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Strength of an Acid
The strength of an acid is a measure of its tendency to donate a hydrogen ion to a solution. The level of equilibrium—the point at which the acid stops giving up hydrogen ions—determines the strength of the acid. The lower the level of acid in a solution when this happens, the stronger the acid is. The strength of an acid is represented by the value pKa. The lower an acid's pKa, the stronger it is.
Okay, stay with us. Don't worry about pKa too much. When most people talk about acidity, they're talking about pH. While pKa measures the strength of an acid itself, pH measures the amount of acid in a solution. As a result, the pH of vinegar and lemon juice doesn't correspond directly to the strength of the acetic and citric acid in them because the amount of acid in the solution varies. For example, two different types of vinegar both contain acetic acid, but they may contain different amounts. Although the acid in both will have the same strength, the two types of vinegar may differ in their pH. As with pKa, the lower the pH of a solution, the more acidic it is.
Lemon juice contains high levels of citric acid, with around .05 grams per milliliter. Citric acid is a weak acid, with a pKa of 2.79. The exact level of citric acid in lemon juice can fluctuate; but in general, lemon juice is highly acidic, with a pH of 2, because of the high percentage of acid it contains. In some cases, lemon juice can have a slightly higher pH of up to 2.6. One way to demonstrate the low pH—and therefore high acidity—of lemon juice is with a litmus strip or indicator solution. These change color when they come in contact with acids. Lemon juice dropped into an indicator solution or onto a litmus strip causes it to turn bright red, indicating its high acid content.
Vinegar is a solution of acetic acid and water. Acetic acid is much weaker than citric acid, with a pKa of 4.74. The pH of vinegar can vary greatly depending on the specific type or composition. Some vinegar can be nearly as acidic as lemon juice, with a pH of around 2.4, while other types are much more basic, with a pH of over 3. It's important to remember that small increments on the pH scale represent large differences in acidity. A solution with a pH of 2 is ten times as acidic as one with a pH of 3. Like lemon juice, vinegar turns a universal indicator red, but the color is usually somewhat lighter orange-red, indicating its slightly higher pH.