Acid-Base Chemistry Lab

The purpose of this lab was to determine the pH and total acidity of grape juice and wine. A second purpose was to experimentally determine the pKa of a monoprotic weak acid, polyprotic weak acid, as well as, a free amino acid, alanine.

Acids and bases were defined by Bronsted-Lowry in 1923, in which acids were described as substances capable of donating a proton and bases are substances capable of accepting a proton (Thompson and Dinh 2009). In addition to defining acids and bases, Bronsted-Lowry further classified acids and bases as being either strong or weak. By definition, a strong acid or base is a substance that is completely dissociated in aqueous solutions and in contrast, weak acids only partially dissociate and reach their equilibrium with their conjugate base (Harris 2003).

Buffers are comprised of weak acids and their conjugate bases. In the food industry, buffers are commonly used to protect changes in pH of food stuffs. The two conjugate components of the buffer resist changes in pH by absorbing the addition of any hydrogen or hydroxyl ions (Christen and Smith 2000). When weak acids and their conjugate bases are at equilibrium, their concentrations can be expressed in terms of the dissociation constant, Ka. For a strong acid, the Ka value is greater than 1 x 10-2M and less than 1 x 10-2M for a weak acid (Thompson and Dinh 2009). For weak acids, the relationship of hydrogen ion concentration and pH can be defined by the Henderson-Hasselbach equation (Thompson and Dinh 2009).

Wine quality greatly depends on the quality of the grapes and the microorganisms involved in the fermentation process (Husnik and others 2006). When compared to lactic acid bacteria, yeasts are better suited for growt...

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Thompson LD, Dinh T. 2009. Acid-Base Chemistry. FDSC 4303/5303 food chemistry laboratory manual. Lubbock, Tx.: Texas Tech University, Department of Animal and Food Sciences.

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