Step 1: Understanding the Concept:
Nephelometry and turbidimetry both measure light interacting with a fine suspended precipitate, either the scattered light (nephelometry) or the transmitted light lost to scattering (turbidimetry). The method needs a reagent that reliably forms this kind of insoluble, evenly dispersed particulate with the analyte.
Step 2: Key Formula or Approach:
Pick the cyanide salt whose cation forms an insoluble compound with the substance being assayed, since a soluble cyanide salt would give no precipitate and therefore no turbidity to measure.
Step 3: Detailed Explanation:
Sodium and potassium cyanide are both freely soluble salts, so mixing either into the test solution would not generate a particulate suspension on its own. Silver cyanide, by contrast, is only sparingly soluble, and under controlled reaction conditions it forms a fine, evenly dispersed precipitate whose particle size stays consistent enough to give reproducible light scattering. This reproducibility is what makes the silver salt suitable for nepheloturbidometric measurement, while gold cyanide is not the reagent conventionally used for this purpose.
Step 4: Final Answer:
Cyanide is used in nepheloturbidometry as the silver (Ag) salt.