The pOH Scale

The pOH scale is analogous to the pH scale, but is defined with respect to [OH ] instead of [H30 ]. 

To find the [OH ] from pOH, we must undo the log function. Use either Method 1 or Method 2. 

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A little trick that you should learn early on (especially if your calculator lacks a log button) is that when the coefficient in your concentration of hydronium ions is 1, as in 1 x 10 8, the pH is equal to the absolute value of the exponent. The pOH scale is used in a similar way to indicate the concentration of hydroxide ions in a solution. A concentration of hydroxide ions of 1 x 103 would correspond to a 3 on the pOH scale. The following table summarizes the method of converting the concentrations of hydronium ions and hydroxide ions to the pH and pOH scales respectively. 

We define pH as a measure of acidity and also introduce the pOH scale. We see that the acidity of a solution depends on the relative concentrations of H and OH ions. (15.3)... 

The pOH scale can also he used to describe acidic and alkaline solutions. 

Equation 16.6 provides another way to express the relationship between the hydronium ion concentration and the hydroxide ion concentration. On the pOH scale, 7.(X) is neutral, numbers greater than 7.00 indicate that a solution is acidic, and numbers less than 7.(X) indicate that a solution is basic. Table 16.5 lists pOH values for a range of hydroxide ion concentrations at 25°C. 

Think About It Remember that the pOH scale is, in essence, the reverse of the pH scale. On the pOH scale, numbers below 7 indicate a basic solution, whereas numbers above 7 indicate an acidic solution. The pOH benchmarks (abbreviated in Table 16.5) work the same way the pH benchmarks do. In part (a), for example, a hydroxide ion concentration between 1 X 10 M and 1 X 10 M con-esponds to a pOH between 4 and 5 ... 

The pOH scale is similar to the pH scale except that pOH is associated with the [OH ] of an aqueous solution. 

Because the pH and pOH scales are based on logarithms, a change in 1 pH or pOH unit represents a change in ion concentration of a factor of ten. Coffee with a pH of 5 has approximately 100 times the hydronium ion concentration as tap water with a pH of 7. 

The pH scale in water is widely used as a measure of acid-base properties in aqueous solutions. It is defined by pH=-log a(H+). In Section 3.1, we dealt with the poH value, defined by poH=-log a(H+), for solutions in amphiprotic and aprotic solvents of high permittivity. Recently, however, the symbol pH has also been used for the value of -log o(H+) in such non-aqueous solutions. Therefore, hereafter, the symbol pH is used instead of paH-... 

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. 

At 298 K, a solution having a pOH less than 7.0 is basic a solution having a pOH of 7.0 is neutral and a solution having a pOH greater than 7.0 is acidic. As with the pH scale, a change of one pOH unit expresses a tenfold change in ion concentration. For example, a solution with a pOH of 2.0 has 100 times the hydroxide ion concentration of a solution with a pOH of 4.0. 

The poH value fails completely to indicate the increased acidity (proton availability) that accompanies the replacement of water by a solvent of lower dielectric constant and probably somewhat lower basicity. Nevertheless, the paH scale can be used to measure acidity in many partially aqueous media. 

The pH and pOH scales are shorthand notations, in a sense. Instead of writing that the concentration of hydronium ions in a particular solution is 0.00001 M (or, 1 X 10 5 M, in scientific notation), we simply say that the pH of the solution is 5. How do we convert between the concentration of the ions and the pH or pOH scales We simply use the following formulas. 

The pH and pOH scales provide a convenient way to express the acidity and basicity of dilute aqueous solutions. The pH and pOH of a solution are defined as... 

To develop familiarity with the pH and pOH scales, consider a series of solutions in which [H3O+] varies from 10 M to 1.0 X 10-1 M. Obviously, [OH-] will vary from 1.0 X 10-1 M to 10 M in these solutions. Table 18-3 summarizes these scales. 

Just as the way the pH has been defined, similarly, the activity of OH" ions is expressed in pOH scale defined as... 

A pOH scale analogous to the pH scale can be devised using the negative logarithm of the hydroxide ion concentration of a solution. Thns we define pOH as... 

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