Modifications of the Micro-Kjeldahl Method Using Phosphinic Acid

Abstract

The kjeldahl metho isl apPlied to organic compounds ontaini: ng nitroge: n-oxygen, or nitrogen-nitrogen bonds by. pre-reduci-ng such types of nitrogen bonds-with phosphinic acid and converting them into. amino nitrogen before kjeldahlization.
When a sample (4f-v8 mg) was pre-reduced with 200 mg of NaH2PO2 in O.5 ml of concentrated H2SO4 over a micro. fia. me for 30 minutes, as shown in. Table 1, most nitro, nitroso, , azo, and aeid hydrazide compo unds could be analyzed accurately. However, hydrazine deriv atives and volatile compounds such as p-chloronitrobenzene an, d m-dinitrQ, benzene gave incortect results, because a large portion of the reducing agent was consumed for reduction of H2SO".
As indicated in Table 2, the use of HsPO2 in acetic acid tended to give better results than when NaH2PO2 was used in H2SO4 for the pre-treatment of 2, 4-dinitrophenylhydrazine, which contain. es nitrogen-oxygen a: nd ni ogen-nitrogen bonds. Further, an addition of HgO()and concentrated HCI as the amino acceptor, in t. he pre-treatment provided effective reduction.
The resultS in Toble 3, which were obtained by pte-treatment with O.2ml of HsPO2, O.2iml of concentrated HCI, O.2ml of AcOH, and 20 mg of HgO(ll) at 130"C for 1 hour, indicated that the pre-treatment gave reasonable data for P-chloronitrobenzene, m-dinitrobenzene, and 2, 4-dinitrophenylhydrazine. Howeveri smaller values were given for typical refractory com"po ds such as 2, 4-dinitrophenylhydra one, semicarbazo: ne, and heterocyclic compounds thah theJ eorresportding theoretical valtiest.
In order to rise the r. educing power, 50 mg of zinc dust was added to the mixture menti6ned.. above;. Highly accurate results were obtained. Not pnly hydrazine derivatives, but hqterocyclic compbunds such as 1, 4-phthalazinediqne, 1, 3, 4 xadiazole, and 1, 2, 3-benzotriazo1e, in which the rings contain nitrogenrnitrogen, bond, s, , . could be, analyzed accurately.