Final answer:
The principle of reagent strips for pH is a double indicator system where the paper contains embedded indicators that change color at certain pH levels, allowing estimation of the solution's pH.
Step-by-step explanation:
The principle of a reagent strip for pH testing is based on the use of a double indicator system. These indicators are selected to exhibit a range of colors that correspond to different pH levels. The typical reagent strip contains embedded indicators, such as a universal indicator, which changes color based on the pH of the solution it comes in contact with. A universal indicator is a mixture of several different indicators resulting in a gradient of color changes over a range of pH values. Substances like phenolphthalein also play a similar role in determining pH, but usually in more controlled experiments such as titrations.
The pH paper, or reagent strip, changes color in response to the concentration of hydrogen ions (H+) in the solution, effectively indicating the pH. The pKin, or dissociation constant (pKa), of the indicators used in pH paper determines at which pH level the indicator will change color, aiding in the estimation of the solution's pH.
Options such as sodium nitroprusside reactions are more relevant to tests for specific substances in solutions like urinalysis test strips and do not relate to pH testing. Therefore, the correct answer for the principle of a pH reagent strip is a) Double indicator system.