1. QUALITATIVE AND QUANTITATIVE ELEMENTAL ANAYSIS
          QUALITATIVE AND QUANTITATIVE ELEMENTAL ANAYSIS by SHITTU T.O. is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
QUALITATIVE AND QUANTITATIVE ELEMENTAL ANAYSIS

2. Learning goals At the end of the lecture, you should be able to:
a). Describe qualitative tests for specific elements in organic sample
b). Carry out basic calculations in quantitative elemental analysis.

3. Qualitative elemental analysis
This method deals with identification of various elements present in an organic sample.
Most organic samples contain C, H ,O and other elements such as N, P, S, halogens and metals.

4. Detection of C and H
C & H can be detected in an organic sample by heating the sample with cupric oxide (CUO) in a combustion tube.
Sample+ CUO CO2(g) + H2O(g)
CO2 turns lime water milky.
H2O turns anhydrous CUSO4 from white to blue or anhydrous CoCl2 fom blue to pink.

5. Detection of N, S and halogens
N, S and halogens can be detected in an organic sample by using Lassaigne’s sodium fusion test. This method converts elements in the organic sample into soluble sodium compound by fusion with sodium metal.
[C,H,N,X,P, S] Na (excess) NaCN + Na2S + NaX
If limited sodium is used, sodiumthiosulphate is formed instead of sodiumcyanide
[C,H,N,X,P, S] Na (limited) NaSCN + NaX Na
  NaCN + Na2S

6. Sodium fusion test cont’d
Further work-ups and analyses will then lead to identification of S, Br, I, Cl or N.

7. Quantitative elemental analysis
This deals with estimation of the proportion by mass or % of the element in a given mass of organic sample.

8. Quantitative determination of C and H
A known mass of the organic compound is burned in excess O2, where C and H are converted to CO2 and H2O respectively.
Apparatus for determining C & H
Source:

9. The CO2 is absorbed in soda lime (KOH) and weighed.
H2O is absorbed in CaCl2 and also weighed.
The % C and H can then be determined.

10. Question 1
1.62g of an unknown organic compound containing only carbon, hydrogen and oxygen, gave 2.64g of CO2 and 0.9g of H2O on complete combustion.
i. Determine the empirical formula of the compound.
ii. What is the molecular formula of the compound given if its relative molecular mass is 180

11. Quantitative determination of nitrogen
i. Kjeldahl method
ii. Dumas method
Kjeldahl method
This method is used for quantitative determination of nitrogen in samples such as protein, amines and other nitrogen containing compounds.
The sample is digested by heating it with sulphuric acid in the presence of a catalyst such as Cu or Se. The resulting mixture is made alkaline by addition of NaOH.
The volume of NH3(g) liberated is then determined by volumetric analysis by titrating NH3 formed with a standard H2SO4 or HCl, using a suitable indicator.

12. Equation of reaction for titration:
NH3(g) + H2SO4 (aq) (NH4)2SO4(aq)

13. Dumas method:
This method converts the nitrogen in the organic sample to gaseous nitrogen(N2). A known mass of the compound is heated with excess Cu2O in the absence of air. The gases produced are passed over heated Cu to decompose any oxide of nitrogen into gaseous nitrogen.
The nitrogen is collected in a nitrometer. The volume of nitrogen collected is then converted to that at S.T.P
Mole= 𝑉𝑁2 𝑎𝑡 𝑆.𝑇.𝑃 𝑑𝑚3 22.4𝑑𝑚3

14. Question
1.4g sample of a newly isolated organic compound was subjected to kjeldahl analysis. The gas produced was neutralized by 26.3cm3 of 0.25M sulphuric acid.
(a). name the gas produced in this analysis
(b). Calculate the percentage of nitrogen in the organic sample. (N= 14.0, H= 1.0)

15. Question 3:
0.14g of an organic sample on suitable treatment gave 27.44cm3 of nitrogen measured at STP. What is the percentage of nitrogen in the sample

16. Quantitative determination of sulphur and halogens
Sulphur and halogens can be quantitatively determined by gravimetric analysis using Carius method.
Under suitable treatment, the S in the sample is converted to BaSO4 and
halogens to AgX (X = Cl, Br, I e.t.c.)

17. A 0. 8g sample gave 1. 2 g of BaSO4 in a gravimetric analysis
A 0.8g sample gave 1.2 g of BaSO4 in a gravimetric analysis. What is the % of S in the sample.

18. Practice question
The molecular mass of an organic compound containing C, H and N was found to be 59. on combustion, 0.13g of the compound gave 0.29g of CO2 and 0.18g of H2O g of the compound on suitable treatment gave 27cm3 of N2 measured at S.T.P. Deduce the structure possible structures of the compound.

19. QUALITATIVE AND QUANTITATIVE ELEMENTAL ANAYSIS

20. Learning goals At the end of the lecture, you should be able to:
a). Describe qualitative tests for specific elements in organic sample
b). Carry out basic calculations in quantitative elemental analysis.

21. Qualitative elemental analysis
This method deals with identification of various elements present in an organic sample.
Most organic samples contain C, H ,O and other elements such as N, P, S, halogens and metals.

22. Detection of C and H
C & H can be detected in an organic sample by heating the sample with cupric oxide (CUO) in a combustion tube.
Sample+ CUO CO2(g) + H2O(g)
CO2 turns lime water milky.
H2O turns anhydrous CUSO4 from white to blue or anhydrous CoCl2 fom blue to pink.

23. Detection of N, S and halogens
N, S and halogens can be detected in an organic sample by using Lassaigne’s sodium fusion test. This method converts elements in the oragnic sample into soluble sodium compound by fusion with sodium metal.
[C,H,N,X,P, S] Na (excess) NaCN + Na2S + NaX
If limited sodium is used, sodiumthiosulphate is formed instead of sodiumcyanide
[C,H,N,X,P, S] Na (limited) NaSCN + NaX Na
  NaCN + Na2S

24. Procedure in Lassaigne’s sodium fusion test
The sample is heated with excess sodium metal to redness. The heated sample is and plugged into water and then ground in a mortar. The content is allowed to cool and then filtered.
Test for nitrogen:
Lassaignes fitrate + FeSO4(aq) Na4Fe(CN)6(aq) + Na2SO4(aq)
3Na4Fe(CN)6(aq) + 4FeCl3 (aq) Fe4[Fe(CN)6]3(aq)+ 12NaCl prussian blue solution

25. Test for sulfur
Na2S (Lassaignes fitrate )+ dil. Pb[CH3COO] dilute CH3COOH PbS (black ppt) CH3COONa

26. Test for halogens
To test for halogens, Lassaignes fitrate is first acidified with few drops of dilute H2SO4 and boiled to remove S And CN in the form of HCN(g) and H2S(g). These S2- and CN- ions can also interfere with the precipitating agents such as AgNO3
NaX + AgNO3 dil.HNO NaNO3 + AgX
White ppt soluble in excess aqueous NH3, it indicates the presence of Cl- present
Pale yellow ppt sparingly soluble in excess aqueous NH3, it indicates the presence of Br- present
yellow ppt insoluble in excess aqueous NH3, it indicates the presence of I- present

27. Quantitative elemental analysis
This deals with estimation of the proportion by mass or % of the element in a given mass of organic sample.
Quantitative determination of C and H
A known mass of the organic compound is burnt in excess O2, where C and H are converted to CO2 and H2O respectively.
The CO2 is absorbed in soda lime (KOH) and weighed.
H2O is absorbed in CaCl2 and also weighed.
The % C and H can then be determined.

28. Question 1
1.62g of an unknown organic compound containing only Carbon, hydrogen and oxygen, gave 2.64g of CO2 and 0.9g of H2O on complete combustion.
i. Determine the empirical formula of the compound.
ii. What is the molecular formula of the compound given if its relative molecular mass is 180

29. Quantitative determination of nitrogen
i. Kjeldahl method
ii. Dumas method
Kjedahl method
this method is used for quantitative determination of nitrgen in samples such as protein, amines and other nitrogen containing compounds. The sample is digested by heating it with sulphuric acid in the presence of a catalyst such as Cu or Se. the digestion converts the nitrogen in the sample to ammonia. The ammonia gas formed is not liberated in acid solution due to the presence of SO42-. The solution is made alkaline by reacting NaOH, which converts it to NH3.
The volume of NH3(g) liberted is then determined by volumetric analysis by titrating NH3 formed with a standard H2SO4 or HCl, using a suitable indicator.
Equation of titration for titration:
NH3(g) + H2SO4 (aq) (NH4)2SO4(aq)

30. Dumas method:
This method converts the nitrogen in the organic sample to gaseous nitrogen(N2). A known mass of the compound is heated with excess Cu2O in the absence of air. The gases produced are passed over heated Cu to decompose any oxide of nitrogen into gaseous nitrogen. All the gases produced are then passes through concentrated KOH to remove all the gases except N2. the nitrogen is collected in a nitrometer. The vloume of nitrogen collected is then converted to that a S.T.P
Mole= 𝑉𝑁2 𝑎𝑡 𝑆.𝑇.𝑃 𝑑𝑚3 22.4𝑑𝑚3

31. Question 2
1.4g sample of a newly isolated organic compound was subjected to kjedahl analysis. The gas produced was neutralized by 26.3cm3 of 0.25M sulphuric acid.
(a). name the gas produced in this analysis
(b). Calculate the percentage of nitrogen in the organic sample. (N= 14.0, H= 1.0)

32. Question 3:
0.14g of an organic sample on suitable treatment gave 27.44cm3 of nitrogen measured at STP. What is the percentage of nitrogen by mass of nitrogen.

33. Quantitative determination of sulphur and halogens
Sulphur and halogen can be quantitatively determined by gravimetric analysis using Carius method. The precipitate formed is washed, filtered, dried and weighed. Under suitable treatment, the S in an sample is converted to BaSO4 and halogens to AgX

34. QUALITATIVE AND QUANTITATIVE ELEMENTAL ANAYSIS by SHITTU T. O
          QUALITATIVE AND QUANTITATIVE ELEMENTAL ANAYSIS by SHITTU T.O. is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Practice questions
The molecular mass of an organic compound containing C, H and N was found to be 59. on combustion, 0.13g of the compound gave 0.29g of CO2 and 0.18g of H2O g of the compound on suitable treatment gave 27cm3 of N2 measured at S.T.P. Deduce the structure possible structures of the compound.