Why pH scale limited to a fixed range

Asked by  | 24th Mar, 2008, 09:18: PM

Expert Answer:

pH is the measure of the acidity or basicity of a solution. It is formally a measure of the activity of dissolved hydrogen ions (H+), but for very dilute solutions, the molarity (molar concentration) of H+ may be used as a substitute with little loss of accuracy.[1] In solution, hydrogen ions occur as a number of cations including hydronium ions (H3O+).[2]

In pure water at 25°C, the concentration of H+ equals the concentration of hydroxide ions (OH-). This is defined as "neutral" and corresponds to a pH level of 7.0. Solutions in which the concentration of H+ exceeds that of OH- have a pH value lower than 7.0 and are known as acids. Solutions in which OH- exceeds H+ have a pH value greater than 7.0 known as bases. Because pH is dependent on ionic activity, a property which cannot be measured easily or fully predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways to do so.Neutral pH at 25 °C is not exactly 7. pH is an experimental value, so it has an associated error. Since the dissociation constant of water is (1.011 ± 0.005) × 10−14, pH of water at 25 °C would be 6.998 ± 0.001. The value is consistent, however, with neutral pH being 7.00 to two significant figures, which is near enough for most people to assume that it is exactly 7. The pH of water gets smaller with higher temperatures. For example, at 50 °C, pH of water is 6.55 ± 0.01. This means that a diluted solution is neutral when its pH at 50 °C is around Neutral pH at 25 °C is not exactly 7. pH is an experimental value, so it has an associated error.

Answered by  | 6th Jun, 2008, 06:20: AM

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