Determining the Acidity of a Solution

The acidity of a solution has pronounced effects on many chemical reactions. It is therefore important to be able to learn and control the hydrogen ion concentration. This control is obtained through application of the Equilibrium Law. Common types of calculation, based on this law, are those needed to determine KA from experimental data and those using KA to find [H+], We will illustrate both of these types, using benzoic acid, QH6COOH, as an example. 

A common laboratory technique for determining the concentration of a solute is titration (Fig. L.2). Titrations are usually either acid-base titrations, in which an acid reacts with a base, or redox titrations, in which the reaction is between a reducing agent and an oxidizing agent. Titrations are widely used to monitor water purity and blood composition and for quality control in the food industry. 

A simple, reliable, and fast method of determining the pH of a solution and of monitoring a titration is with a pH meter, which uses a special electrode to measure H 0+ concentration. An automatic titrator monitors the pH of the analyte solution continuously. It detects the stoichiometric point by responding to the characteristic rapid change in pH (Fig. 11.9). Another common technique is to use an indicator to detect the stoichiometric point. An acid-base indicator is a water-soluble organic dye with a color that depends on the pH. The sudden change in pH... 

The low concentration of an indicator also explains why the presence of this weak acid does not change the pH of the solution. Indicators are always present as minor species in solution, never as major species. Thus, the dominant equilibrium that determines the pH of a solution never involves the indicator. The of the substance being titrated establishes the equilibrium concentration of hydronium ions. This, in turn, establishes whether the... 

Two questions should immediately arise, (a) How can you know exactly when to stop the titration so that the number of moles of NaOH is equal to the number of moles of HCI (b) What is the use of determining the concentration of a solution of NaOH when the NaOH has now been used up reacting with the HCI (a) An indicator is used to tell us when to stop the titration. Typically an indicator is a compound that is one color (or colorless) in an acidic solution and a second color in a basic solution. Thus, we add NaOH slowly, drop by drop toward the end, until a permanent color change takes place. At that point, the end point has been reached, and the titration is complete. The HCI has just been used up. (b) The purpose of doing a titration is to determine the concentration of a solution. If the concentration of a liter of NaOH is to be determined, a small portion of it is used in the titration. The rest has the same concentration, and although the part used in the titration is no longer useful, the concentration of the bulk of the solution is now known. 

The acidity of a solution is determined by the hydronium ion concentration of the solution. The greater [H,0+], the more acidic the solution the lower [H,0+], the more basic the solution. Other substances, for example, OH, affect the acidity of a solution by affecting the concentration of H, 0+. The presence of OH in water in greater concentration than H30+ makes the solution basic. If the relative concentrations are reversed, the solution is acidic. 

The activities a, of dilute solutions are simply the concentrations of the solutes and the equilibrium constant can be used to determine the pH of a solution when a known amount of acid is dissolved in water. The proton concentration and hence pH is given by the solution of the general quadratic ... 

The acidity of an aqueous solution is determined by the concentration of HsO ions. Thus, the pH of a solution indicates the concentration of hydrogen ions in the solution. The concentration of hydrogen ions can be indicated as [H ] or its solvated form in water as [H3O ]. Because the [HsO ] in an aqueous solution is typically quite small, chemists have found an equivalent way to express [H30 ] as a positive number whose value normally lies between 0 and 14. The lower the pH, the more acidic is the solution. The pH of a solution can be changed simply by adding acid or base to the solution. Do not confuse pH with pK. The pH scale is used to describe the acidity of a solution. The pK is characteristic of a particular compound, and it tells how readily the compound gives up a proton. 

Hydrolysis of Ortho- and Pyrophosphoric Acid Salts. Use a universal indicator to determine the pH of a solution of mono-, di-, and trisubstituted sodium salts of orthophosphoric acid, and also of di- and tetrasubstituted sodium salts of pyrophosphoric acid. Write the equations of the reactions and explain why a different medium is obtained when the given salts are dissolved in water. 

Equation (4-11) illustrates the role of solvent basicity in determining the strength of a solute acid. Equation (4-11) is, however, of little direct use in analytical acid-base measurements because we have no methods of evaluating the absolute constants acidity basicity suppose we wish to Compare the ionization constants of two acids HA and HA" in a solvent SH. From (4-11)... 

Stoichiometry provides the basis for a procedure called titration, which is used to determine the concentrations of acidic and basic solutions. Titration is a method for determining the concentration of a solution by reacting a known volume of the solution with a solution of known concentration. If you wished to find the concentration of an acid solution, you would titrate the acid solution with a solution of a base of known concentration. You also could titrate a base of unknown concentration with an acid of known concentration. How is an acid-base titration carried out Figure 19-16 illustrates the equipment used for the following titration procedure using a pH meter. 

Titration is the process in which an acid-base neutralization reaction is used to determine the concentration of a solution of unknown concentration. 

The tendency of a solvent to accept or donate protons determines the strength of a solute acid or base dissolved in it. For example, perchloric and hydrochloric acids are strong acids in water. If anhydrous acetic acid, a weaker proton acceptor than water, is substituted as the solvent, neither of these acids undergoes complete dissociation instead, equilibria such as the following are established ... 

Because they are different colors at different pHs, indicators can help you determine the pH of a solution. Some indicators, such as litmus, are soaked into paper strips. When litmus paper is placed in an acidic solution, it turns red. When placed in a basic solution, litmus paper turns blue. Some indicators can change through a wide range of colors, with each different color appearing at a different pH value. 

Mixtures of various dyes have been used often to permit a preliminary determination of pH. The components of such mixtures are chosen so that different color tints appear over a wide range of pH (3 to 11), making it possible to obtain an approximate value for the acidity of a solution. These universal indicators are not intended for accurate pH determinations because the color difference attending a given pH increment is much less than for single unmixed indicators. The author himself has never considered such universal indicators very important because pH... 

A convenient system for studying substituent effects is the equilibrium between meta- and />ara-substituted benzoic acids and their corresponding anions (reaction 3.5). The acids are straightforward to synthesize, and the acidities in water at 25 °C are readily determined the pH of a solution containing equal molar quantities of the acid and its fully ionized sodium salt will be equal to — log A, where KA is the dissociation constant of the acid. Ortho substituents are not considered because of complications caused by steric effects. 

In general terms, alkalinity is the Acid Neutralizing Capacity of a solution, that is, the quantity of acid required to neutralize the solution. The acidity is similarly the Base Neutralizing Capacity , the quantity of base required to neutralize the acidity of a solution. Alkalinity and acidity are determined by titrating a sample of solution with an acid (such as HC1) or a base (such as NaOH) of known concentration. However, the variety of ways in which these simple concepts can be defined and interpreted has led to much confusion. Several modeling programs now do not allow input of acidity or alkalinity, as such, partly because of this confusion. However, others do, and in any case users will still have to deal with these concepts if they appear in their analyses. 

Having discussed buffer solutions, we can now look in more detail at the quantitative aspects of acid-base titrations, which we discussed briefly in Section 4.6. Recall that titration is a procedure for determining the concentration of a solution using another... 

After adding a base, colorimetric determination is performed of the concentrations of the base B and the conjugate acid BH+ in the studied solution. The concentrations found allow one to estimate the acidity of a solution according to the following equation ... 

The method used most often for determining the molarity of a solution (as well as numbers of moles of solid reagents) is titration. For example, a solution of HCl, whose concentration is known, and a solution of NaOH, whose concentration is to be determined, are titrated. The NaOH solution is added to a carefully measured volume of HCl solution, the addition being stopped when exactly the correct stoichiometric quantity has been added according to the balanced chemical equation, and the volume of the NaOH solution is carefully measured. An indicator, a chemical that changes color at the point where the proper quantity of one chemical has been added to the other, signals the end of the titration. An acid-base indicator is a compound that has a very intense color in acidic or basic solution, or a different intense color in each. The end point is a point in the titration at the point where the ratio of moles of the reactants added is the same as that ratio in the balanced chemical equation. 

SAMPLE PROBLEM 18.9 Determining the pH of a Solution of A Problem Sodium acetate (CH3COONa, or NaAc for this problem) has applications in photographic development and textile dyeing. What is the pH of 0.25 M NaAc of acetic acid (HAc) is 1.8X10 ... 

The traditional way of determining the pH of a solution is by using indicators—substances that exhibit different colors in acidic and basic solutions. We can represent a generic indicator as a weak acid HIn. Let s assume that for a particular indicator HIn is red and the anion In is blue. 

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