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# Kjeldahl's method cannot be used for estimation of nitrogen in $(i)\; C_6H_5NO_2 ;\; (ii)\;Pyridine; \;(iii)\;C_6H_5 - N = N -C_6H_5 ;\;(iv)\;C_6H_5NHCOCH_3$

$(a)\;(i),(ii) and\;(iii) \\ (b) \; (ii),(iii)\;and \;(iv) \\ (c) \;(i),(ii)\;and \;(iv) \\ (d)\;(i),(iii)\;and (iv)$

Kjeldahl method is not applicable to compounds containing nitrogen in nitro and azo groups and nitrogen present in rings (e.g. pyridine) as nitrogen of these compounds does not convert to ammonium sulfate under the conditions of this method.
Since these compounds are not completely converted into $(NH_4)_2\;SO_4$ during digestion.
$(i),(ii) and\;(iii)$ cannot be used for estimation of nitrogen using Kjeldahl's method.
Hence a is the correct answer.
edited Mar 20, 2014