Adding AMMONIA to COPPER ION

In this experiment, why do the directions tell you to add ammonia, NH3, to the copper solutions?

The answer is in the two flasks on the left.

Both contain 0.1 M "Cu(II)".

Flask A is a simple aqueous solution. It has a pale blue color.

Flask B is that same 0.1 M solution to which some ammonia was added.

A B
The chemistry of going from Flask A to Flask B is:

Cu(H2O)6+2

+ 6 NH3

--->

Cu(NH3)6+2

+ 6 H2O

Flask A

Flask B

Per atom of copper present, Flask B will absorb hundreds times more photons of orange light than Flask A.
Using the copper ammine complex rather than the copper aqua complex increases the sensitivity of this analysis.
Small changes in the copper concentration will cause big changes in the amount of light absorbed.

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USING COPPER(II) as the INDICATOR
in an
ACID-BASE TITRATION

In this experiment you will dissolve a weighed piece of copper wire in an excess of concentrated Nitric Acid.

Cu + 2H+ + 2 HNO3 -----> 2 NO2 + 2 H2O + Cu+2

The excess Nitric Acid is then neutralized with 6 M Ammonia. You use this slightly dilute (6M) ammonia rather than concentrated (14 M) ammonia in order to reduce the reaction's exotherm.

(Recall CALORIMETRY where you measured Molar Heat of Neutralization)

...
NH3 + HNO3 ------> NH4NO3

Copper dissolving in HNO3

Excess HNO3 almost neutralized by 6M NH3

Excess HNO3
just neutralized by the 6M NH3

Large excess of concentrated NH3 added

The dissolved copper acts as an indicator, turning greenish as the neutralization point is approached, precipitating Cu(OH)2 at the equivalence point then redissolving and turning very dark blue after an excess of NH3 is added.
While there are many better indicators than copper for acid base titrations, it is sometimes worthwhile to look at things from a different view.

RWK