Question

At physiological \(\mathrm{pH}\), which two forms of phosphoric acid have the highest concentrations? a. \(\mathrm{H}_{3} \mathrm{PO}_{4}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) b. \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) and \(\mathrm{HPO}_{4}^{2-}\) c. \(\mathrm{HPO}_{4}^{2-}\) and \(\mathrm{PO}_{4}^{3-}\) d. \(\mathrm{PO}_{4}^{3-}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\)

Short answer

b. H_2PO_4^{-}and HPO_4^{2-}

Step-by-step solution

- Understanding Physiological pH

Physiological pH typically refers to the pH level of human blood, which is approximately 7.4. Understanding this context is crucial because the forms of phosphoric acid present will depend on the pH.

- Determine pKa Values of Phosphoric Acid

Phosphoric acid (H_3PO_4) has three pKa values: {pKa_1} = 2.15, {pKa_2} = 7.20, and pKa_3} = 12.38. At a physiological pH of 7.4, compare this pH to these pKa values to see which forms will dominate.

- Analyze Forms Present Around pKa Values

When pH is close to a pKa value, the two forms around that pKa will be present in comparable amounts. At pH 7.4, which is close to pKa_2 (7.20), the forms H_2PO_4^{-}and HPO_4^{2-} will have the highest concentrations.

- Deciding the Dominant Forms

Since pH 7.4 is nearer to pKa_2, H_2PO_4^{-}and HPO_4^{2-} will be the dominant species. Therefore, the correct answer is (b).

Key concepts

pH Levels

The concept of pH is essential in understanding many chemical and biological systems. pH measures the acidity or basicity of a solution.
It is defined as the negative logarithm of the hydrogen ion concentration, represented as: \(\text{pH} = -\text{log}[H^+]\text{.}\) The pH scale ranges from 0 to 14, where 7 is neutral, values below 7 are acidic, and values above 7 are basic.
Physiological pH typically refers to the pH of human blood, which is tightly regulated around 7.4. This slight alkaline pH is crucial for various bodily functions, including enzyme activity and oxygen transport.
Understanding pH levels helps us predict the behavior of different species in solution, such as phosphoric acid forms at physiological conditions.

pKa Values

In acid-base chemistry, the pKa value is a critical concept to grasp. pKa is the negative logarithm of the acid dissociation constant (Ka), giving insight into the strength of an acid. A lower pKa value indicates a stronger acid, which dissociates more readily in solution. For phosphoric acid \((H_3PO_4)\), there are three pKa values: \(\text{pKa}_1 = 2.15, \text{pKa}_2 = 7.20,\) and \(\text{pKa}_3 = 12.38\text{.}\) These values correspond to the sequential loss of hydrogen ions:
• \(\text{H}_3\text{PO}_4 \rightleftharpoons H_2\text{PO}_4^- + H^+\)
• \(\text{H}_2\text{PO}_4^- \rightleftharpoons \text{HPO}_4^{2-} + H^+\)
• \(\text{HPO}_4^{2-} \rightleftharpoons \text{PO}_4^{3-} + H^+\) Comparing the pH of a solution to these pKa values helps us determine the predominant forms of the acid-present.

Phosphoric Acid Species

Phosphoric acid (\(\text{H}_3\text{PO}_4\)) can exist in several forms, depending on the pH of the environment. These species include:
• \(\text{H}_3\text{PO}_4\) (phosphoric acid)
• \(\text{H}_2\text{PO}_4^-\) (dihydrogen phosphate)
• \(\text{HPO}_4^{2-}\) (hydrogen phosphate)
• \(\text{PO}_4^{3-}\) (phosphate)
The relative concentrations of these species can be predicted using their respective pKa values. For instance, at a physiological pH of 7.4, which is close to \(\text{pKa}_2\) (7.20), the forms \(\text{H}_2\text{PO}_4^-\) and \(\text{HPO}_4^{2-}\) will dominate. This insight is crucial for biochemical pathways and buffering systems in the body.

Acid-Base Chemistry

Acid-base chemistry revolves around the principles of acids donating protons (H+) and bases accepting protons. The Brønsted-Lowry definition is widely used, where an acid is a proton donor, and a base is a proton acceptor. Strong acids, like HCl, fully dissociate in water, while weak acids partially dissociate.
Buffer solutions resist changes in pH upon addition of a small amount of acid or base and are vital in biological systems. Phosphate buffers, involving species from phosphoric acid, play a significant role in maintaining the physiological pH.
Understanding acid-base equilibria helps predict the behavior of substances in various pH conditions, as demonstrated with the different species of phosphoric acid.