Experiment I - Separation of Basic Constituent into Groups

Learning Objectives

This experiment will be an opportunity to learn about:

• The methods of separating basic constituents into groups according to the reactions and properties of their ions; and
• identify the precipitating reagent of each group of cation.

Key Concept

Elements in the periodic table can be separated into two different ways.

• According to their atomic numbers.
• According to the reactions and the properties of their ions.

This experiment will shed some light on the second method.

Experiment Walkthrough

The experiment involves the use of reagents to react with the solution’s constituents to form salts or ionic compounds.

This experiment involves mixing your cations with reagents to initiate a precipitation reaction from the cation of your mixture.

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First, place 5 drops ( $5\text{gtts}$ ) of each of the following test solutions: ${\text{Ag}}^{+}$, ${\text{Cu}}^{2+}$, ${\text{Zn}}^{2+}$, ${Ba}^{2+}$, and $Na+$ Cation in a test tube.

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Then drop 1-2 drops ( $1–2\text{gtts}$ ) of $2\text{N}\text{HCl}$ at a time until precipitation is complete, then stir and centrifuge.

To check if the precipitation is complete, add 1-2 more drops ( $1–2\text{gtts}$ ) of $2\text{N}\text{HCl}$, if no more precipitation is formed then it is deemed complete, then stir and centrifuge one last time.

In the end you will have formed a white precipitate of silver (I) chloride ( $\text{AgCl}$ ) in your test tube.

Cations who are in the same group precipitate as chlorides by adding $2\text{N}\text{HCl}$.

Reaction of $\text{HCl}$ and ${\text{Ag}}^{+}$ Cation ( $\text{Ag}\left({\text{NO}}_3\right)$ reagent ).

$\text{Ag}\left({\text{NO}}_3\right)+\text{HCl}⟶\text{AgCl}\downarrow +{\text{NHO}}_3$

Transfer your filtrate to another test tube.

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Place the filtrate in your other test tube in an evaporating dish then evaporate almost to dryness.

Once the optimal dryness is reached add 1 drop ( $1\text{gtt}$ ) of concentrated $\text{HCl}$ and 39 drops ( $39\text{gtts}$ ) of distilled water; stir then transfer into a test tube.

Proceed to saturate it with ${\text{H}}_2\text{S}$; stir and centrifuge.

In the end you will have formed a dark gray to black precipitate of copper (II) sulfide.

$\text{HCl}$ is the selected reagent because it serves as a source of $C{l}^{-}$ ions which makes insoluble salts with this group of cations.

$\text{Cu}{\left({\text{NO}}_3\right)}_2+{\text{H}}_2\text{S}⟶\text{CuS}\downarrow +2\text{H}{\text{NO}}_3$

Transfer your filtrate to another test tube.

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To your new test tube, add 5 drops ( $5\text{gtts}$ ) of ${\text{NH}}_4\text{Cl}$, followed by concentrated ${\text{NH}}_4\text{OH}$ to make your solution alkaline.

Proceed to saturate it with ${\text{H}}_2\text{S}$; stir and centrifuge.

In the end you will have formed a white precipitate of zinc (I) sulfide.

${\text{H}}_2\text{S}$ in an alkaline medium is the reagent for selective precipitation of cations in this group. The pH of your medium is maintained at ~9 by ${\text{NH}}_3$/${\text{NH}}_4^{+}$ buffer ( ${\text{NH}}_4^{+}$ in this case ).

$\text{Zn}(\text{OH}{)}_2+{\text{H}}_2\text{S}⟶\text{ZnS}\downarrow +2{\text{H}}_2\text{O}$

Transfer your filtrate to another test tube.

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To your new test tube, place it in an evaporating dish and evaporate almost to dryness.

Once the optimal dryness is reached, add 3 drops ( $3\text{gtts}$ ) of $6\text{N}{\text{NH}}_4\text{OH}$ then heat on a water bath for 3-5 minutes ( $3–5\text{m}$ ).

After heating, add 15 drops ( $15\text{gtts}$ ) of $({\text{NH}}_3{)}_2{\text{CO}}_3$ and 15 drops ( $15\text{gtts}$ ) of $95\%$ ethyl alcohol; stir and centrifuge.

In the end you would have the white precipitate of barium (II) carbonate.

The carbonate ion ( $C{O}_3^{2-}$ ) is what precipitates the cations of this group.

${\text{BaCl}}_2+({\text{NH}}_4{)}_2{\text{CO}}_3⟶{\text{BaCO}}_3+2{\text{NH}}_4\text{Cl}$

Transfer your filtrate to another test tube.

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The remains of your test tube should be your sodium cation ( ${\text{Na}}^{+}$ ).

Cations that are in the same group as sodium do not form insoluble salts and therefor stay in the form of a solution.

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Conclusions

Cations are divided into 5 distinct groups, labeled 1 to 5 in Roman Numerals.

Group Name	Alternate Name	Members	Precipitating reagents and conditions
Group I	Insoluble chlorides	${\text{Ag}}^{+}$, ${\text{Hg}}_2^{2+}$, ${\text{Pb}}^{2+}$	$\text{HCl}$
Group II	Insoluble sulfides in an acidic medium	${\text{Hg}}^{2+}$, ${\text{Pb}}^{2+}$, ${\text{Cu}}^{2+}$, ${\text{Bi}}^{3+}$, ${\text{Cd}}^{2+}$, ${\text{As}}^{3+}$, ${\text{Sb}}^{3+}$, ${\text{Sb}}^{4+}$	${\text{H}}_2\text{S}$, in an acidic medium.
Group III	Insoluble sulfides or hydroxides in an alkaline medium	${\text{Al}}^{3+}$, ${\text{Fe}}^{3+}$, ${\text{Co}}^{2+}$, ${\text{Ni}}^{2+}$, ${\text{Cr}}^{3+}$, ${\text{Zn}}^{2+}$, ${\text{Mn}}^{2+}$	${\text{H}}_2\text{S}$, in an alkaline medium maintained at ~9 by ${\text{NH}}_3$/${\text{NH}}_4^{+}$
Group IV	Carbonate precipitates	${\text{Ca}}^{2+}$	$({\text{NH}}_4{)}_2{\text{CO}}_3$
Group V	Soluble salts	${\text{Mg}}^{2+}$, ${\text{Na}}^{+}$, ${\text{K}}^{+}$	$N/A$