Calculate the Ph of titrant solution
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Titration of a Weak Base with a Strong Acid
Last updatedJun 6, 2019
Titration of a Weak Acid with a Strong Base
Titration of a Weak Polyprotic Acid
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Data obtained through the process of titration can be used to compute the molarity and the correlated acidity of a solution at various times of the titration. The initial and final volumes of the analyte and titrant solutions, as well as the pH, or measure of acidity, are essential in calculating the total number of moles of analyte present. Once this information is determined, the molarity of the analyte, which was unknown before the titration, can then be computed, because its volume was measured beforehand. However, chemists are often interested in the data collected at various points during the titration as well, not just at the beginning and the end. These data can then be translated to points on a graph, resulting in an informational titration curve.
Introduction
Chemists are typically interested in calculating volume and acidity data for the following critical points: at the starting point before any titrant is added, at the midpoint, at a point before the equivalence point (excluding the initial condition), at the equivalence point, and past the equivalence point.
Figure 1. Conventional setup of a lab titration. In this particular case, the weak base (colored in green), is being titrated by the strong acid (colored in red). In a typical titration, a few drops of indicator, such as phenolphthaelein, is added. The indicator causes the solution in the flask to undergo a color change that signifies the equivalence point has been reached. Image created by Christine Chang.
Before titrant is added
Before the stopcock on the buret containing the strong acid is released, the analyte in the flask is completely unreacted. Calculating the pH of this initial solution allows chemists to analyze the changes in acidity, as well as the acidic strength of the titrant after the titration is complete. To calculate the pH, an ICE (Initial, Change, Equilibrium) table is used. When titrating weak bases, water is always a reactant in this initial step, and its conjugate base, hydroxide, in the products.
Table 1. ICE table at initial conditions. Drawing a chart like the one above is a useful step in calculating the concentration of hydroxide before any acid is added. The concentration can then be used to determine the starting pH. Since the quantity of water does not directly affect the reaction, its column is left blank. Table created by Christine Chang.
At this point, there is no BH+ or OH- in the analyte solution—the molarities of these species are zero, as observed in the ICE table. Because the stopcock has not yet been released, there is no acid in the flask to react with the base and yield products BH+ and OH-. Once the strong acid is released into the flask, however, the BH+ and OH- begin to form.
The next step in determining the initial concentration of OH- is to use the information from the ICE table to set up an equilibrium expression with Kb. Kb is used in this case, instead of Ka, because the analyte being titrated is a base.
Kb=x2[B]−x
Since Kb and [B] are given, the only variable left to solve for is x, which is equal to [OH-]. This requires the quadratic equation:
−b±b2−4ac−−−−−−−√2a
Once x is obtained, the pOH can be determined using the relation pOH=−logx . The pH can then be derived from the pOH using pH=14−pOH .
At the midpoint