In the Mohr’s method for detecting chloride concentration, what is the indicator used and the observed color change at the end point?
- AEriochrome Black T; Blue to wine red
- BPotassium Chromate; Yellow to reddish-brownCorrect
- CPhenolphthalein; Pink to colorless
- DStarch; Colorless to blue
Solution
Concept
Mohr method for chloride determination
Formula
2 Ag+ + CrO4(2-) -> Ag2CrO4 (red-brown ppt)
Step-by-step solution
1. Mohr's titration uses standard AgNO3 solution to precipitate Cl- as white AgCl. 2. Potassium chromate (K2CrO4) indicator provides chromate ions in solution. 3. After all chloride ions react, excess Ag+ reacts with chromate: 2 Ag+ + CrO4(2-) -> Ag2CrO4. 4. Silver chromate forms a brick reddish-brown precipitate signaling the endpoint. Answer: B
Explanation
Mohr's method is an argentometric precipitation titration used to determine chloride ion concentration in water using standard silver nitrate (AgNO3). Potassium chromate (K2CrO4) serves as the indicator, imparting an initial yellow color. When all chloride ions have precipitated as white silver chloride, excess silver ions react with chromate ions to form a distinct reddish-brown precipitate of silver chromate (Ag2CrO4) marking the end point.
Why the other options are wrong
A) Eriochrome Black T is used for EDTA hardness titration, changing from wine red to blue. C) Phenolphthalein is an acid-base indicator for alkalinity. D) Starch indicator turns blue in iodometric dissolved oxygen determinations.
Shortcut
Mohr's chloride test = Potassium chromate turning yellow to reddish-brown.
Common mistake
Confusing with the wine-red to blue color change of EDTA total hardness testing using EBT.
Correct answer: B) Potassium Chromate; Yellow to reddish-brown