H and pOH

The pH values produced by 1 mol dm-3 solutions of HC1 and NaOH, i.e. 0 and 14 respectively, define the practical range in which activity coefficients of H + (aq) and OH-(aq) may be ignored for general purposes, and subsidiary definitions of pH and pOH may be used pH = —logi0[H+] and pOH = —logi0[OH-]. The square brackets indicate the molar concentration of the species enclosed as a ratio to the standard molar concentration of I mol dm-3. 

Find the pH and pOH of solutions with die following [H+]. Classify each as acidic or basic. 

Figure 2.1 will serve as a useful mnemonic for the relation between [ H + ], pH, [OH-], and pOH in acid and alkaline solution. 

Using these definitions, the pH and pOH of a neutral solution at 25 °C are both equal to 7. We can see from the expression for that [H+] and [OH ] are inversely related, and consequentially pH and pOH are inversely related. We can picture pH and pOH as sitting on opposite sides of a seesaw, as one goes up, the other always goes down. The product of the hydrogen and hydroxide concentrations will be equal to 1.0 X 10 ", while the sum of the pH and pOH will be equal to 14. In an acidic solution, the hydrogen ion concentration increases above 1.0 X 10 , the hydroxide concentration decreases, and the pH value gets smaller. The relationship between the type of solution, pH, pOH, and ion concentrations is shown in Table 13.6. The pHs of a number of common substances are presented in Table 13.7. 

Before going on to the next step, we need to look at a new variable. So far, you ve learned about pH and pOH. Remember, these were obtained by taking the —log of the [H+] and [OH-], respectively. Here, we re going to introduce two new terms pKa, and pKb. These terms are used to describe the Ka and Kb in small, positive numbers, rather than very small exponents (much like the pH values of 0-14 represent the small [H+] ion amounts of 1.0 X 10-14). pKa is calculated as the -log Ka, and pKb is the -log Kb. You will need these terms to understand the rest of the derivation, which we ll continue now. 

C) We can solve this two different ways. To start either way we use the value for the molar solubility to determine the [OH-]. Once [OH-] is known, we convert it to [H+] and solve for pH. The other way is to calculate pOH and convert this to pH. Both methods are shown below. 

Thus, if any one of the values [H ], [OH ], pH, or pOH is known, the other three can be calculated easily. At concentrations of H and OH" greater than 0.1 M, the activity coefficients must be taken into account. 

Using pOH Sometimes chemists find it convenient to express the basicity, or alkalinity, of a solution on a pOH scale that mirrors the relationship between pH and [H+]. The pOH of a solution is the negative logarithm of the hydroxide ion concentration. 

Figure 19-12 illustrates the relationship between pH and [H+] and the relationship between pOH and [OH ] at 298 K. Use this diagram as a reference until you become thoroughly familiar with these relationships. 

Study this diagram to sharpen your understanding of pH and pOH. Note that at each vertical position, the sum of pH (above the arrow) and pOH (below the arrow) equals 14. Also note that at every position the product of [H+] and [OH ] equals 10". ... 

You have been given the pH of a solution and must calculate [H+] and [OH ]. You can obtain [H+] using the equation that defines pH. Then, subtract the pH from 14.00 to obtain pOH. The pH is close to 7 but greater than 7, so [H+] should be slightly less than 10 and [OH ] should be greater than 10 . ... 

Wong, I.H. and T.Y. Poh, Effects of Jet Grouting on Adjacent Ground and Structures, Journal of Geotechnical and Environmental Engineering, March 2000, pp 247-256, ASCE, Reston, VA. 

Methyl orange. Similar studies were made with the sodium salt of dimethylamino-azo-benzene sulfonic acid. The transformation started after the addition of 0.5-0.6 c.c. of 0.1 N HC1.[H+] = 2.2 X 10-4 or pH 3.66, and pOH of 8.54. The dissociation of this indicator too increases with temperature. 

Tropeolin 00. 45 c.c. of water was boiled with three drops of 0.1% tropeolin solution, and titrated with 0.1 N hydrochloric acid. The interval began after 5 c.c. of acid had been added, or at [H+] = 10-, pH = 2, and pOH = 10.2. The end of the region was too difficult to observe. The dissociation constant of tropeolin 00 also increases with temperature since, at 18°, the initial color appears at a pH of 3.1. 

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