EXPERIMENT 6Elemental Analysis

By: Abnasan, Stephanie Joy

B.Paragas, Regina Monica

Noreen

Cl

NH2

S

NH2CH3

OOH

OHH

HO H

OH

OH

H

HCH2OH

Elemental Analysis

• used to determine the presence of atoms in a given sample of organic compound

• used to determine the structure of a compound and detect the functional groups present

• used to test for the purity of a given sample

EA can detect the following elements:

• Carbon• Hydrogen• Nitrogen• Sulfur• Oxygen• Phosphorus• Halogens ( F, Cl, Br, I )

Methodology

Fig 1.1 Setup Apparatus for Sodium Fusion

Test tube holder

5-mL test tube

Burner

Method RationaleA. Sodium Fusion A small piece of sodium was placed in a 5-mL test tube and was heated

using Bunsen burner.In order for sodium to vaporize

The flame was removed and 0.3 grams of the unknown was added.

Test tube was heatedIn order for sodium to react with the

unknown

Method RationaleA. Sodium Fusion

A small piece of sodium was placed in a 5-mL test tube

and was heated using Bunsen

burner.

In order for sodium to vaporize

The flame was removed and 0.3

grams of the unknown was

added.

Test tube was heated

In order for Sodium to react

with the unknown

Sodium Fusion• Compounds with Nitrogen

• Compounds with a Halogen

• Compounds with Sulfur

B. Qualitative Tests

1. Sulfur Test

5 drops of the fusion solution and 3 drops of water were placed in a 5mL test tube.

2 drops of 2% aqueous sodium nitroprussside

solution was then added. Presence of a

deep blue-violet confirms the presence

of sulfur.

Thioacteamide: +Glucose: -

p-Choloroaniline: -

Test tube was dropped in a beaker containing 10 mL of cold dH20; test tube

is expected to shatter

The liquid molecules expand when placed in an environment of lower temperature.

Because the container can no

longer accommodate the size of the

molecules, causing it to crack. Upon

breakage, excess sodium can react with the unknown

Beaker was placed on a hot plate.

To allow solution to boil; completion of

reactionSolution was filtered. Filtrate was used a fusion solution for

Part B.

Experimental

Theoretical

p-chloroaniline

yellow No change

thioacetamide

Dark violet Dark violet

Glucose White solution

No change

SULFUR TEST: Experimental vs. Theoretical Observation

2. Nitrogen Test

A pinch of FeSO 4 was mixed with 1 mL of

the fusion solution in a test tube

In order for these reactions to occur:

2 NaCN + FeSO4 Fe (CN)2 +

Na2SO4

Fe (CN)2 + 4 NaCN Na4Fe(CN)6

5 drops of 10% solution of KF was

added.

To intensify blue coloration.

The resulting mixture was boiled and then

2 drops of 5% solution of ferric

cyanide was added.

3Na4Fe(CN)6 + 4FeCl3 Fe4[Fe(CN)6]3 + 12 NaCl (blue ppt.)

6M H2SO4 was added. An indication of the presence of nitrogen

is a Prussian blue precipitate

To dissolve the insoluble iron

hydroxides and make the solution acid to

litmus.Thioacteamide: +

Glucose: -p-Choloroaniline: +

The mixture was filtered and the filter

paper was washed with distilled water (if it is impossible to

determine a blue coloration).

to check whether the residue was blue or

not

Experimental

Theoretical

p-chloroaniline

yellow blue

Thioacetamide

blue blue

Glucose White solution

No change

NITROGEN TEST: Experimental vs. Theoretical Observation

3. Halogen Test10 drops of the

fusion solution was placed into a small

test tube.

2 M HNO3 was added

To acidify solution NaX + AgNO3

AgX + NaNO3

The solution was heated to the gentle

boil for about 1 minute and then cooled to room temperature

to expel HCN or H2S that might be

present- if nitrogen and sulfur were

present, the compounds AgCN (white) and Ag2S

(black) would also precipitate, making it hard to see the presence of silver halide precipitate.

After which, 4 drops of 0.1 M AgNO3 was

added to the solution. A heavy

curdy-type precipitate indicates

the presence of a halogen. A faint

turbidity is a negative test.

a solubility test was conducted using 0.5 mL of 2M NH4OH and

a stirring rod

to determine whether the solution

is solubleif Soluble: AgBrnot soluble: AgI

Thioacteamide: +Glucose: -

p-Choloroaniline: +

SampleSufur Test

Nitrogen Test

Halogen Test

Unknown 1 (Glucose)

- - -

Unknown 2 (p-

chloroaniline)

- + +

Unknown 3 (Thioacetami

de)+ + -

Summary of the Qualitative Tests for the Unknowns

Experimental

Theoretical

p-chloroaniline

Heavy curdy ppt.

White ppt. (for

chlorine)thioacetamide

Black ppt. No ppt.

Glucose No ppt. No ppt.

HALOGEN TEST: Experimental vs. Theoretical Observation

1. Outline the procedures for detecting the presence of oxygen

and phosphorus in organic compounds. a. Oxygen

• Ferrox TestSoak filter paper in methanol containing equal

amounts of KCNS and ferric chloride 

In the presence of oxygen-containing compounds, the complex ion (FeCNS++) distributes itself between the filter paper and the test

compound

Deep-red colored of solution is observed. Solutions of non-oxygen compounds remain colorless.

• Winkler Method

Manganese(II) ions liberated from the Manganese Sulfate are loosely bound with excess hydroxide

Manganese(II) is oxidized to Manganese(III) in the presence of a strong base and binds the dissolved

oxygen 

Free iodine is produced upon acidification of the sample at a rate of one I2 moleculefor each atom of

oxygen. 

Free iodine complexes with excess iodide ions.

The iodine/iodide complex is reduced to iodide with thiosulfate

b. Phosphorus• Dip nickel chromium loop into the sample

Conduct flame test

Using a blue cobalt glass, filter out the yellow sodium

Check for the positivity of phosphorous by checking for the presence of a pale

bluish green color

•   Add about 8 drops of HCl to 5 mg of unknown

in a 3 mL screw cup vial

Close tightly and place it into a 100oC preheated oven overnight

 Remove the vial from the oven a cool to room

temperature

Remove the vial cap and place it again into a 100⁰C oven for an hour to evaporate the acid

Add 0.5mL of distilled water to the residue. Then mix it and transfer to a microcentrifuge tube.

After a minute of centrifugation, transfer 5 drops of clear supernatant to

a 3 mL vial

Add 5 drops of molybdic acid to the solution. Mix the solution and add 2

drops of Fiske-Subarrow reducer

Observe for the color changes after 10 minutes. The presence of blue color

signifies presence of phosphorus

• La Motte Model

Fill the test tube to line 6 with the Phosphorus Extracting Solution

Use the 0.5 g spoon to add three measures of the sample

Cap and shake for 1 minute

Remove cap.  Allow to stand and sample to settle until liquid above the soil is clear

Use a pipet, dropper with the red bulb, to transfer the clear liquid to a second clean test tube

Add six drops of the Phosphorus Indicator Reagent to sample extract in the second Tube

Cap and shake to mix the contents

Add one phosphorus test tablet

Cap and shake vigorously until the tablet is dissolved.  A blue color will develop

Match the test color with the phosphorus color chart.  Record this phosphorus level.

2. Discuss other methods of detecting the presence of sulfur, nitrogen and halogen in organic compounds.

• Detection of Sulfur

In chemistry, the Schöniger oxidation (also known as the Schöniger Flask Test) is a test in qualitative elemental analysis, developed by Wolfgang Schöniger.

sample (with C,N,S,H,X) + oxygen (O2) + NaOH → Na+X- + Na+NO-

2 + Na+2SO-

3

(X = Cl, Br, F, I)

• Detection of Nitrogen

1. Benzidine Test – in the presence of cyanide ions, benzidine is oxidized to give the intensely colored benzidine blue. Oxidation is effected in the presence of cupric ions. The strength of cupric ions as oxidizing agents is greatly reduced in the presence of cuprous ions. Thus, the cuprous ions formed are removed from the test solution. Insoluble cuprous cyanide can be removed by filtration.

2. This test is based on the formation of a red color when the cyanide ion is converted to FeCNS++.(NH4)2SX + NaCN NaCNS + (NH4)2S

CNS- + Fe+++ FeCNS++ (red color)

3. Soda Lime Test (Will Varentrop method). Compounds which contain nitrogens liberate ammonia when heated with soda lime. The ammonia liberated is detected with litmus paper or with HCl.

CONCLUSION AND RECOMMENDATIONS

• The detection of various elements present in an organic compound is called elemental analysis. Carbon and hydrogen are present in almost all the organic compounds. Other commonly present elements in organic compounds are oxygen, nitrogen, halogens, sulfur and sometimes phosphorus. Some of the most common methods included in identifying such are sodium fusion, sulfur test, nitrogen test and halogen test.

• It was found out that p-chloroaniline turned out to be positive for nitrogen and halogen test (i.e. chlorine); thioacetamide C2H5NS showed positive results for nitrogen and sulfur; and glucose C6H12O6 obtained negative tests for S, N and halogens.

• For safety measures, students should observe strict precautionary measures. Treat every reagent as toxic that one should never touch it with his/her bare hands. Also, during sodium fusion, one should not allow sodium to be in contact with water as its reaction is highly exothermic. And finally, always do heating procedures inside a hood.

-=Fin =-