Kjeldahl percentage nitrogen comes out half of the key, and why does Dumas work for nitro compounds but Kjeldahl doesn't?

8 answers & replies

• [+][–]Siddharth RaoDropperSample· 1mo ago

  The silly thing is N vs M. That formula wants normality, and $H_2SO_4$ is dibasic. 0.1 M means 0.2 N. So $1.4 \times 0.2 \times 15 / 0.5 = 8.4\%$.

  If you don't trust the formula, do it from scratch. Moles of acid = $0.1 \times 0.015 = 1.5\times10^{-3}$. Each mole of acid takes 2 moles of $NH_3$, so $NH_3 = 3\times 10^{-3}$ mol. Nitrogen mass = $3\times10^{-3} \times 14 = 0.042$ g. $0.042/0.5 = 8.4\%$.

  I lost marks on exactly this in a test last year. Now I never use the shortcut without writing the moles once.

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  • [+][–]Yash Tiwari11thSample· 1mo agoAuthor

    omg bhai. Normality, yaar. Thanks, 8.4 aa gaya 😭

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• [+][–]Farhan ShaikhDropperSample· 1mo ago

  Kjeldahl works by converting N in the compound to ammonium sulphate with conc. $H_2SO_4$ (plus $K_2SO_4$ and a little $CuSO_4$ as catalyst), then NaOH releases ammonia. I think it fails for aromatic amines too like aniline, since the N is attached to the ring? Atleast that's what our sir said.

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  • [+][–]Tanvi Deshmukh12thSample· 1mo ago

    Farhan, aniline is fine in Kjeldahl, that one gets converted OK. What fails: nitro compounds, azo compounds, and nitrogen in the ring (like pyridine). In these N isn't converted to ammonium sulphate under those digestion conditions, so you under-measure.

    Dumas has no such issue because it doesn't depend on that chemistry at all. You heat the compound with excess CuO in a $CO_2$ atmosphere. Carbon and hydrogen go to $CO_2$ and $H_2O$, and the nitrogen comes out as $N_2$ (with a bit of oxides of nitrogen, which get reduced by a hot copper gauze). The gas mixture goes over aqueous KOH, which absorbs the $CO_2$, and the $N_2$ is collected and its volume measured.

    $\%N = \frac{28 \times V_1 \times 100}{22400 \times m}$

    where $V_1$ is volume of $N_2$ at STP in mL.

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    • [+][–]Farhan ShaikhDropperSample· 1mo ago

      ohh okay, so it's only the nitro/azo/ring ones. I'll fix my notes. thanks tanvi didi

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• [+][–]Ishita GoyalDropperSample· 1mo ago

  Small plan for this chapter, since it keeps getting asked in tests: keep three "families" in your head.

  1. Which method for N: Kjeldahl (not for nitro, azo, ring N) or Dumas (any).
  2. Lassaigne's test: sodium fusion makes NaCN, $Na_2S$, NaX. N gives Prussian blue (with $FeSO_4$, then conc. $H_2SO_4$ on boiling), S gives black PbS with lead acetate or violet with sodium nitroprusside. If both N and S are present you get blood-red $Fe(SCN)^{2+}$ instead of Prussian blue.
  3. Halogens: boil the fusion extract with dil. $HNO_3$ first to decompose NaCN and $Na_2S$, then add $AgNO_3$. White ppt (Cl, soluble in $NH_4OH$), pale yellow (Br, sparingly soluble), yellow (I, insoluble).

  Nitric acid step is the one everyone forgets.

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  • [+][–]Yash Tiwari11thSample· 1mo agoAuthor

    ishita didi ye wali list screenshot kar liya hai. nitric acid wala step pakka yaad rakhunga

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• [+][–]Lakshmi TripathiDropperSample· 1mo ago

  Also the purification part, in case your test has it. Steam distillation: for liquids that are immiscible with water and steam volatile (aniline is the textbook example). Distillation under reduced pressure: for liquids that decompose near their boiling point (glycerol). Fractional distillation: when boiling points are close. Simple distillation: when the difference is large and the liquid doesn't decompose.

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