Question

Tollen's reagent is used for the detection of aldehyde when a solution of $AgNO _{3}$ is added to glucose with $NH _{4} OH$ then gluconic acid is formed
$ Ag ^{+}+e^{-} \rightarrow Ag ; E_{ red }^{+}=0.8 \,V$
$ C _{6} H _{12} O _{6}+ H _{2} O \rightarrow $ Gluconic acid $\left( C _{6} H _{12} O _{7}\right)+2 H ^{+}+2 e^{-} $
$E_{ red }{ }^{\circ}=-0.05 \,V $
$ Ag \left( NH _{3}\right)_{2}{ }^{+}+e^{-} \rightarrow Ag ( s )+2 NH _{3} ; E_{ red }{ }^{\circ}=0.337 \,V$
[Use $2.303 \times \frac{R T}{F}=0.0592$ and $ \frac{F}{R T}=38.92$ at $398\, K ]$
Ammonia is always is added in this reaction. Which of the following must be incorrect?

• A. $NH _{3}$ combines with $Ag +$ to form a complex.
• B. $Ag \left( NH _{3}\right)_{2}^{+}$is a stronger oxidising reagent than $Ag ^{+} .$
• C. In absence of $NH _{3}$, silver salt of gluconic acid is formed.
• D. $NH _{3}$ has affected the standard reduction potential of glucose/ gluconic acid electrode.

Answer 1

Answer: (D)

It is clear from the given reactions that $NH _{3}$ combines with $Ag$ to give $Ag \left( NH _{3}\right) .$ Also, the value of $E_{\text {red }} 0$ of $Ag ^{+}$is greater than $Ag \left( NH _{3}\right)$, which means it is a stronger oxidising agent than $Ag ^{+}$and in absence of ammonia, gluconic acid is formed. Therefore, the statement $(D)$ is incorrect as ammonia has no effect on the standard reduction potential of glucose/gluconic electrode.