What is the role of rochelle salt in fehling's reagent? How about Sodium citrate in benedict's reagent?

Question

This is about the tests for differentiating aldehydes or ketones. Thanks in advance!

Answer 1

From http://columbia.thefreedictionary.com/Fehling's+solution

Fehling's solution is prepared just before use by mixing equal volumes of two previously prepared solutions, one containing about 70 grams cupric sulfate pentahydrate per liter of solution
and the other containing about 350 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 100 grams sodium hydroxide per liter of solution. The cupric ion (complexed with tartrate ion) is reduced to cuprous ion by the aldehyde (which is oxidized) and precipitates as cuprous oxide (Cu2O).

So the tartrate part of Rochelle salt is the key reactant.

In the case of sodium citrate: the cupric ion (complexed with citrate ions) is reduced to cuprous ion by the aldehyde group (which is oxidized), and precipitates as cuprous oxide, Cu2O.

Thus the citrate is key to the reaction.

Answer 2

Sodium Citrate and Potassium Tartarate (Rochelles) salt play similar roles in qualitative analysis.
Fehling solution A is copper sulphate (Cu2+), when aldehyde is added with water if there was no Rochelle salt Cu(OH)2 would be precipitated at any case. It would not serve its purpose. Rochelle salt forms a complex with Cu preventing its precipitation as Copper Hydroxide allowing Cu2O to form as precipitate.