Strong bases titration

In a strong acid-strong base titration, pH changes very rapidly near the end point. 

A weak acid-strong base titration. The curve represents the titration of 50.00 mL of 1.000 M acetic acid, HC2H3O2. with 1.000 /W NaOH. The solution at the equivalence point is basic (pH = 9.22). Phenolphthalein is a suitable indicator. Methyl red would change color much too early, when only about 33 mL of NaOH had been added. Bromthymol blue would change color slightly too quickly. 

From Figure 14.5 and Example 14.7, we conclude that any indicator that changes ccdor between pH 4 and 10 should be satisfactory for a strong acid-strong base titration. Bromthymol blue (BB end point pH = 7) would work very well, but so would methyl red (MR end point pH = 5) or phenolphthalein (PP end point pH = 9). 

A typical weak acid-strong base titration is that of acetic acid with sodium hydroxide. The net ionic equation for the reaction is... 

From Figure 14.6 and Example 14.8, it should be clear that the indicator used in this titration must change color at about pH 9. Phenolphthalein (end point pH = 9) is satisfactory. Methyl red (end point pH = 5) is not suitable. If we used methyl red, we would stop the titration much too early, when reaction is only about 65% complete. This situation is typical of weak acid-strong base titrations. For such a titration, we choose an indicator that changes color above pH 7. 

Strategy Once you realize that this is a weak acid-strong base titration, the problem unravels follow the rules cited in Table 14.3. 

HOWTO CALCULATE THE pH DURING A STRONG ACID-STRONG BASE TITRATION... 

EXAMPLE 11.4 Sample exercise Calculating points on the pH curve for a strong add-strong base titration... 

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

Strong Acid-Weak Base and Weak Acid-Strong Base Titrations... 

Figures 11.6 and 11.7 show the different pH curves that arc found experimentally for these two types of titrations. Notice that the stoichiometric point does not occur at pH = 7. Moreover, although the pH changes reasonably sharply near the stoichiometric point, it does not change as abruptly as it does in a strong acid-strong base titration.

A common laboratory application of acid-base reactions is a titration. A titration is a laboratory procedure in which a solution of known concentration is used to determine the concentration of an unknown solution. For strong acid/strong base titration systems, the net ionic equation is ... 

The curve for a weak acid-strong base titration The weak acid here is propanoic acid, CH3CH2COOH. 

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... 

There are only a few direct strong acid strong base titrations carried out in pharmacopoeial assays. 

Strong acid/strong base titrations are used in pharmacopoeial assays of perchloric acid, hydrochloric acid, sulphuric acid and thiamine hydrochloride. 

Weak acid/strong base titration is used in the pharmacopoeial assays of benzoic acid, citric acid, chlorambucil injection, mustine injection, nicotinic acid tablets and undecanoic acid. 

One of the most common type of titration is the strong acid and strong base titration. In the titration process, equal number of moles of strong acids and strong bases neutralize each other, and this can be seen with a suitable indicator. 

The complete titration curve in Figure 11-1 exhibits a rapid change in pH near the equivalence point. The equivalence point is where the slope (dpH/dVf) is greatest (and the second derivative is 0, which makes it an inflection point). To repeat an important statement, the pH at the equivalence point is 7.00 only in a strong-acid-strong-base titration. If one or both of the reactants are weak, the equivalence point pH is not 7.00. 

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