Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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control, analysis, and testing pH AND ORP BY MICHAEL BANHIDI CONSULTANT, LEONIA, N.J. PH The term "pH" is used to express the degree of acidity or alkalinity of a solution. Although many different ions may be formed in solution those that establish whether the solution is acid or alkaline are the hydrogen (H+ ) and the hydroxyl (OH) ions, respectively. When the hydrogen ions exceed the hydroxyl ions in num- ber, the solution is acid. When they are equal, the solution is neutral. Alkalinity (or basicity) denotes that the hydroxyl ions exceed the hydrogen ions. Acids and alkalis vary in the degree to which they form ions in solution. Those that almost completely ionize are called strong acids or bases. Hydrochloric acid and sodium hydroxide are examples. Other acids and bases ionize to only a small degree and are called weak acids or bases. Acetic acid and ammonium hydroxide are typical examples. Acids and bases may also be classified according to the number of H+ ions they can donate per molecule. If an acid can donate only one H+ ion it is known as monofunctional. If the available H+ ions are more than one a di- (two) or tri- (three) classification is used. Even pure water dissociates to a minute degree into hydrogen and hydroxyl ions. Pure water at 25O C always contains 0.0000007 or, in simple form, 1 10-7 gram-equivalents per liter of hydrogen ions and, since pure water is neutral, an equal concentration of hydroxyl ions. The product of these concentrations is a constant equal to 1.0 10-14 Thus, if the hydrogen-ion concentration of a solution were 1.0 10-4 at 25O C. gram- equivalents per liter, the hydroxyl-ion concentration would be equal to 1 10-10 gram-equivalents per liter. Because, in this case, the hydrogen-ion concentration is greater than the hydroxyl-ion concentration, the solution is acidic. To express hydrogen-ion concentration more conveniently than by the use of decimals or by negative exponents, the term "pH" was adopted. Expressed math- ematically, pH is equal to the negative logarithm to the base of 10 of the hydro- gen-ion concentration. This is expressed as follows: hydrogen-ion concentration = 1 10-7 acid solutions contain more than 1 10-7 gram-equivalents of hydrogen ion per liter or has a pH value of 7. Since gram-equivalents of hydrogen ion per liter, the pH values of these solutions are less than 7. Conversely, alkaline solu- tions have pH values greater than 7. It is also helpful to remember that a unit change in pH represents a tenfold change in acidity or alkalinity. Thus, compared with a solution of 5 pH, a solu- tion of 4 pH is ten times as acidic—of 3 pH, a hundred times as acidic—and 2 pH, a thousand times as acidic. The commonly accepted range within which pH values are expressed covers the scale 0 to 14. In plating solutions pH determinations below 1 and above 12 are of little or no value. Outside these limits acidities or alkalinities are more con- veniently expressed as percent concentrations. Nonaqueous solvents may provide a pH outside the 0 to 14 range normally encountered in aqueous solutions. Many digital meters are capable of -19.99 to 505

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