Which instrument is used to measure monovalent ions via photometric techniques?

• A. Flame photometry
• B. AAS
• C. UV-Vis spectrophotometry
• D. Ion-selective electrode

Answer: (A)

Measuring monovalent ions with light emission from a flame is the idea here. Flame photometry works by aspirating the sample into a flame where the atoms are excited and emit light at specific wavelengths. The intensity of that emitted light at those characteristic lines (for example, sodium around 589 nm and potassium around 766–769 nm) is proportional to the ion’s concentration in the sample. This direct emission, or photometric, detection makes flame photometry especially well-suited for monovalent alkali metals, which give strong, clean emission signals.

The other techniques involve different detection principles. Atomic absorption spectroscopy measures how much light is absorbed by atoms in a sample, not how much they emit, which is a different photometric approach. UV-Vis spectrophotometry relies on absorbance of colored species and is not inherently tailored to alkali metals unless you form specific complexes, which adds steps and isn’t the standard method for these ions. An ion-selective electrode uses electrochemical potential to determine ion activity, not light emission, so it isn’t photometric either.

So the instrument best suited for measuring monovalent ions via photometric emission is flame photometry.