The alkalinity due to bicarbonate ion when P < 1/2M will be ____________a.M-2Pb.2(M-P)c.2P-Md.Nil

To determine the alkalinity due to the bicarbonate ion when $P < \frac{1}{2}M$, we need to analyze how the balance between bicarbonate ion concentration and the parameters $M$ and $P$ interacts.

Breakdown of the Problem

1. Define the terms:

   • $M$: Total alkalinity or total concentration of bicarbonate and carbonate ions.
   • $P$: A parameter that can represent a specific concentration in relation to bicarbonate ions.

2. Focus on the condition $P < \frac{1}{2}M$.

Relevant Concepts

1. Alkalinity contributions from bicarbonate ions primarily depend on their concentration relative to $M$ and $P$.
2. When the concentration of bicarbonate ions is affected by the parameter $P$, we must interpret the options provided in the context of $P$ being lower than half of $M$.

Analysis and Detail

1. When $P < \frac{1}{2}M$, the bicarbonate ions will dominate the alkalinity calculations. In situations where $P$ is low compared to $M$, we can derive that:

   • The concentration of the bicarbonate ion will depend on the total alkalinity $M$ and the lesser influence of $P$.

2. Evaluating the options:

   • a. $M - 2P$: This doesn’t fit as it suggests a reduction that isn’t applicable.
   • b. $2(M - P)$: Suggests more contribution which might be complex under the limits given.
   • c. $2P - M$: This indicates an imbalance since $P$ is small.
   • d. Nil: Likely indicates no contribution under the specificity that $P$ is significantly less than $M$.

Verify and Summarize

Given the analysis of the options with emphasis on $P < \frac{1}{2}M$, the concentration of bicarbonate does not reach a level to contribute positively to the alkalinity.

Final Answer

Thus, the alkalinity due to bicarbonate ion when $P < \frac{1}{2}M$ will be d. Nil.