The pH scale, numbered from 0 to 14, describes the relative acidity or alkalinity of chemicals, with the middle of the scale, pH 7, being neutral (pure water). Substances from pH 0 to pH 7 are acids; those from pH 7 to pH 14 are alkaline. The scale is logarithmic; for example, a chemical with a pH of 5 is 100 times less acidic than one at pH 3, but it’s 10 times more acidic than a substance with a pH of 6. The same goes for the alkaline end of the scale.
Cleaning surfaces with liquids is really a neutralization process — the balancing of acids with alkalies, and vice versa.
Oil, grease, and many bodily wastes — most of which are, or contain, fatty acids — are best cleaned up with alkaline solutions such as detergents or commercial all-purpose cleaners. Lye, a common name for sodium hydroxide, at the extreme end of the scale at about pH 13, is used for heavy-duty degreasing tasks such as oven and drain cleaning.
Acids such as white vinegar, lemon juice, and phosphoric acid are best for cleaning alkaline stains or deposits such as those from alcoholic beverages, coffee, tea or mineral scale from hard water. Sulfuric and hydrofluoric acids, at the low end of the scale at pH 1-2, are useful for rust removal or etching surfaces.
Always wear skin, eye and respiratory protection when you must work with harsh cleaning agents at the extreme ends of the scale. Most of these chemicals can burn skin or eyes quickly, and their vapors can cause permanent damage to nasal passages and lungs. Store these products out of the reach of children. Always read and follow label directions carefully for safety and best results. Never mix any chemicals together in an attempt to boost cleaning power.