Question

The \(K_{\mathrm{a} 1}\) of ascorbic acid is \(7.94 \times 10^{-5}\). Would you expect ascorbic acid dissolved in blood plasma (pH 7.35-7.45) to exist primarily as ascorbic acid or as ascorbate anion? Explain.

Short answer

Answer: Ascorbic acid dissolved in blood plasma primarily exists as the ascorbate anion.

Step-by-step solution

Calculate the pKa value of ascorbic acid

To find the pKa value, we can use the equation: pKa = -log(Ka) We are given the Ka value of ascorbic acid as \(7.94 \times 10^{-5}\). Plugging this value into the equation, we get: pKa = -log(\(7.94 \times 10^{-5}\))

Calculate the pKa value

Using a calculator to find the value of pKa, we get: pKa ≈ 4.10

Compare pH and pKa values

Now we have the pKa value of ascorbic acid (4.10) and the pH range of blood plasma (7.35-7.45). If the pH is greater than the pKa value, the ascorbic acid will primarily exist as its conjugate base, ascorbate anion (A-). If the pH is less than the pKa, the ascorbic acid will primarily exist in its acidic form (HA). In this case, the pH values (7.35-7.45) are greater than the pKa value (4.10). Therefore, we can conclude the following:

Conclusion

As the pH of the blood plasma (7.35-7.45) is greater than the pKa value of ascorbic acid (4.10), ascorbic acid dissolved in blood plasma would primarily exist as the ascorbate anion.