To achieve our goal of understanding how acids and bases work, we must first define what acids and bases are. There are three distinct conceptions of acids and bases that will be considered in this SparkNote--the Arrhenius model, the Bronsted-Lowry model, and the Lewis model. Each of these models describe an acid-base reaction as a process by which a transfer occurs between two partner reagents, the acid and the base. For our purposes, the most useful model is the Bronsted-Lowry model, because we will be considering reactions involving proton transfers. Bronsted and Lowry described acids as proton donors and bases as proton acceptors. Calculations and measurements of pH are relevant to the Bronsted-Lowry conception of acid-base reactions. We will discuss each of the three models of acid-base reactions using representative equations.
To speak of acid or base strength, we need scales for acidity and basicity. pH and pOH scales are quantitative representations of these values for acidic and basic solutions. Next, we will define the acid dissociation constant, Ka, and the base dissociation constant, Kb, to quantify the strengths of particular acids and bases. These terms allow us to process acid and base strength mathematically and package them into values that we can gage conceptually. Using reference values, we can then see that a solution with pH 6 is weakly acidic, since it is slightly lower in pH than water at pH 7.