experiment16

Cation Analysis

Beautiful Colors

and Chemical Principles

Qualitatively identify which cations are present in an unknown via the use of wet chemical techniques. Group II: Pb2+, Cu2+ , Bi3+ , Cd2+

Group III: Co2+, Ni2+ , Fe3+ , Mn2+, Cr3+, Zn2+

Group IV: Ba2+, Sr2+ , Ca2+

Group V: Na+, Li+, K+

Purpose

Ions in Groups II - IV are separated on the basis of selective precipitation techniques. Analysis of solubility product constants

(Ksp) and making use of the Le Châtelier’s Principle to shift the direction in which reactions that are at equilibrium proceed are

showcased in the selective separation of these ions.

Ions that form water soluble compounds are identified via flame testing.

Topics that will be explored: Methods used to mix, heat, evaporate, safely and properly handle and dispose of chemical reagents and waste Chemical Reactions

Ionic Equilibria Fractional Precipitation (pptn) Reactions

Complete precipitation Washing a precipitate (ppt)

Redox and Non-Redox Reactions Complex Ions / Coordination Complexes

Water Solubility / Miscibility or Lack Thereof Separation Techniques and their Pros and Cons

Centrifugation and Decantation Filtration

Indicator Papers (e.g., litmus and methyl violet paper): Their Utility and Role in pH Adjustment Solution Dilution Flame Testing

What’s in the Hood?

Test Tube Centrifuge

Test Tube Heating Block

Know Solutions Used for Flame Testing

Bunsen Burner

Hot Plate to Evaporate Solutions

Nichrome Wire

used in Flame Testing

Cobalt Glasses [Used to filter out interference of the

yellow flame emitted by the sodium ion during

flame testing.]

Sign of the Times: Cleanliness is Next to Godliness

Top Loader Balance (3 decimal place precision)

Eye Wash Fountain (Since you’re always wearing safety

glasses while you are in the lab you won’t need to use it!)

Paper Towel Dispenser

Assorted Brushes

De-Ionized (DI) Water Since tap water has ions that are being tested for in this experiment, to prevent

sample contamination, it is important that after you’ve cleaned your glassware that

you then rinse it with DI H2O. S

O A

P

Coronavirus 2020

Water soluble solids and aqueous solution disposal in the sink

Paper and Plastic Waste Receptacle

Broken Glass

Receptacle

There’s a Place for Everything and Everything has a (Proper) Place Stock Solutions of Acids and Bases are kept in one of the safety hoods.

Concentrated Solutions

15.1 M NH4OH 17.4 M HC2H3O2 11.7 M HCl 15.8 M HNO3 18.0 M H2SO4

Dilute Solutions

(All are 6 M)

If a procedure calls for an acid or base solution that is not in stock (e.g., 0.5 M HCl), you must prepare it yourself via dilution (see Appendix IV).

V1 x C1 = V2 x C2

Solution Dilution Preparation of 10 mL of 0.5 M HCl from available stock.

Available stock: 6 M HCl (dilute) and 12 M HCl (concentrated) Safer to use the more dilute (i.e., 6 M) solution

V1 x C1 = V2 x C2 V1 x 6 M = 10 mL x 0.5 M

To determine the volume (i.e, V1) of the stock, 6 M HCl, solution needed use the dilution formula:

V1 = 10 mL x 0.5 M 6 M

= 0.8 mL

Since it is difficult to measure 0.8 mL we will use the following rough conversion: 1 mL ≈ 20 drops

16 drops of 6 M HCl Add enough DI water until the total volume is 10 mL (i.e., 9.2 mL of DI water added)

Mix well Result: 10 mL of 0.5 M HCl

There’s a Place for Everything and Everything has a (Proper) Place Reagents other than acids and bases specifically used in this experiment are kept in three strategic locations throughout the lab.

Liquid reagents are kept in dropper bottles. Each has a number and are kept in alphabetical order. Review your nomenclature.

Reagent Bottle 53 (Thioacetamide) plays a prominent role in this experiment.

Each solid reagent bottle has a letter.

Return reagents to their proper place when you are finished using them.

Use a clean spatula Wash any excess reagent down the sink with copious amounts of water.

To prevent contamination, never return excess reagent to its stock bottle.

Other Materials Needed for the Experiment

Student “personal” drawer for the semester has the remaining requisite glassware/

materials needed to perform this experiment.

Vial containing unknown cation

solution is obtained from course

instructor

Porcelain Evaporating

Dish

Graduated cylinder with 3 mL of

unknown

Methyl Violet Paper Red Litmus Paper Blue Litmus Paper

Specific glassware/ materials needed for

students to perform this experiment in a laboratory

work bench by their drawer. Squeeze bottle with DI water

Spatula

Test tube holder

Stirring rod

Test tube rack

Disposable “Eye” Dropper (aka, transfer pipette)

Test tube with

stopper

Separation via Centrifugation and Decantation Centrifugation and Decantation will be the technique of choice used in this experiment to separate a suspended precipitate (ppt) inside a test tube (tt) from a liquid medium.

Test tube with suspended ppt

Test tube with equal volume of water MUST

be placed directly across to serve as a

counterbalance

Opening the lid will cause the rotor to slow

down and eventually stop spinning. You can speed this process by GENTLY pressing on

the rotor.

After centrifugation the solid (aka, ppt) is packed at the

bottom of the test tube which then makes it easy to separate it from the liquid

via decantation.

NOTE
 Test tube holders are being used strictly to more clearly illustrate

the process of decantation.

Precipitate has now been separated from the liquid (aka, supernate or supernatant).

Decantation involves carefully pouring the liquid above the

packed ppt into another test tube.

1 3 4

5 6

When the lid is closed the rotor will start

spinning. Centrifuge for ~45 s.

2

Professor, is there a ppt inside the test tube?

Get thee to a Centrifuge!

The answer is: Centrifuge

Clear vs. Cloudy

Which tt doesn’t have contents that are clear?

Cloudy (ppt suspended

in a liquid)

Clear and Colorless

Clear doesn’t mean the absence of color; it means

that it is not cloudy.

Clear green liquid

4321

Answer: tt#4

Complete Precipitation Complete participation precipitation involves making sure that the addition of more reagent doesn’t lead to further solid formation [i.e., precipitation (pptn)].

The mixture is so cloudy that it is impossible to

tell if the addition of more reagent leads to the

formation of more ppt.

Centrifuge

When the addition of more reagent doesn’t

cause more ppt to form, complete

precipitation has been achieved.

If more ppt forms continue adding more reagent.

Centrifuge when it becomes difficult to see if

the addition of reagent leads to further pptn.

Indicator Paper - Methyl Violet and Litmus Paper

Blue Litmus Paper

Red Litmus Paper

Indicator Paper indicates what the general pH of a solution is. An acid will lower the pH of a solution. A base will raise the pH of a solution.

B as

ic o

r al

ka lin

e to

li tm

us

(p H

> 7

)

A ci

di c

to li

tm us

(p

H <

7 )

N eu

tr al

to li

tm us

(p H

~ 7

)

Bases turn red litmus paper blue. Acids turn blue litmus paper red.

pH ~

0 .5

(d

ar k

gr ee

n or

b lu

e- gr

ee n

co lo

r)

pH > 0.5 (purplish color)

pH < 0.5 (yellowish color)

Methyl Violet Paper (MVP)

To make a solution just basic or alkaline to litmus, carefully

add a base drop-by-drop (e.g., dilute NH4OH) until the first drop causes red litmus

paper to turn blue.

If the pH needs to be adjusted to ~0.5, use MVP. If MVP turns dark green or blue-green then

the pH ~ 0.5. If MVP turns purplish, the solution’s pH > 0.5.

The pH must be lowered via addition of an acid (e.g., HCl) until the solution turns MVP dark green or blue green.

Scenarios

Washing a Precipitate

Place 2 - 3 mL of de-ionized water into a packed ppt

Carefully mix contents

Centrifuge

Carefully Decant

NOTE
 Test tube holders are being used strictly to more clearly

illustrate the process of decantation.

Secret to Success

from IIIA

from IVB

from IIA

Chemistry

Aside from having good experimental technique, to successfully complete this experiment, you must:

Carefully read and understand the whole procedure for one step before proceeding to the next step (i.e., don’t read the procedure one sentence at a time). Failure to read and understand the whole procedure for one step may lead you to throw away a liquid or solid that needs to be saved for another step.

Meticulously label the contents of your test tubes. You will need to save solutions and solids to use in other steps in the procedure that will occur 1 - 3 weeks in the future.

Summarize, in your own words, the experimental procedure in your notebook.

The Implications Behind Color Masking Darker color precipitates and solutions mask the presence of those that have a lighter color.

Once the contents are mixed, it would be impossible to tell that the liquid was light blue. Once mixed, centrifugation would help determine the color of the liquid.

Centrifugation reveals that the liquid or supernate is yellow

Conventions and Picking Up Clues Procedures are written to inform you what happens if the ion being tested for is present (i.e., a positive test). If the ion is not present then you won’t see the indicated result (i.e., a negative test).

Step Purpose Procedure Results Sep. of

Fe3+ and Mn2+ from Zn2+ and Cr3+

F. Mn(OH)3 MnO2 blackbrown

Fe(OH)3 red-brown

Zn(OH)42+ colorless

CrO42-

yellow

The chemical formula of the species formed after undergoing the procedure in each step is included in the results along with the species physical state

[if the ion precipitates out of solution it is underlined and if the ion stays in solution it is not underlined.]

Observation Inference ppt is red-brown Fe3+ present and

Mn2+ absent

liquid is yellow Cr3+ present and Zn2+ can’t tell

Observation Inference ppt is blackish Mn2+ present and

Fe3+ can’t tell due to color masking

liquid is colorless Cr3+ absent and Zn2+ can’t tell if Zn2+ was absent

the liquid would still be colorless.

S ce

na ri

o A

S ce

na ri

o B

Confirmatory Tests are also provided for situations where observations are ambiguous.

Fractional Precipitation The decomposition of thioacetamide in an acidic medium produces H2S, which serves as a source of sulfide (S2-) ions.

H2S smells like rotten

eggs.

H2S (aq) 2 H + (aq) + S2– (aq)

At very low pH’s (pH ~ 0.5), more H+ is present. In accordance with the Le Châtelier’s Principle, this drives the equilibrium to the left reducing [S2-]. Only the more insoluble Group II sulfides will precipitate at low pH’s.

Group II Ions Pb2+, Cu2+ , Bi3+ , Cd2+

The sulfides of these ions have a very small molar solubility and selectively precipitate out first.

At higher pH’s, less H+ (i.e., more OH–) is present. In accordance with the Le Châtelier’s Principle, this drives the

equilibrium to the right increasing [S2-]. This causes the precipitation of the less insoluble Group III sulfides. Some

ions, in this group precipitate out as hydroxides [i.e., Cr(OH)3 ].

Group III Ions Co2+, Ni2+ , Fe3+ , Mn2+, Cr3+, Zn2

The sulfides (or hydroxides) of these ions have a higher molar solubility

The pH that causes selective precipitation of Group III sulfides (or hydroxides) occurs when the solution is “just” alkaline to litmus. If

the pH is too high (i.e., pH > 9), then Group IV sulfides or hydroxides would also unselectively start to precipitate out as well.

This equilibrium reaction really occurs in 2 steps. This equation, however, indicates

the overall reaction that is occurring.

H3C C

NH2

S Δ

H2S +

thioacetamide hydrogen

sulfide acetic acid

H+ + H2O + NH4+

ammonium ion

H3C C

OH

O

Water Solubility Rules

The Dissolution Process and Water Solubility Two conditions favor dissolution (formation of a homogenous mixture or solution): i. a decrease in the energy of the system, which corresponds to an exothermic process ii. an increase in entropy (i.e., disorder) of the system – which is usually the case.

The relative strength of the following interactions affects the dissolution of a solute in a solvent:

a) solute-solute interaction b) solvent-solvent interaction c) solvent-solute interaction [if c > a and b then dissolution will occur]

to break these interactions requires energy (⨁∆H)

Like dissolves Like - If two substances have similar intermolecular forces (IMF), chances are that the solute will have a high solubility in the solvent

The following type of solutes that have an appreciable solubility in water: Ionic compounds having low ion charges, Organic compounds having less than 6 C that are capable of H-bonding or have a dipole-dipole IMF. NOTE: carbohydrates (e.g., monosaccharides ⇒ CxH2xOx) have many –OH groups and as such are even more H2O soluble. A few polar gases (HF, NH3, HCl, HBr, HI, H2S).

Reduction - Oxidation Reduction is defined as the gain of electrons (e.g., Cu2+ + 2 e- → Cu)

Oxidation is defined as the loss of electrons (e.g., Zn → Zn2+ + 2e–)

There can be no oxidation without an accompanying reduction or vice-versa. Often times spectator ions (e.g., Na+, K+, Cl–, NO3–) are not shown in redox reactions.

To determine which species in a redox reaction has been oxidized and which has been reduced, one must first determine the change in oxidation number that is occurring.

Cu2+ + Zn → Cu + Zn2+

The copper(II) ion has been reduced (oxidation number has been reduced from +2 to 0, i.e., it has gained 2e–). The reagent that gets reduced is known as the oxidizing agent or oxidant.

Zinc metal has been oxidized (oxidation number has increased from 0 to +2, i.e., it has lost 2e-). The reagent that gets oxidized is known as the reducing agent or reductant.

Complex Ions / Coordination Complexes A complex ion has a transition metal ion at its center and several molecules or ions surrounding it.

The surrounding molecules or ions (called ligands) are bonded to the metal ion by a coordinate covalent (dative) bond; therefore, ligands have at least one lone electron pair.

The ligand acts as a Lewis base (electron pair donor) while the transition metal ion acts as a Lewis acid (electron pair acceptor).

Cu O

H

H

O H

H

O HH

O H H

O H

H

O

H

H

2+

Cu2+(aq) really exists as the complex ion, Cu(H2O)62+, that has an octahedral geometry

Ligands encountered in this experiment are shown in the table below. NOTE: Only one lone pair of electrons is shown.*

Name water ammonia thiocyanate cyanide chloride hydroxide

Formula :OH2 * :NH3 :SCN– * :CN– * :Cl– * :OH– *

Charge neutral neutral -1 -1 -1 -1

Co(SCN)42–

Co2+ complex ion

The Cation Schematic: An Experimental Road Map

Pb2+, Cu2+ , Bi3+, Cd2+

Group II

Co2+, Ni2+, Fe3+, Cr3+, Mn2+, Zn2+

Group III

Ba2+, Sr2+, Ca2+

Group IV

Li+, Na+, K+

Group V

pH = 0.5, H2S (thioacetamide), Δ

PbS, CuS, Bi2S3, CdS

black ppt brown-orange or yellow ppt

Group III, IV, V

So yellow or white ppt (Discard)

Pb2+, Bi3+, Cd2+, Cu2+ light blue

white ppt

yellow ppt

Bi3+, Cd2+, Cu2+

white ppt

Bio

black ppt

Cu(NH3)42+, Cd(NH3)42+

dark blue colorless

maroon ppt

Cuo

black ppt H2S (thioacetamide), Δ

brown-orange or yellow ppt

pH = basic, H2S (thioacetamide), Δ

CoS, NiS, FeS, Cr(OH)3, MnS, ZnS

black gray-green pink white ppt

0.5 M HCl

CoS, NiS

black ppt conc. HNO3, HCl

So yellow or white ppt

(Discard) Co2+, Ni2+

dimethylglyoxime (DMG)

red ppt

KSCN isoamyl alc.-ether

blue or blue-green in alcohol-ether layer

Fe2+, Mn2+, Cr3+, Zn2+

NaOH, Na2O2

MnO2, Mn(OH)3, Fe(OH)3 black brown red-brown ppt

HNO3, NaNO2

Discard (if ppt remains) Mn

2+, Fe3+

purple soln

FeSCN2+

NaBiO3 HNO3

blood red soln

CrO42-, Zn(OH)42-

colorlessyellow

HCl ethylenediamine

K4Fe(CN)6

white ppt

HC2H3O2 Pb(C2H3O2)2

yellow ppt

Group IV, V

Original Soln Flame Test

Na+, Li+, K+

orange yellow

cobalt glasses used when Na+ is present

red light violet

NH4OH, (NH4)2CO3

Group V (Discard)

BaCO3, SrCO3, CaCO3

white ppt

HC2H3O2, NH4C2H3O2, K2CrO4

yellow ppt

Sr2+, Ca2+, (CrO42-)

white ppt

flame test

yellow-green

NH4OH, ethyl alcohol

yellow ppt

flame test

red

white ppt

flame test

red-orange

basic (NH4OH)

HNO3

(NH4)2SO4

PbSO4

NH4C2H3O2

PbCrO4

NH4OH

Bi(OH)3

Na2SnO2

K4Fe(CN)6

Cu2Fe(CN)6

Na2S2O4

Cd2+

CdS

Co(SCN)42-

MnO4-

KSCN

Zn2Fe(CN)6PbCrO4

K2CrO4

Pb(C2H3O2)2

BaCrO4

HCl

Ba2+

H2SO4

BaSO4

SrCrO4

HCl

Sr2+

Ca2+

(NH4)2C2O4

CaC2O4

HCl

Ca2+

Ni(DMG)2

ppt liquid

Group II Schematic Groups II - V

PbS, CuS, Bi2S3, CdS

black ppt brown-orange or yellow ppt

Group III, IV, V

So yellow or white ppt (Discard)

Pb2+, Bi3+, Cd2+, Cu2+ light blue

HNO3

white ppt

yellow ppt

PbSO4

NH4C2H3O2

PbCrO4

K2CrO4

Pb(C2H3O2)2

pH = 0.5, H2S (thioacetamide), Δ

(NH4)2SO4

NH4OH

Bi3+, Cd2+, Cu2+

white ppt

Bio

black ppt

Bi(OH)3

Na2SnO2

Cu(NH3)42+, Cd(NH3)42+

dark blue colorless

maroon ppt

Cuo

black ppt H2S (thioacetamide), Δ

brown-orange or yellow ppt

K4Fe(CN)6

Cu2Fe(CN)6

Na2S2O4

Cd2+

CdS

⊕ test for

Cu2+

⊕ test for

Cu2+

⊕ test for

Cu2+

⊕ test for

Cu2+

⊕ test for Bi3+

⊕ test for

Bi3+

⊕ test for

Cd2+

⊕ test for

Pb2+

⊕ test for

Pb2+ light blue

Soln has not been throughly

mixed

Group III Schematic Groups III - V

CoS, NiS

black ppt

HCl ethylenediamine K4Fe(CN)6

white ppt

HC2H3O2 Pb(C2H3O2)2

yellow ppt

Zn2Fe(CN)6 PbCrO4

conc. HNO3, HCl

So yellow or white ppt

(Discard)

Co2+, Ni2+

dimethylglyoxime (DMG)

red ppt

KSCN isoamyl alc.-ether

blue or blue-green in alcohol-ether layer

basic (NH4OH)

Co(SCN)42-Ni(DMG)2

Fe2+, Mn2+, Cr3+, Zn2+

Discard (if ppt remains) Mn

2+, Fe3+

purple soln

FeSCN2+

NaBiO3 HNO3

blood red soln

MnO4-

KSCN

MnO2, Mn(OH)3, Fe(OH)3 black brown red-brown ppt

CrO42-, Zn(OH)42-

colorlessyellow

NaOH, Na2O2

CoS, NiS, FeS, Cr(OH)3, MnS, ZnS

black gray-green pink white ppt Group IV, V

pH = basic, H2S (thioacetamide), Δ

Fe3+ contamination

due to sloppiness

NH4HF2 removes

contamination

bl oo

d re

d

pu rp

le ⊕

test for

Ni2+

⊕ test for

Co2+

⊕ test for

Mn2+

⊕ test for

Fe2+

⊕ test for

Cr3+

⊕ test for

Zn2+

Group IV Schematic

BaCO3, SrCO3, CaCO3

Groups IV - V NH4OH, (NH4)2CO3

white ppt

Group V (Discard)

yellow ppt

white ppt

BaCrO4

HCl

Ba2+

H2SO4

BaSO4

flame test

yellow-green

HC2H3O2, NH4C2H3O2, K2CrO4

Sr2+, Ca2+, (CrO42-)

yellow ppt

flame test

red

SrCrO4

HCl

Sr2+

NH4OH, ethyl alcohol

white ppt

flame test

red-orange

Ca2+

(NH4)2C2O4

CaC2O4

HCl

Ca2+

⊕ test for

Sr2+ ⊕

test for

Ca2+

⊕ test for

Ba2+

⊕ test for

Ba2+

⊕ test for

Ba2+ ⊕ test for

Sr2+

⊕ test for

Ca2+

short-lived flame short-lived

flame short-lived

flame

Group V Schematic

Na+ Yellow-orange flame

that persists for a minimum of 30 s

K+ Violet/Lilac flame This color is very

short-lived

Li+ Red flame

that persists for a minimum of 10 s

Cobalt glasses are used when Na+ is present to see if

K+ is also present in the unknown.

Cobalt glasses are used when Na+ is present to see if

Li+ is also present in the unknown.

Original Soln Flame Test

Na+, Li+, K+

orange yellow

cobalt glasses used when Na+ is present

red light violet