Titration calculations weak base

Bell has calculated Hq values with fair accuracy by assuming that the increase in acidity in strongly acid solutions is due to hydration of hydrogen ions and that the hydration number is 4. The addition of neutral salts to acid solutions produces a marked increase in acidity, and this too is probably a hydration effect in the main. Critchfield and Johnson have made use of this salt effect to titrate very weak bases in concentrated aqueous salt solutions. The addition of DMSO to aqueous solutions of strong bases increases the alkalinity of the solutions. 

Titrations of weak bases with strong acids can be treated using the procedures we have introduced previously. As always, you should first think about the major species in solution and decide whether a reaction occurs that runs essentially to completion. If such a reaction does occur, let it run to completion and then do the stoichiometric calculations. Finally, choose the dominant equilibrium and calculate the pH. 

Titrating a Weak Acid with a Strong Base For this example let s consider the titration of 50.0 mL of 0.100 M acetic acid, CH3COOH, with 0.100 M NaOH. Again, we start by calculating the volume of NaOH needed to reach the equivalence point thus... 

HOWTO CALCULATE THE pH DURING A TITRATION OF A WEAK ACID OR A WEAK BASE... 

LI 6 Calculate the pH at any point in a strong base-weak acid and weak base-strong acid titration (Toolbox 1 1.2 and Examples 11.5 and 1 1.6). 

Ephedrine, a weak base, is the active ingredient in many commercial decongestants. To analyze a sample of ephedrine dissolved in 0.200 L of water, a chemist carries out a titration with 0.900 M HCl, monitoring the pH continuously. The data obtained in this titration are shown in Figure 18-6. Calculate Zj, for ephedrine and determine the pH of the solution at the stoichiometric point. 

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. 

The next point in the titration curve is the equivalence point. At this point, both the material added and the material originally present are limiting. At this point, neither of the reactants will be present and therefore will not affect the pH. If the titration involves a strong acid and a strong base, the pH at the equivalence point is 7. If the titration involves a weak base, only the conjugate acid is present to affect the pH. This will require a Ka calculation. If the titration involves a weak acid, only the conjugate base is present to affect the pH. This will require a Kb calculation. The calculation of the conjugate acid or base will be the moles produced divided by the total volume of the solution. 

This technique uses both direct and back titrations of weak acids and bases. Values of are obtained directly. In purely aqueous media, over the pH range 2-10, the titration of dilute (0.005 to 0.05 M) solutions of weak monovalent acids and bases with a glass electrode can lead to reliable thermodynamic pKs. Over this pH interval, the activity coefficients of the ionic species can be calculated by means of the Debye-Hiickel equation. Also, the activity coefficients of the neutral species remain essentially constant and... 

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. 

We can calculate pH titration curves using the principles of aqueous solution equilibria. To understand why titration curves have certain characteristic shapes, let s calculate these curves for four important types of titration (1) strong acid-strong base, (2) weak acid-strong base, (3) weak base-strong acid, and (4) polyprotic acid-strong base. For convenience, we ll express amounts of solute in millimoles (mmol) and solution volumes in milliliters (mL). Molar concentration can thus be expressed in mmol/mL, a unit that is equivalent to mol/L ... 

Because Na+ is neither an acid nor a base and CH3C02 is a weak base, we have a basic salt solution (Section 15.14), whose pH can be calculated as 8.72 by the method outlined in Worked Example 15.15. For a weak monoprotic acid-strong base titration, the pH at the equivalence point is always greater than 7 because the anion of the weak acid is a base. 

Figure 16.9 shows the pH titration curve for a typical weak base-strong acid titration, the titration of 40.0 mL of 0.100 M NH3 with 0.100 M HC1. The pH calculations are simply outlined to save space you should verify the results yourself. 

Before Addition of Any HCl The equilibrium problem at the start of the titration is the familiar one of calculating the pH of a solution of a weak base (Section 15.12). The principal reaction at this point is the reaction of ammonia with water ... 

The calculations for the titration of a weak base with a strong acid are illustrated by the following titration of 100.0 mL of 0.050 M NH3 with 0.10 M HC1. The strategies needed at several key areas in the titration will he described qualitatively. The actual calculations are summarized in Table f. ... 

Novocain, the commonly used local anaesthetic, is a weak base with K, = 7 X 10 M. (a) If you had a 0.0200 M solution of Novocain in water, what would be the approximate concentration of OH and the pH (b) Suppose that you wanted to determine the concentration of Novocain in a solution that is about 0.020 M by titration with 0.020 M HCl. Calculate the expected pH at the equivalence point. 

Four distinctly different types of calculations are needed to derive a titration curve for a weak acid (or a weak base) ... 

In the previous chapter we calculated the acidity or basicity of aqueous solutions of strong acids, strong bases, weak acids, weak bases, and their salts. In this chapter we will study (1) solutions that have both weak acids and weak bases present, (2) indicators, and (3) titration curves. 

In titration curves for weak acids and weak bases, pH changes near the equivalence point are too small for color indicators to be used. The solution is buffered both before and after the equivalence point. Figure 19-6 shows the titration curve for 100.0 mL of 0.100 M CH3COOH solution titrated with 0.100 M aqueous NH3. The calculation of values on the curve in Figure 19-6 other than the initial pH and the pH at the equivalence point is beyond the scope of this text. 

HCl] is sufficiently high for non-ideality to be significant. Such considerations have to borne in mind when calculations of the pA[ of a fairly strong weak acid or a fairly strong weak base are made from pH titrations (see Section 8.25.3 and Worked Problems 8.21 and 8.22), or from emf measurements (see Worked Problem 9.23). 

Further we introduced buffer chemistry and saw how pH may be calculated in buffer solutions and on how the buffer equation is often used in practice. When one has a solution of a weak acid and its corresponding weak base, both in concentrations of the same magnitude, one has a buffer system and the buffer equation may be used to calculate pH. Lastly, we looked at titration and on pH curves exemplifyed through examples of titration of monovalent weak acid with strong base and titration of divalent acid likewise with strong base NaOH. In the end we saw how colour indicator work. 

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