Step 1: Understanding the Concept:
Rochelle salt is a double salt of tartaric acid and is widely used in chemical analysis and as a mild laxative.
Step 2: Detailed Explanation:
1. Chemical Name: It is Sodium Potassium Tartrate tetrahydrate (\(KNaC_4H_4O_6 \cdot 4H_2O\)).
2. Pharmacy Use: It is an essential component of Fehling's solution (used to test for reducing sugars) where it acts as a sequestering agent to prevent the precipitation of copper ions.
3. History: It was first synthesized by Elie Seignette in La Rochelle, France, which is how it got its name.
Step 3: Final Answer:
Chemically, Rochelle salt is defined as sodium potassium tartrate.