak-base titration is acidic because the salt formed is itself a weak acid. Thus the salt solution has a pH lower than 7 at the equivalence point. 

Figure 17.5 Titration curve of a strong acid-weak base titration. A 0.100 M HCl solution is added from a buret to 25.0 mL of a 0.100 A/ NH3 solution in an Erlenmeyer flask. As a result of salt hydrolysis, the pH at the equivalence point is lower than 7.

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