The sensitivity of a spectrophotometer allows detection of 1 percent of one colo

Question

The sensitivity of a spectrophotometer allows detection of 1 percent of one colored form of an indicator (for example H In) in the presence of 99 percent of the other colored form (In). Based on this fact, over what pH range relative to its pK can an indicator be used if a spectrophotometer is employed? A major buffering system in the blood is H_2CO_3/HCO_3. At first glance this would appear to be a poor choice. At the normal blood pH (7.4) a solution of H_2CO_3 and HCO_3 will change pH abruptly if small amounts of acid or base are added since the pK_a of H_2CO_3 is 6A. Offer an explanation for this apparent contradiction. Support this argument with appropriate chemical equations.

Explanation / Answer

3. When the sensitivity of a spectrophoometer allows detection of 1 percent of one colored forms of an indicator, in presence of 99 percent of the other colored form of the same indicator, the pH range applicable for the indicator relative to the pK of the indicator would be in the range of +/-2.0 units.

4. Buffering system for blood is carbonate buffer of composition, H2CO3/HCO3-.

When an acid is added,

HCO3- + H+ <==> H2CO3

and, when a base is added,

H2CO3 + OH- <==> HCO3- + H2O

Thus, the pH is well maintained by the buffer system on addition of an acid or a base externally.

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