Experiment #: 8
Title: Synthesis of Methyl Orange and Investigation of Acid Base Indicating Properties
Purpose:
Is to synthesize the azo dye methyl orange and investigate its behavior as an acid-base indicator.
Through the process of using diazotization and coupling reactions and develop an understanding
of how molecular structure and conjugation affect the color and pH sensitivity of organic dyes.
Procedure:
1. In a 600 mL beaker equipped w/a magnetic stirrer, prepare a solution of 5%
sodium carbonate.
2. Add 5.2 g of sulfanilic acid to beaker and stir until dissolved, using a small amount
of water as needed and rinsing down sides.
3. Add a small additional quantity of sodium carbonate and allow bubbling to subside
completely.
4. In a separate container, dissolve 1.8 g of sodium nitrite in 10 mL of water and slowly
add this to sulfanilic acid solution.
a. Cool mixture in an ice-water bath until temperature reaches 0–5°C.
5. Slowly acidify cooled solution by adding 3 moles of concentrated HCl dissolved in 5 mL
of water, dropwise over several minutes.
6. Separately, prepare a solution of 3 g of N,N-dimethylaniline acidified w/1.5 mL of
glacial acetic acid.
7. Combine diazonium salt solution w/dimethylaniline solution to allow coupling reaction
to form methyl orange.
8. Remove reaction mixture from ice bath and add a sodium hydroxide solution (prepared
by dissolving 3.5 g of NaOH in 10 mL water).
9. Precipitate methyl orange by adding 10 g of solid sodium chloride.
10. Isolate solid product using Büchner funnel suction filtration, washing w/ cold water and
ice-cold ethanol to purify.
11. Allow methyl orange to dry.
12. Test acid-base indicator properties by preparing a methyl orange solution and
alternating between acidic (1 M HCl) and basic (1 M NaOH) environments, observing
color changes.
Data/Results/Calculations:
• Synthesis Outcome:
o Orange-red solid methyl orange successfully synthesized and isolated.
• Color Changes Observed:
o In acidic solution (1 M HCl):
▪ Red color.
o In basic solution (1 M NaOH):
▪ Orange color.
• pH Indicator Range:
o Approximately pH 3.2 to 4.4.
• Observation of Color Change:
Title: Synthesis of Methyl Orange and Investigation of Acid Base Indicating Properties
Purpose:
Is to synthesize the azo dye methyl orange and investigate its behavior as an acid-base indicator.
Through the process of using diazotization and coupling reactions and develop an understanding
of how molecular structure and conjugation affect the color and pH sensitivity of organic dyes.
Procedure:
1. In a 600 mL beaker equipped w/a magnetic stirrer, prepare a solution of 5%
sodium carbonate.
2. Add 5.2 g of sulfanilic acid to beaker and stir until dissolved, using a small amount
of water as needed and rinsing down sides.
3. Add a small additional quantity of sodium carbonate and allow bubbling to subside
completely.
4. In a separate container, dissolve 1.8 g of sodium nitrite in 10 mL of water and slowly
add this to sulfanilic acid solution.
a. Cool mixture in an ice-water bath until temperature reaches 0–5°C.
5. Slowly acidify cooled solution by adding 3 moles of concentrated HCl dissolved in 5 mL
of water, dropwise over several minutes.
6. Separately, prepare a solution of 3 g of N,N-dimethylaniline acidified w/1.5 mL of
glacial acetic acid.
7. Combine diazonium salt solution w/dimethylaniline solution to allow coupling reaction
to form methyl orange.
8. Remove reaction mixture from ice bath and add a sodium hydroxide solution (prepared
by dissolving 3.5 g of NaOH in 10 mL water).
9. Precipitate methyl orange by adding 10 g of solid sodium chloride.
10. Isolate solid product using Büchner funnel suction filtration, washing w/ cold water and
ice-cold ethanol to purify.
11. Allow methyl orange to dry.
12. Test acid-base indicator properties by preparing a methyl orange solution and
alternating between acidic (1 M HCl) and basic (1 M NaOH) environments, observing
color changes.
Data/Results/Calculations:
• Synthesis Outcome:
o Orange-red solid methyl orange successfully synthesized and isolated.
• Color Changes Observed:
o In acidic solution (1 M HCl):
▪ Red color.
o In basic solution (1 M NaOH):
▪ Orange color.
• pH Indicator Range:
o Approximately pH 3.2 to 4.4.
• Observation of Color Change:
