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348-EN, General section

Copper sulphate as alternative to mercury monoxide catalyst in the Kjeldahl reaction


The  Kjeldahl reaction developed to analyse nitrogen content of organic substances has been reviewed many times to optimize it and make safer for the users. A comprehensive handbook on various Kjeldahl analyses includes a chapter on catalysts that presents alternatives to mercury monoxide.

Substituted substance(s)

  1. Mercury monoxide

    CAS No. 21908-53-2 EC No. 244-654-7 Index No. 

Alternative substance(s)

  1. Copper sulfate pentahydrate

    CAS No. 7758-98-7 EC No. 231-847-6 Index No. 029-004-00-0

Other type of alternative

Reliability of information

Internet information: data are from an internet document and only a basic and partial evaluation could be performed

Hazard assessment

Mercury monoxide is  not included in the EU harmonized classification.  It is self classified as very toxic for aquatic environment,  and toxic to humans (mercury itself being classified EU reprotoxic 1B).

Copper sulphate is classified as  harmful if swallowed, skin and eye irritant  and very toxic to aquatic environment.


» Check the Substance Database according to SUBSPORT Screening Criteria (SDSC)

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Substitution description

Kjeldahl reaction, very used in studying protein content in organic matter, initially used mercury monoxide to catalyse sample digestion. Awareness on the hazards for health and environment as well as some process drawbacks lead to the substitution of mercuric monoxide (mercuric oxide). Copper and selenium compounds or their combinations and more recently titanium catalysts are  considered safer alternative, with copper sulphate currently being the most commonly used.
The handbook containing information on catalyst properties notes that the alternatives give good results, even if the reaction time might be slightly longer than for mercury.
Other practical data on the various application of the Kjeldahl reaction and how to carry on different steps of the analysis are presented in the handbook.

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Case/substitution evaluation

The substitution eliminates the use of a mercury compound that is fatal by inhalation, ingestion and skin contact and that has negative impact on the environment. Since the alternative is hazardous to the aquatic environment, protective measures need to be taken.

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Other solutions

Selenium reaction mixture (see Further information)

Alternative analytic methods ( see Further information)

Further information

– Mercury monoxide MSDS from Sigma Aldrich

– Selenium reaction mixture MSDS from Sigma Aldrich

– Alternative method : 

Further languages available

Who provided the information

Type of information supplier
Producer / distributor


FOSS, DK-3400 Hilleroed, Denmark,

Publication source

The description is based on the document ’Handbook for Kjeldahl Digestion A recent review of the classical method with improvements developed by FOSS’ , published by FOSS, DK-3400 Hilleroed, Denmark , on its website, in 2008.

Type of publication and availability 

Internet brochure, freely available

Original document: click here

Comments from SUBSPORT users

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Publication or last update: January 2013