Which is the suitable method for detection of Nitrogen present in food and fertilizers?

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Q. Which is the suitable method for detection of Nitrogen present in food and fertilizers?

Tamil Nadu DgeTamil Nadu HSC First Year (DGE) Board 2019Subjective· 2mImportance★★★★★

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Concept understanding — Estimation of Elements in Organic Compounds

Once an element's presence is confirmed qualitatively, its percentage BY WEIGHT is found by a dedicated quantitative method. Oxygen alone has no direct method -- its percentage is always found by DIFFERENCE (100% minus every other measured element).

Carbon and hydrogen are estimated together by combustion: a known weight $w$ of sample is burnt completely in excess oxygen, and the masses of water ( $x$ g) and $CO_2$ ( $y$ g) formed are weighed via an absorption train (conc. $H_2SO_4$ -soaked pumice for water, then KOH bulbs for $CO_2$ ). Since 18 g $H_2O$ contains 2 g H, $\%H = \frac{2}{18}\times\frac{x}{w}\times100$ ; since 44 g $CO_2$ contains 12 g C, $\%C = \frac{12}{44}\times\frac{y}{w}\times100$ .

Sulphur (Carius method): the sample is heated with fuming $HNO_3$ in a sealed tube, oxidising S to $H_2SO_4$ , which is precipitated with excess $BaCl_2$ as $BaSO_4$ (mass $x$ from sample mass $w$ ). Since 233 g $BaSO_4$ contains 32 g S, $\%S = \frac{32}{233}\times\frac{x}{w}\times100$ .

Halogens (Carius method): heated with fuming $HNO_3$ + $AgNO_3$ ; the halogen precipitates directly as $AgX$ . $\%Cl = \frac{35.5}{143.5}\times\frac{a}{w}\times100$ ; $\%Br = \frac{80}{188}\times\frac{b}{w}\times100$ ; $\%I = \frac{127}{235}\times\frac{c}{w}\times100$ (a/b/c = mass of AgCl/AgBr/AgI).

Phosphorus (Carius method): P oxidises to phosphoric acid, precipitated either as ammonium phosphomolybdate, $(NH_4)_3PO_4{\cdot}12MoO_3$ (1877 g/mol; $\%P = \frac{31}{1877}\times\frac{x}{w}\times100$ ), or, after igniting a magnesium-ammonium phosphate precipitate, as magnesium pyrophosphate, $Mg_2P_2O_7$ (222 g/mol; $\%P = \frac{62}{222}\times\frac{y}{w}\times100$ ). …

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Kjeldahl's method is used to quantitatively estimate nitrogen in samples like food and fertilizers.

While Lassaigne's/Duma's methods are used to detect or estimate nitrogen in general organic compounds, Kjeldahl's method is specifically well suited to nitrogen-containing biological and agricultural materials such as food, milk, and fertilizers, because it works reliably on proteins, amides, and similar nitrogenous compounds (it is not suitable for compounds where nitrogen is present as a nitro group, azo group, or in a ring, e.g. pyridine).

Procedure:

Digestion: The sample is heated with concentrated sulfuric acid (often with a catalyst such as CuSO4/K2SO4), which converts the organically bound nitrogen into ammonium sulfate, (NH4)2SO4.
Distillation: The digested mixture is then treated with excess sodium hydroxide, which liberates ammonia gas (NH3). This ammonia is distilled over and collected/absorbed in a known, excess volume of standard acid (e.g. HCl or H2SO4). …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 mark
$Q. Write true or false: Kjeldahl's method is estimation method of halogen.$
›Reveal solutionSolution

False. Kjeldahl's method estimates nitrogen content in organic compounds; halogens are estimated by the Carius method.

In Kjeldahl's method, a known mass of an organic compound is heated with concentrated sulphuric acid, which converts the nitrogen present into ammonium sulphate. This is then treated with excess sodium hydroxide to liberate ammonia gas, which is absorbed in a known volume of standard acid; back-titration gives the amount of nitrogen present. This method is specific for nitrogen (it fails for nitro compounds and compounds with nitrogen in the ring, like pyridine, which need a modified procedure).

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Halogen is estimated by:$
(a) Carius method
(b) Duma method
(c) Lassaigne test
(d) Kjeldahl method
›Reveal solutionSolution
Halogens are quantitatively estimated by Carius method; Duma is for nitrogen, Lassaigne's test is a qualitative test, and Kjeldahl is for nitrogen estimation.

In the Carius method, a known mass of the organic compound is heated with fuming HNO3 in the presence of AgNO3 in a sealed hard glass (Carius) tube. The halogen present is converted to silver halide (AgX), which is filtered, washed, dried, and weighed to calculate the percentage of halogen.

Duma's method estimates nitrogen (by volume of N2 gas). …
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CBSE 2020Set ANNUAL1 mark
$Q. Answer in one word/sentence: By which method is the estimation of Nitrogen carried out?$
›Reveal solutionSolution

Estimation of nitrogen in an organic compound is carried out by Kjeldahl's method, which converts the compound's nitrogen into ammonium sulfate and measures the ammonia released on treatment with alkali.

In Kjeldahl's method, a known mass of the organic compound is heated with concentrated sulfuric acid, which converts the nitrogen present into ammonium sulfate, (NH4)2SO4. This solution is then heated with excess sodium hydroxide, which liberates ammonia gas. The liberated ammonia is passed into and absorbed by a known, excess volume of a standard acid, and the unreacted acid is then determined by back-titration with a standard alkali. From this, the mass of nitrogen (and hence its percentage) in the original compound is calculated.

…

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CBSE 2019Set ANNUAL1 markMCQ
$Q. Match the columns — Column A: (D) Kjeldahl's method. Choose the correct matching term from Column B.$
(i) Estimation of nitrogen
(ii) Estimation of halogen
(iii) V ∝ T
(iv) Concept of Ionisation
(v) Gain of electrons
›Reveal solutionSolution

Kjeldahl's method is used for the quantitative estimation of nitrogen in an organic compound, by converting the nitrogen to ammonium sulphate and then to ammonia, which is estimated by titration.

In Kjeldahl's method, a known mass of the organic compound is heated with concentrated sulphuric acid, which converts the nitrogen present into ammonium sulphate. This is then heated with excess NaOH to liberate ammonia gas, which is absorbed in a known volume of standard acid; the amount …

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CBSE 2019Set ANNUAL1 markMCQ
$Q. Match the columns — Column A: (E) Carius method. Choose the correct matching term from Column B.$
(i) Estimation of nitrogen
(ii) Estimation of halogen
(iii) V ∝ T
(iv) Concept of Ionisation
(v) Gain of electrons
›Reveal solutionSolution

Carius method estimates halogens (also sulphur and phosphorus) in an organic compound by heating it with fuming nitric acid and silver nitrate in a sealed tube, converting the halogen into a precipitate of silver halide that is weighed.

In the Carius method, a known mass of the organic compound is heated with fuming nitric acid in the presence of silver nitrate inside a sealed Carius tube. This oxidises the organic matter and converts the halogen present into the corresponding silver halide (AgCl, AgBr, or …

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CBSE 2018Set ANNUAL1 markMCQ
$Q. Which one of the following method is used to estimate Nitrogen?$
(a) Kjeldahl method
(b) Carius method
(c) Lassaigne's test
(d) Liebig's combustion method
›Reveal solutionSolution

The Kjeldahl method is used to estimate nitrogen in an organic compound.

In the Kjeldahl method, a known mass of the organic compound is heated with concentrated sulphuric acid (along with K2SO4 and a trace of CuSO4 as catalyst), which converts the nitrogen present into ammonium sulphate. This is then heated with excess NaOH to liberate ammonia gas, which is absorbed in a known, excess volume of a standard acid (e.g., HCl). The unreacted acid is back-titrated with a standard alkali, and the amount of acid neutralised by the liberated ammonia gives the percentage of nitrogen in the compound. ( …

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Q.Which is the suitable method for detection of Nitrogen present in food and fertilizers?

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