Question

Phosphate potential \((pK_{sp})\) = ?

• A. \(pK_{sp} = pCa + 1/2 pH_2PO_4\)
• B. \(pK_{sp} = 1/2 pCa + pH_2PO_4\)
• C. \(pK_{sp} = pCa + pH_2PO_4\)
• D. \(pK_{sp} = pCa + 1/4 pH_2PO_4\)

Hint:

Remember that the calcium ion (\(\text{Ca}^{2+}\)) carries a \(2+\) charge, requiring a factor of \(\frac{1}{2}\) to balance the monovalent \(\text{H}_2\text{PO}_4^-\) ion in the potential expression:
\[ \frac{1}{2}\text{pCa} + \text{pH}_2\text{PO}_4 \]

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Phosphate potential is an index used in soil chemistry to measure the activity or availability of orthophosphate ions in the soil solution.
It is derived from the solubility product of monocalcium phosphate, which is the primary form of soluble phosphorus absorbed by plants in neutral to acidic soils.

Step 2: Detailed Explanation:
The solubility of monocalcium phosphate, \(\text{Ca(H}_2\text{PO}_4)_2\), is governed by the concentrations of calcium ions (\(\text{Ca}^{2+}\)) and dihydrogen phosphate ions (\(\text{H}_2\text{PO}_4^-\)) in the soil solution.
According to Schofield's phosphate potential concept, the activity of monocalcium phosphate can be expressed as:
\[ \text{Phosphate Potential} = \frac{1}{2} \text{pCa} + \text{pH}_2\text{PO}_4 \]
Where:
- \(\text{pCa}\) is the negative logarithm of the calcium ion activity:
\[ \text{pCa} = -\log[\text{Ca}^{2+}] \]
- \(\text{pH}_2\text{PO}_4\) is the negative logarithm of the dihydrogen phosphate ion activity:
\[ \text{pH}_2\text{PO}_4 = -\log[\text{H}_2\text{PO}_4^-] \]
This term combines the calcium and phosphate activities to provide a index of phosphorus availability that is independent of slight variations in soil-to-water ratios.
This corresponds to Option B.

Step 3: Final Answer:
The phosphate potential is defined as \(pK_{sp} = 1/2 pCa + pH_2PO_4\).