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pH Scale

Introduction and Definitions:

Acidic and basic are two extremes that describe a chemical property chemicals. Mixing acids and bases can cancel out or neutralize their extreme effects. A substance that is neither acidic nor basic is neutral.

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8.

Pure water is neutral. But when chemicals are mixed with water, the mixture can become either acidic or basic. Examples of acidic substances are vinegar and lemon juice. Lye, milk of magnesia, and ammonia are examples of basic substances.

Practice:

 Ionization of Water: Water molecules exist in equilibrium with hydrogen ions and hydroxide ions. H2O <--> H+ + OH- The water equilibrium constant is written as: Kw = [H+] [OH-] Experimentally, it has been found that the concentration of: H+ = OH- = 10-7 Therefore: Kw = [10-7][ 10-7] = [10-14] (To multiply exponential numbers - simply add the exponents.) The values for Kw, H+, OH- concentration all indicate that the equilibrium favors the reactant (water molecules). In other words, only very small amounts of H+ and OH- ions are present. Effect of Acids and Bases on Water Equilibrium: If an acid (H+) is added to the water, the equilibrium shifts to the left and the OH- ion concentration decreases. Water Equilibrium: H2O <--> H+ + OH- If base ( OH-) is added to water, the equilibrium shifts to left and the H+ concentration decreases. Water Equilibrium Principle: The multiplication product (addition of exponents) of H+ and OH- ion concentration must always be equal to 10-14. BOTH H+ and OH- ions are ALWAYS PRESENT in any solution. A solution is acidic if the H+ are in excess. A solution is basic, if the OH- ions are in excess.

 Number Representation and Logarithms Number Exponent Notation Log of the Number 1000 103 3 100 102 2 10 101 1 1 100 0 0.1 10-1 -1 0.01 10-2 -2 0.001 10-3 -3 0.0001 10-4 -4

Definition of pH, pOH, and pKw:

The concentrations of hydrogen ions and indirectly hydroxide ions are given by a pH number. pH is defined as the negative logarithm of the hydrogen ion concentration. The equation is:

pH = - log [H+]
similarly, pOH = - log [OH-]
and p Kw = - log [Kw] .

Logarithms of numbers that are multiples of ten are merely the exponents of the number including the sign. See the table on the left for a review. The method to find logs of numbers that are not multiples of ten are found by using a calculator. The method is not discussed here.

 Example: If an acid has an H+ concentration of 0.0001 M, find the pH. Solution: First convert the number to exponential notation, find the log, then solve the pH equation. H+ = 0.0001M = 10-4; log of 10-4 = -4; pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH The purpose of the negative sign in the log definition is to give a positive pH value.

 Example: If the base has an OH- concentration of 0.001M, find the pH. Solution: First find the pOH, (similar to finding the pH,) then subtract the pOH from 14. OH- = 0.001M = 10-3; pOH = -log [OH-] = -log (10-3) = +3 = pOH pH = 14 - pOH; pH = 14 - 3 = 11 = pH

pH Principle: pH and pOH must always equal pKw (14).

Click for larger image

pH Scale:

The pH scale, (0 - 14), is the full set of pH numbers which indicate the concentration of H+ and OH-ions in water. The diagram on the left gives some relationships which summarizes much of the previous discussion.

pH Scale Principle:
H
+ ion concentration and pH relate inversely.
OH
- ion concentration and pH relate directly.