What is a pH test?

a pH test is a test for acidity To first answer this question, I'd like to explain what pH is. The pH is a number, by convention from 0-14, that describes the acid-base properties of some solution. The lower the number, the more acidic the substance, while the higher the number, the more basic. The terms "acidic" and "basic" are chemically relative here, as certain substances (water included) can be both acids and bases (in fact, pure water, by definition, has a pH of 7, which is considered neutral). The pH is derived by first taking the log to the base of 10 of the concentration of Hydrogen ions, H+, that is: log [concentration of H+] This will give a negative value because this concentration is usually miniscule. Since we want a positive value, we multiply by the negative sign, or: -log [concentration of H+] There is also a value called the pOH, which is based on the concentration of hydroxide ion, OH-, and is derived in an analogous way, but the scale is reversed, where small values of pOH signify lessacidity and more basicity. In practice, we use either an electronic device or a liquid indicator which changes color to indicate a certain pH value. Now it is important to understand one major aspect of the pH scale. Because we are taking the logarithm of the concentration of H+ ions, and then mutiplying it by the negative sign, when we go from a value of, say, 1 to 2 on the pH scale, the concentration of H+ decreases by a factor of 10. Conversely, if we go from a value of 2 to 1, the concentration of H+ increases by a factor of 10. And if we go from 3 to 1 on the pH scale, we actually have an increase in H+ (that is, acidity), of 100. The simple rule is to find the integer difference between the larger and smaller pH value, and then raise 10 to that power. This is the factor by which the smaller value is more acidic than the larger one. Again, keep in mind here that the smaller the pH the more acidic, but that this value increases by 10-fold from each value going down. The human blood is slightly basic, or about 7.4 on the pH scale. However, the stomach is usually very acidic (about a pH of 2) while the small intestine is slightly more basic (>7). Most natural organisms contain buffers, which are pairs of weak conjugate acid/bases that act to "buffer" or minimize the change in pH whenever a base or acid is added. However, all buffers have a pH range in which they effectively buffer; outside that range (either too acidic or too basic) they are not as useful. Acids (pH lower than 7) and bases (pH over 7) must be handled very carefully. To reduce acidity or basicity, one must reduce the H+ or OH- ion concentration, which is usually done by adding copious amounts of water. More water, less concentration, and therefore less acidity/basicity. People can also ingest some sort of buffer or "antacid", which is actually a weak base that "neutralizes" the acid.