Which instrument is commonly used to measure the concentrations of sodium (Na) and potassium (K)?

Explanation

Sodium (Na) and potassium (K) levels are typically measured using a flame photometer, as it is specifically designed to detect and quantify these alkali metals based on their emission spectra. Hydrometers measure liquid density, and spectrophotometers assess light absorbance, making them less suitable for direct Na and K determination.