Hydrolysis

A good buffer mixture will have about equal concentrations of the acid or base plus the corresponding salt. When about 90 percent of the buffer pair has been used up, the buffer becomes much less effective. For example, with acetic acid buffer systems, the initial pH will be about 4.75. When there is a situation in which the acetic acid concentration is about 10 percent of the acetate concentration, the pH will change by 1 pH unit. Past that, the pH will increase dramatically. This means that it will work until 90 percent of the mixture is sodium acetate and not acetic acid.

When the pH of the system is less than 7, weak acids and salts are better buffers. When the pH of the system is greater than 7, weak bases and their salts are better buffers. In blood systems, when the pH is around seven, the carbonic acid (H2CO3) and bicarbonate ion (HCO3-) system is prominent. In such cases, the pH is about 7.4 with pH changes greater than 0.4 being potentially fatal.

HYDROLYSIS

Remember that a salt is made when an acid and base are mixed together and when hydroxide ions and hydrogen ions mix to form water. The bystander in all of this is the salt ions and salt solution. When weak acids and weak bases react, the relative strength of the conjugated acid-base pair determines the pH of the solution.

A salt solution can be neutral, acidic, or basic by itself because of the nature of its ions. In other words, there are neutral salt ions, acidic ions, and basic ions. This is in keeping with the idea that, according to the Lewis theory, molecules not directly having H+ or OH- ions will have acidic or basic potential because they can donate or need electron pairs.

Neutral cations include sodium, potassium, rubidium, cesium, magnesium, strontium, barium, and calcium. Neutral anions include chloride, bromide, iodide, and perchlorate. Acidic cations include ammonium, lead (II), and aluminum (III). Acidic anions include HSO4- and H2PO4. Basic anions include (HPO2)4, (PO3)4, F. NO2, CN, and HCO3. There are no basic cations.

Any salt made from a strong acid and a weak base will be an acidic salt. An example of this is ammonium chloride (made from HCl and NH4OH). The ammonium chloride

leaves behind an acid cation in the form of NH4+ that itself interacts with water in an activity called “hydrolysis”, which leads to ammonia plus H3O+.

A basic salt is formed between a weak acid and a strong base. An example of this is sodium acetate, which is made from the weak acid acetic acid and the strong base sodium hydroxide. Acetate is itself a basic anion that undergoes hydrolysis to make acetic acid plus hydroxide ion (and leads to a basic situation).

As mentioned before, the salt of a weak acid and a weak base might be neutral, acidic, or basic, depending on the K constant. If the K constant of the acidic cation is greater than that of the basic anion, this will be an acidic salt. If the K constant of the acidic cation is less than that of the basic anion, the salt will be a basic salt. If the K constant of the acid is the same as the K constant of the base, the resultant salt will be a neutral salt.