Using pH to Determine

In Chapter 15 we learned that we can determine the value of an equilibrium constant using equilibrium concentrations [ Section 15.2].Usingasimilarappioach,wecanusethepHofaweak acid solution to determine the value of K. Suppose we want to determine the /fa of a weak acid (HA) and we know that a 0.25 M solution of the acid has a pH of 3.47 at 25°C. The first step is to use pH to determine the equilibrium hydronittm ion concentration. Using Equation 16.3, we get 

We use the starting concentration of the weak acid and the equilibrium concentration of the hydro-nium ion to constmct an equilibrium table and determine the equilibrium concentrations of all 

These equilibrium concentrations are substituted into the equilibrium expression to give 

Aspirin (acetylsalicylic acid, HC9H7O4) is a weak acid. It ionizes in water according to the equation 

Strategy Determine the hydronium ion concentration from the pH. Use the hydronium ion concentration to determine the equilibrium concentrations of the other species, and plug the equilibrium concentrations into the equilibrium expression to evaluate Kg. 

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Just as we can use pH to determine the of a weak acid, we can also use it to determine the Kf, of a weak base. Sample Problem 16.15 demonstrates this piocedure. 

Strategy Use pH to determine pOH, and pOH to determine the hydroxide ion concentration. From the hydroxide ion concentration, use reaction stoichiometry to determine the other equilibrium concentrations and plug those concentrations into the equilibrium expression to evaluate K. ... 

A combination pH electrode is the most commonly used electrode to determine soil pH. It is illustrated in Figure 9.5 and shown in Figure 9.6 (D), which shows the pH sensing bulb and reference side. As an alternative to the... 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

In this section, you learned why solutions of different salts have different pH values. You learned how to analyze the composition of a salt to predict whether the salt forms an acidic, basic, or neutral solution. Finally, you learned how to apply your understanding of the properties of salts to calculate the pH at the equivalence point of a titration. You used the pH to determine a suitable indicator for the titration. In section 9.2, you will further investigate the equilibria of solutions and learn how to predict the solubility of ionic compounds in solution. 

Potentiometric titration of an aqueous suspension of oxides in the presence of varying concentrations of indifferent electrolyte has been used successfully to determine the zero point of charge (z.p.c.) and the variation in excess surface charge with pH (I, 8). The variation in excess surface charge (rH+-r0H-) with pH and NaCl concentration is shown for goethite in Figure 4. 

This is a very useful procedure to determine when protected secondary structure is formed. But it is quite difficult to perform quantitatively in practice. Individual time points, each of not particularly high precision, are measured separately, and a very large number of these points is required to analyze multiphasic kinetics. Also, folding intermediates can be very labile and become unprotected in the labeling pulse, especially at high pH. 

Two methods are commonly used to determine the endpoint of an acidity titration. The potentiometric method titrates to a predetermined pH and the colorimetric method uses an indicator that changes color at a particular pH to determine the endpoint. Other methods define the endpoint as the inflection of a titration curve, i.e., plots of pH value versus milliliter of NaOH used (Sadler and Murphy, 1998 Hand et al., 1993). However, the increased precision... 

This is the Henderson-Hasselbach equation, or simply the buffer equation. We can use the buffer equation to calculate the pH of a buffer. We can also use it to determine the ratio of weak acid to conjugate base at a given pH. 

Answer There is more than one way to solve this problem. One possible solution would use the given [OH ] to calculate the [H+], use the calculated [H+] to determine the pH, and then use the pH to determine the pOH. Another would be to use the [OH-] to determine the pOH, then use the pOH to determine the pH, and finally use the pH to determine the [H+], While there are still other variations possible, we will select the second one and solve the problem ... 

Candida albicans cells were prepared and used immediately to determine the minimum inhibitory concentration (MIC). RPMI-1640 was used as medium with L-glutamine buffered to pH 7 with 0.15 M 3-(Ar-morpholino)propane sulphonic acid solution. Candida albicans cells, 1.5 x 103 cells/ml, were added to the wells of a 96-well plate containing RPMI-1640 and treated with selected experimental... 

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