Warning:

Do not attempt to perform any of the following experiment(s) without taking extreme caution.
Always add acid to water, never the reverse.
Chemicals that fall under the categories below must be handled under a fume hood:

• Chemicals with a National Fire Protection Association (NFPA) Health rating of 3 or 4
• Toxic volatile chemicals
• Flammable chemicals
• Carcinogens
• A procedure that creates a toxic aerosol compound
• Reactive (highly exothermic) chemicals that may spatter
• Toxic gases (NH3, CO, F2, Cl2, H2S, NO2, etc.)
• Odorous chemicals

Always wear proper PPE, including:

• Safety goggles
• Disposable (nitrile) gloves
• Closed toed shoes
• long sleeved shirt and pant

The Reduction of Dichromate
6/10/2018

Abstract:
The reduction of dichromate from Cr2O72-  to Cr23+ occurs only in acidic aqueous environment, and accompanied by a visible change of color. The process occurs in a reduction-oxidation reaction. Although the redox process occurs in acidic media, the type of acid and reducing agent affects the color of the resulting solution. This phenomenon may be explained by the nephelauxetic series, and the allowed d-d electron transitions.

Procedure:

Required materials and chemicals: ammonium dichromate, ethanol, hydrochloric acid, sulfuric acid, sodium sulfite, 4 test tubes, water, corck stoppers, volummetric pipettes (1mL and 5mL) or 4 serological pipettes, (optional) extra large test tube for control, and pipette for quickening the reactions

Part 1: Effects of varying the acid media with a constant reducing agent

1.       Add approximately 50mg of ammonium (or potassium dichromate) into two large test tubes

2.       To one test tube add 10mL of 10% w/w sulfuric acid (H2SO4), and to another add 10mL of 10% w/w hydrochloric acid (HCl). This is done by:

·         With 97-98% w/w H2SO4 pipette approximately 9mL water into the first large tube, then pipette 1 mL of H2SO4 into the same large tube

·         Label the first large tube

·         With 32-34% w/w HCl pipette approximately 7mL water into a second large tube, then pipette 3mL HCl into the same tube

·         Label the second large tube

3.       Observe and note the exothermic process, as heat evolves

4.       To each test tube, add approximately 1 mL ethanol

5.       Observe and record the observations (color change)

Part 2: Effects of varying the reducing agent with a constant acid media

1.       Add approximately 50mg of ammonium (or potassium dichromate) into two large test tubes

2.       Add 10mL of 10% w/w H2SO4 to both test tubes

·         Note the exothermic process again

3.       Add 50mg of sodium sulfite to one test tube

·         Label this test tube

2.       Add approximately 1 mL of ethanol to the second test tube

·         Label this test tube

3.       Observe and record the observations (color change)

MSDS:

Ammonium dichromate:
Chemical formula: (NH4)2Cr2O7
Molar mass: 252.07 g/mol
Toxicity: hazardous to skin cotnact, eye contact (irritant), ingestion, inhalation, carcinogenic, mutagenic for bacteria
flammability: may be combustible at high temp (190 degrees Celsius)
Color: orange/ red
Odor: odorless
Classification: oxidizing material
Other names: diammonium salt
Physical appearance: solid
Solubility: Soluble in cold water, hot water, alcohol, insoluble in acetone

Sodium Sulfite:
Chemical formula: Na2SO3
Molar mass: 126.04 g/mol
Toxicity: Hazardous in case of ingestion, inhalation (lung irritant), slightly hazardous to skin and eye contact
flammability: non-flammable
Color: white
Odor: odorless
Classification: not a DOT controlled material
Other names:  Sulfurous acid, disodium salt
Physical appearance: solid
Solubility: Soluble in cold water, hot water, glycerol. Almost insoluble in alcohol

Hydrochloric Acid:
Chemical formula: HCl
Molar mass: 36.46 g/mol
Toxicity: very hazardous to skin contact, eye contact, ingestion, slightly hazardous in case of inhalation (lung sensitizer) non-carcinogenic
flammability: non-flammable
Color: colorless
Odor: pungent, irritating (strong)
Classification: class 8 corrosive material
Other names: muriatic acid
Physical appearance: liquid
Solubility: soluble in cold water, hot water, diethyl ether

Sulfuric Acid:
Chemical formula: H2SO4
Molar mass: 98.079 g/mol
Toxicity: very hazardous to skin contact, eye contact, ingestion, inhalation, possible carcinogen
flammability: non-flammable
Color: odorless
Odor: odorless
Classification: class 8 corrosive material
Other names: sulphuric acid
Physical appearance: liquid
Solubility: soluble in cold water, ethyl alcohol

Ethanol:
Chemical formula: C2H6O
Molar mass: 46.07 g/mol
Toxicity: hazardous to ingestion, skin contact, and eye contact
flammability: Flammable (363 degrees Celsius)
Color: colorless
Odor: alcohol-like, ethereal, pleasant
Classification: principle alcohol found in alcohol drinks
Other names: Ethyl alcohol
Physical appearance:
Solubility: easily soluble in cold water, hot water, methanol, acetone, diethyl ether

Formulas:

 

Data:

Part 1:

Control Test Tube:
Chemicals: ammonium dichromate .0456g, water 10 mL

Results: color change to yellow/gold color

Test Tube 1:

Chemicals: dilute sulfuric acid 10 mL (1:10), ethanol 1mL, ammonium dichromate 0.0545g

Results: Release of heat upon addition of acid to water, a fast color change from yellow/gold to blue shortly after adding ethanol. Complete reaction in approx. 5 mintues

 

Test Tube 2:

Chemicals: dilute hydrochloric acid 10 mL (3:10), ethanol 1mL, dichromate .0595mg

Results: Release of heat upon addition of acid to water, a slow color change from yellow/gold to bright green after adding ethanol. Complete reaction in approx. 20 minutes

Part 2:

Control Test Tube:
Chemicals: ammonium dichromate .0456g, water 10 mL

Results: color change to yellow/gold color

Test Tube 1:

Chemicals: dilute sulfuric acid 10 mL (1:10), ethanol 1 mL, ammonium dichromate 0.0560g

Results: Release of heat upon addition of acid to water, a fast color change from yellow/gold to blue shortly after adding ethanol. Complete reaction in approx. 6 mintues

 

Test Tube 2:

 

Chemicals: dilute sulfuric acid 10 mL (1:10), sodium sulfite .0550g, ammonium dichromate 0.0520g

Results: Release of heat upon addition of acid to water, a slow color change from yellow/gold to dull green after adding ethanol. Complete reaction in approx. 10 minutes

 

Discussion/Results:

The oxidation-reduction reaction indicates ethanol as the reducing agent of dichromate.

Cr2O7 + CH3CH2OH + 10H+ à Cr2 + 6H2O + CHCOOH

 

However, ethanol and chromium are not the only reagents necessary for the reaction, since protons are part of the reaction equation. The protons must come from an acid media, that is, the reduction of dichromate only occurs at an acidic pH, which necessitates the use of acid. This requirement can also be predicted from using a Pourbaix diagram of chromium’s varying oxidized/reduced states.

Likewise, reduced chromium (Cr+³ ) is a a stable species under normal conditions. This stability can be predicted from a Frost diagram. The diagram below shows a comproportionation between Cr 2+ and Cr2O72- .

 

Reference:

  1. Sciencelab.com
  2. https://commons.wikimedia.org/wiki/File:Ammonium-dichromate-sample.jpg