All pH values less than 7 indicate acidic solutions, and all values greater than 7 indicate basic solutions. Aneutral solution has a pHof7 since the H and OH on concentrations are equal. It should also be emphasized that each pH unit on the scale means a tenfold increase or decrease in H ion concentration from the previous number. Thus, a pH range of 0 to 14, for example, represents a range of concentration of H (or OH ions from 1 to 10-14 tremendous range Finally, the pH scale is not restricted to whole numbers. Calculation offractional pH values requires the use of logarithms. Some fractional pH values and the corresponding concentra- tions of H+ or OH- ions are shown in table 12.2. Indicators Acid-base indicators are compounds which exhibit one color in an acid solution and a different color in a more basic solution. Not all indicators change color at the same pH. Some indicators change colors at pH 7, others at pH 4, pH 5, or pH 6, and some at pH 8, pH 9, or pH 10. The color shift of an indicator, from its "acidic color" to its "basic color," requires about 2 pH units, or 1 full pH unit on either side of the midpoint. Indicators are useful for telling us when solu- TABLE 12.2 Some Fractional pH Values