98 Principles of Food Science blue at pH 7.5 and pH 10.0. To get a more accurate reading, you need a pH meter. The meter has a probe that you insert into a solution. Once calibrated, most pH meters will give accurate readings to a tenth of a pH unit. See 6-8. Food scientists often use another method to measure the pH of foods. This method is called titration. It is the process of adding a base with a known pH to an acid. Titration is also the process of adding an acid with a known pH to a base. An indicator is added to the unknown. Using a buret, the base or acid whose pH is known is dispensed into the unknown a drop at a time. Scientists look for a color change to occur. Then they can calculate how many acid molecules had to be present to neutralize all the base molecules. The point at which there are equal numbers of acid and base molecules is called the endpoint. This is also called the equivalence point. The endpoint is where neutralization has occurred. Neutralization is the point at which all ions in a solution have combined chemically. For example, suppose you have a solution with 20 hydronium ions. You would have to add 20 hydroxide ions before neutralization would occur. In neutralization processes, the reactants are an acid and a base. The products are water and a salt. The equation below shows what happens when two common chemical compounds, hydrochloric acid (HCl) and sodium hydroxide (NaOH), are combined. HCl + NaOH H+ + Cl– + Na+ + OH– HOH + Na+Cl– acid + base forms ions in solution water + salt are experimenting with acids and bases. If you splash any strong acid or base on skin, immediately flush the area with running water. This will dilute the acid or base and help prevent damage. Measuring pH Scientists have two methods of measuring pH. The most economical is using organic dyes. These organic dyes are called indicators. An indicator demonstrates through color change the degree of acidity of a solution. The problem with organic dyes is accuracy. Scientists know that litmus is blue at pH 8.0. However, it is also pH of Some Common Foods lemon juice 2.2—2.4 vinegar 2.4—3.4 apples 2.9—3.3 blueberries 3.2—3.4 cheese 4.0—4.5 rye bread 4.3—5.4 molasses 5.0—5.4 white bleached flour 5.7—5.9 Dutch cocoa 6.0—6.4 whole eggs 6.4—9.7 crackers 7.0—8.5 devil’s food cake 7.5—9.0 Source: E.J. Pyler, Baking Science and Technology. Merriam, Kansas Sosland Publishing Company, 1988, p. 251. 6-6 Foods fall at a range of pH levels. The pH Scale pH Concentration of Hydrogen or Hydronium Ions (molecules per liter) Concentration of Hydroxide Ions (molecules per liter) 1 0.1 or 10-1 10-13 2 0.01 or 10-2 10-12 3 0.001 or 10-3 10-11 4 0.0001 or 10-4 10-10 5 0.00001 or 10-5 10-9 6 0.000001 or 10-6 10-8 7 0.0000001 or 10-7 10-7 8 0.00000001 or 10-8 10-6 9 0.000000001 or 10-9 10-5 10 0.0000000001 or 10-10 10-4 11 0.00000000001 or 10-11 10-3 12 0.000000000001 or 10-12 10-2 13 0.0000000000001 or 10-13 10-1 14 0.00000000000001 or 10-14 10-0 6-5 The pH scale is based on the number of hydronium ions present in a solution. Copyright Goodheart-Willcox Co., Inc.