Which of the following is a spectrophotometric method for quantifying serum chloride?

• A. Ferric perchlorate
• B. Ammonium molybdate
• C. Bathophenanthroline
• D. Cresolphthalein complexone

Answer: (A)

Chloride can be measured by a colorimetric, spectrophotometric approach that relies on a redox reaction involving ferric ions. In this method, ferric perchlorate provides Fe3+ which can react with thiocyanate to form a deeply colored ferric thiocyanate complex. Chloride ions reduce some of the Fe3+ to Fe2+, so less ferric is available to form the red complex. The amount of red color remaining (read by the spectrophotometer) is thus related to how much chloride is present—the more chloride, the less color develops. The other reagents listed are used for different assays—ammonium molybdate for phosphate, bathophenanthroline for iron, and cresolphthalein complexone for calcium—so they aren’t appropriate for quantifying chloride.