Kjeldahl nitrogen calculation method

Kjeldahl nitrogen calculation method, Total:177 items.

The international standard classification for "Kjeldahl nitrogen calculation method" includes: food products in general , Chemical laboratories. Laboratory equipment , Animal feeding stuffs .

The Chinese standard classification for "Kjeldahl nitrogen calculation method" includes: Food processing and products in general , Analysis methods for toxic substances in water , Analysis methods for environmental quality of soils , Electrochemical, thermochemistry and optical profile type analyzer , Livestock and poultry feed and additive .

Jilin Provincial Standard of the People's Republic of China

DB22/T 1932-2013 Determination method of total nitrogen content in peat (Kjeldahl method)

Group Standards of the People's Republic of China

T/NAIA 001-2020 Determination of ammoniacal nitrogen in fermented feed Kjeldahl method
T/NAIA 044-2021 Urine Total Nitrogen Determination in Livestock Kjeldahl Method
T/NAIA 060-2021 Determination of true protein in feed Kjeldahl method
T/QAS 071-2021 Determination of total nitrogen in brine Kjeldahl method

General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China

GB/T 5511-2008 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method
GB/T 11891-1989 Water quality--Determination of Kjeldahl nitrogen

未注明发布机构

ASTM RR-D19-1194 2016 D8083-Test Method for Total Nitrogen, and Total Kjeldahl Nitrogen (TKN) by Calculation, in Water by High Temperature Catalytic Combustion and Chemiluminescence Detection
DIN EN ISO 8968-1 E:2013-04 Determination of nitrogen content of milk and milk products Part 1: Principles of Kjeldahl method and calculation of crude protein (draft)
DIN EN ISO 8968-1:2002 Milk — Determination of nitrogen content — Part 1: Kjeldahl method (ISO 8968-1:2001)
DIN EN ISO 20483 E:2012-07 Determination of nitrogen content and calculation of crude protein content in cereals and legumes Kjeldahl method (draft)
ASTM RR-D31-1008 1996 D2868-Standard Test Method for Nitrogen Content (Kjeldahl) and Hide Substance Content of Leather

Ningxia Provincial Standard of the People's Republic of China

DB64/T 1734-2020 Soil Hydrolyzable Nitrogen Determination Kjeldahl Method

中华人民共和国环境保护部

GB 11891-1989 Water quility-Determination of Kjeldahl nitrogen

Professional Standard - Environmental Protection

HJ 717-2014 Soil quality - Determination of total nitrogen - Modified Kjeldahl method

中华人民共和国国家质量监督检验检疫总局、中国国家标准化管理委员会

GB/T 33862-2017 Automatic(semi) Kjeldahl analyzer

国家市场监督管理总局、中国国家标准化管理委员会

GB/T 6432-2018 Determination of crude protein in feeds—Kjeldahl method

Taiwan Provincial Standard of the People's Republic of China

CNS 3944-1976 Method for Determination of Total Kjeloahl Nitrogen in Polluted and Waste Water

IN-BIS

IS 5194-1969 Determination method of nitrogen-Kjeldahl method

Danish Standards Foundation

DS 242:1988 Water quality. Kjeldahl-nitrogen. Kjeldahl-method
DS/EN ISO 20483:2007 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method
DS/EN ISO 5983-1:2005 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
DS/EN ISO 5983-1/AC:2009 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
DS/EN ISO 8968-1:2002 Milk - Determination of nitrogen content - Part 1: Kjeldahl method
DS/EN 12135:1998 Fruit and vegetable juices - Determination of nitrogen content - Kjeldahl method
DS/ISO 5663:1994 Water quality - Determination of Kjeldahl nitrogen - Method after mineralization with selenium
DS/EN ISO 3188:1995 Starches and derived products - Determination of nitrogen content by the Kjeldahl method - Titrimetric method
DS/EN 25663:1994 Water quality-Determination of Kjeldahl nitrogen-Method after mineralization with selenium
DS/EN 13342:2001 Characterisation of sludges - Determination of total Kjeldahl nitrogen

British Standards Institution (BSI)

DD ISO/TS 17837:2008 Milk and milk products. Determination of nitrogen content and crude protein calculation. Kjeldahl method
BS EN ISO 5983-1:2005(2009) Animal feeding stuffs — Determination of nitrogen content and calculation of crude protein content — Part 1 : Kjeldahl method
BS 7755-3.7:1995 Soil quality. Chemical methods. Determination of total nitrogen. Modified Kjeldahl method
BS EN ISO 8968-1:2014 Milk and milk products. Determination of nitrogen content. Kjeldahl principle and crude protein calculation
BS EN 25663:1993 Water quality. Determination of Kjeldahl nitrogen. Method after mineralization with selenium
BS EN ISO 3188:1994(1999) Starches and derived products — Determination of nitrogen content by the Kjeldahl method — Titrimetric method
BS EN ISO 5378:1994(1999) Starches and derived products — Determination of nitrogen content by the Kjeldahl method — Spectrometric method
BS ISO 6851:2001 Photography. Processing waste. Determination of total amino nitrogen (microdiffusion Kjeldahl method)

SCC

BS DD ISO/TS 17837:2008 Milk and milk products. Determination of nitrogen content and crude protein calculation. Kjeldahl method
DANSK DS/EN ISO 20483:2014 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method
BS EN ISO 8968-1:2002 Milk. Determination of nitrogen content-Kjeldahl method
UNE-EN ISO 20483:2007 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method (ISO 20483:2006)
DANSK DS/EN ISO 5983-1:2005 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
NS-EN ISO 20483:2013 Cereals and pulses — Determination of the nitrogen content and calculation of the crude protein content — Kjeldahl method (ISO 20483:2013)
NS-EN ISO 20483:2006 Cereals and pulses — Determination of the nitrogen content and calculation of the crude protein content — Kjeldahl method (ISO 20483:2006)
12/30251275 DC BS EN ISO 20483. Cereals and pulses. Determination of the nitrogen content and calculation of the crude protein content. Kjeldahl method
AS 1871.11:1976 List of methods for examination of waters Nitrogen (total Kjeldahl incl. ammoniacal)
DANSK DS/EN 25663:1994 Water quality - Determination of Kjeldahl nitrogen - Method after mineralization with selenium
DANSK DS/ISO 5663:1994 Water quality - Determination of Kjeldahl nitrogen - Method after mineralization with selenium
DIN EN ISO 20483 E:2012 Draft Document - Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method (ISO/DIS 20483:2012); German version prEN ISO 20483:2012
DANSK DS/EN ISO 8968-1:2014 Milk and milk products - Determination of nitrogen content - Part 1: Kjeldahl principle and crude protein calculation
NS-EN 12135:1997 Fruit and vegetable juices - Determination of nitrogen content - Kjeldahl method
DANSK DS/EN 12135:1997 Fruit and vegetable juices - Determination of nitrogen content - Kjeldahl method
DANSK DS/EN ISO 3188:1994 Starches and derived products - Determination of nitrogen content by the Kjeldahl method - Titrimetric method
NS-EN ISO 8968-1:2014 Milk and milk products — Determination of nitrogen content — Part 1: Kjeldahl principle and crude protein calculation (ISO 8968-1:2014)
NS-EN 13342:2000 Characterization of sludges — Determination of Kjeldahl nitrogen
DANSK DS/ISO 1871:2009 Food and feed products - General guidelines for the determination of nitrogen by the Kjeldahl method
AENOR UNE-EN 13342:2001 Characterization of sludges - Determination of Kjeldahl nitrogen
AENOR UNE-EN 25663:1994 WATER QUALITY. DETERMINATION OF KJELDAHL NITROGEN. METHOD AFTER MINERALIZATION WITH SELENIUM. (ISO 5663:1984).
BS 5697-1:1979 Methods of test for starch and its derivatives and by-products. Determination of nitrogen conteny by the Kjeldahl method ( titrimetric method)
NS-EN 25663:1993 Water quality — Determination of Kjeldahl nitrogen — Method after mineralization with selenium (ISO 5663:1984)
NS-EN ISO 3188:1994 Starches and derived products — Determination of nitrogen content by the Kjeldahl method — Titrimetric method (ISO 3188:1978)
DIN EN ISO 8968-1 E:2013 Draft Document - Milk and milk products - Determination of nitrogen content and crude protein calculation - Part 1: Kjeldahl principle (ISO/DIS 8968-1:2013); German version prEN ISO 8968-1:2013

International Organization for Standardization (ISO)

ISO/TS 17837:2008 | IDF/RM 25: 2008 Processed cheese products — Determination of nitrogen content and crude protein calculation — Kjeldahl method
ISO 332:1974 Title missing - Legacy paper document
ISO 332:1981 Coal — Determination of nitrogen — Macro Kjeldahl method
ISO 5983:1997 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Kjeldahl method
ISO 8968-1:2014 | IDF 20-1:2014 Milk and milk products — Determination of nitrogen content — Part 1: Kjeldahl principle and crude protein calculation
ISO 8968-1:2001 | IDF 20-1: 2001 Milk — Determination of nitrogen content — Part 1: Kjeldahl method
ISO 5663:1984 Water quality; Determination of Kjeldahl nitrogen; Method after mineralization with selenium
ISO 8968-1:2014 Milk and milk products - Determination of nitrogen content - Part 1: Kjeldahl principle and crude protein calculation

NL-NEN

NEN 3760-1984 Cheese - Determination of the nitrogen content according to Kjeldahl - Calculation of the crude protein content
NEN 3198-1984 Milk, evaporated milk and dried milk - Determination of the nitrogen content according to Kjeldahl - Calculation of the crude protein content (reference method)

SE-SIS

SIS SS 18 71 63-1984 Coal - Determination of nitrogen - Macro Kjeldahl method

BG-BDS

BGC ISO 5983:2001 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Kjeldahl method

GSO

GSO ISO 20483:2013 Cereals and pulses -- Determination of the nitrogen content and calculation of the crude protein content -- Kjeldahl method
OS GSO ISO 20483:2013 Cereals and pulses -- Determination of the nitrogen content and calculation of the crude protein content -- Kjeldahl method
OS GSO ISO 5983-1:2007 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
GSO ISO 5983-1:2007 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
OS GSO ISO 8968-1:2014 Milk and milk products -- Determination of nitrogen content -- Part 1: Kjeldahl principle and crude protein calculation
OS GSO ISO 11261:2015 Soil quality -- Determination of total nitrogen -- Modified Kjeldahl method
GSO ISO 11261:2015 Soil quality -- Determination of total nitrogen -- Modified Kjeldahl method
BH GSO ISO 11261:2017 Soil quality -- Determination of total nitrogen -- Modified Kjeldahl method
GSO ISO 1871:2008 Agricultural food products — General directions for the determination of nitrogen by the Kjeldahl method
GSO ISO 5663:2015 Water quality -- Determination of Kjeldahl nitrogen -- Method after mineralization with selenium
BH GSO ISO 5663:2016 Water quality -- Determination of Kjeldahl nitrogen -- Method after mineralization with selenium
OS GSO ISO 1871:2008 Agricultural food products — General directions for the determination of nitrogen by the Kjeldahl method
GSO ISO 3188:2015 Starches and derived products -- Determination of nitrogen content by the Kjeldahl method -- Titrimetric method

Association Francaise de Normalisation

NF EN ISO 20483:2014 Cereals and legumes - Determination of nitrogen content and calculation of crude protein content - Kjeldahl method
NF V04-211:1971 MILK. DETERMINATION OF TOTAL NITROGEN CONTENT. (KJELDAHL METHOD).
NF EN ISO 5983-1:2005 Animal feed - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method
NF EN 12135:1997 Fruit and vegetable juices - Nitrogen dosage - Kjeldahl method
NF V03-750:2007 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method.
NF V04-221-1*NF EN ISO 8968-1:2014 Milk and milk products - Determination of nitrogen content - Part 1 : Kjeldahl principle and crude protein calculation
NF EN ISO 8968-1:2014 Milk and dairy products - Determination of nitrogen content - Part 1: Kjeldahl method and calculation of crude protein content
NF EN 25663:1994 Water quality - Kjeldahl nitrogen dosage - Method after selenium mineralization
NF ISO 11261:1995 Soil quality - Total nitrogen dosage - Modified Kjeldahl method
NF EN ISO 3188:1994 Starches and derived products - Determination of nitrogen according to the Kjeldahl method - Titrimetric method
NF EN 13342:2000 Characterization of sludge - Determination of Kjeldahl nitrogen
NF X31-111*NF ISO 11261:1995 Soil quality. Determination of total nitrogen. Modified kjeldahl method.

German Institute for Standardization

DIN EN ISO 20483:2014-03 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method (ISO 20483:2013); German version EN ISO 20483:2013
DIN EN ISO 5983-1:2005-10 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method (ISO 5983-1:2005); German version EN ISO 5983-1:2005
DIN EN ISO 5983-1 Berichtigung 1:2009-07 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method (ISO 5983-1:2005); German version EN ISO 5983-1:2005, Corrigendum to DIN EN ISO 5983-1:2005-10; German version EN ISO 5983-1:20...
DIN EN ISO 8968-1:2014-06 Milk and milk products - Determination of nitrogen content - Part 1: Kjeldahl principle and crude protein calculation (ISO 8968-1:2014); German version EN ISO 8968-1:2014
DIN EN 12135:1997-12 Fruit and vegetable juices - Determination of nitrogen content - Kjeldahl method; German version EN 12135:1997
DIN EN 25663:1993-11 Water quality; determination of Kjeldahl nitrogen; method after mineralization with selenium (ISO 5663:1984); German version EN 25663:1993
DIN EN ISO 20483:2014 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method (ISO 20483:2013); German version EN ISO 20483:2013
DIN EN 13342:2001-01 Characterization of sludges - Determination of Kjeldahl nitrogen; German version EN 13342:2000
DIN 51722-1:1990-07 Testing of solid fuels; determination of nitrogen content; semi-micro Kjeldahl method

Indonesia Standards

SNI 4146-2013 Method for determination of total nitrogen content in sediment by Kjeldahl titration method

American Society for Testing and Materials (ASTM)

ASTM D3590-17 Standard Test Methods for Total Kjeldahl Nitrogen in Water
ASTM D3590-11 Standard Test Methods for Total Kjeldahl Nitrogen in Water
ASTM D3590-02 Standard Test Methods for Total Kjeldahl Nitrogen in Water
ASTM E147-61(2017) Standard Specification for Apparatus for Microdetermination of Nitrogen by Kjeldahl Method
ASTM D8001-16e1 Standard Test Method for Determination of Total Nitrogen, Total Kjeldahl Nitrogen by Calculation, and Total Phosphorus in Water, Wastewater by Ion Chromatography
ASTM D8001-16 Standard Test Method for Determination of Total Nitrogen, Total Kjeldahl Nitrogen by Calculation, and Total Phosphorus in Water, Wastewater by Ion Chromatography
ASTM D8083-16 Standard Test Method for Total Nitrogen, and Total Kjeldahl Nitrogen (TKN) by Calculation, in Water by High Temperature Catalytic Combustion and Chemiluminescence Detection
ASTM D8083-16(2023) Standard Test Method for Total Nitrogen, and Total Kjeldahl Nitrogen (TKN) by Calculation, in Water by High Temperature Catalytic Combustion and Chemiluminescence Detection

International Dairy Federation (IDF)

IDF/RM 25-2008 Milk and milk products - Determination of nitrogen content and crude protein calculation - Kjeldahl method
IDF 20-1-2014 Milk and milk products - Determination of nitrogen content - Part 1: Kjeldahl principle and crude protein calculation
IDF 20-1-2001 Milk - Determination of nitrogen content - Part 1: Kjeldahl method

RU-GOST R

GOST ISO/TS 17837-2013 Processed cheese products. Determination of nitrogen content and crude protein calculation. Kjeldahl method
GOST 34111-2017 Juice products. Determination of nitrogen content by Kjeldahl method
GOST 26889-1986 Food-stuffs and food additives. General directions for the determinations of nitrogen content by the Kjeldahl method
GOST R 55246-2012 Milk and milk products. Determination of non-protein nitrogen content by Kjeldahl method

Lithuanian Standards Office

LST EN ISO 20483:2007 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method (ISO 20483:2006)
LST EN 12135-2001 Fruit and vegetable juices - Determination of nitrogen content - Kjeldahl method
LST EN ISO 5983-1:2005 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method (ISO 5983-1:2005)
LST EN ISO 5983-1:2005/AC:2009 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method (ISO 5983-1:2005/Cor 1:2008)
LST EN 13342-2002 Characterization of sludges - Determination of Kjeldahl nitrogen
LST EN ISO 3188:2000 Starches and derived products - Determination of nitrogen content by the Kjeldahl method - Titrimetric method (ISO 3188:1978)
LST EN ISO 8968-1:2002 Milk - Determination of nitrogen content - Part 1: Kjeldahl method (ISO 8968-1:2001)

Korean Agency for Technology and Standards (KATS)

KS E ISO 333-2018 Coal－Determination of nitrogen－Semi-micro Kjeldahl method
KS E ISO 333-2003(2013) Coal－Determination of nitrogen－Semi-micro Kjeldahl method
KS I ISO 11261-2008(2018) Soil quality－Determination of total nitrogen－Modified Kjeldahl method
KS I ISO 11261-2008(2021) Soil quality－Determination of total nitrogen－Modified Kjeldahl method
KS I ISO 11261-2021 Soil quality－Determination of total nitrogen－Modified Kjeldahl method
KS H ISO 3188-2023 Starches and derived products－Determination of nitrogen content by the Kjeldahl method－Titrimetric method
KS H ISO 3188-2008(2018) Starches and derived products－Determination of nitrogen content by the Kjeldahl method－Titrimetric method
KS H ISO 1871-2023 Agricultural food products ― General directions for the determination of nitrogen by the Kjeldahl method
KS I ISO 5663-2007(2017) Water quality－Determination of Kjeldahl nitrogen－Method after mineralization with selenuium
KS H ISO 5378-2023 Starches and derived products－Determination of nitrogen content by the Kjeldahl method－Spectrophotometric method
KS H ISO 5378-2008(2018) Starches and derived products－Determination of nitrogen content by the Kjeldahl method－Spectrophotometric method
KS I ISO 5663:2007 Water quality－Determination of Kjeldahl nitrogen－Method after mineralization with selenuium

AENOR

UNE-EN ISO 20483:2014 Cereals and pulses - Determination of the nitrogen content and calculation of the crude protein content - Kjeldahl method (ISO 20483:2013)
UNE-EN ISO 5983-1:2007 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method (ISO 5983-1:2005)
UNE 77318:2001 Soil quality. Determination of total nitrogen. Modified Kjeldahl method.
UNE-EN 12135:1998 Fruit and vegetable juices - Determination of nitrogen content - Kjeldahl method
UNE 32013:1995 COAL. DETERMINATION OF NITROGEN. SEMI-MICRO KJELDAHL METHOD.
UNE-EN ISO 8968-1:2014 Milk and milk products - Determination of nitrogen content - Part 1: Kjeldahl principle and crude protein calculation (ISO 8968-1:2014)
UNE-EN 25663:1994 WATER QUALITY. DETERMINATION OF KJELDAHL NITROGEN. METHOD AFTER MINERALIZATION WITH SELENIUM. (ISO 5663:1984).
UNE-EN ISO 3188:1995 STARCHES AND DERIVED PRODUCTS. DETERMINATION OF NITROGEN CONTENT BY THE KJELDAHL METHOD. TITRIMETRIC METHOD. (ISO 3188:1978).

TH-TISI

TIS 2255.1-2005 Animal feeding stuff s-determination of nitrogen content and calculation of crude protein content.part 1: kjeldahl method

ES-UNE

UNE-EN ISO 5983-1:2007/AC:2009 Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method (ISO 5983-1:2005/Cor 1:2008)
UNE-EN 13342:2001 Characterization of sludges - Determination of Kjeldahl nitrogen

ZA-SANS

SANS 333:1996 Coal - Determination of nitrogen - Semi-micro Kjeldahl method
SANS 8968-1:2006 Milk - Determination of nitrogen content Part 1: Kjeldahl method

Association of Official Analytical Chemists (AOAC)

AOAC 991.20 AOAC Official Method 991.20 Nitrogen (Total) in Milk Kjeldahl Methods

PL-PKN

PN C04576 ArkusZ12-1973 Water and waste water Tests for nitrogenDetermination of total nitrogen of Kjeldahl

VN-TCVN

TCVN 6498-1999 Soil quality.Determination of total nitrogen.Modifierd Kejldahl method
TCVN 4328-1-2007 Animal feeding stuffs.Determination of nitrogen content and calculation of crude protein content.Part 1: Kjeldahl method
TCVN 6014-2007 Hard coal.Determination of nitrogen.Semi-micro Kjeldahl method
TCVN 5987-1995 Water quality.Determination of Kjeldahl nitrogen.Method after mineralization with selenium

KR-KS

KS H ISO 1871-2013(2023) Agricultural food products ― General directions for the determination of nitrogen by the Kjeldahl method
KS H ISO 3188-2008(2023) Starches and derived products－Determination of nitrogen content by the Kjeldahl method－Titrimetric method
KS I ISO 5663-2023 Water quality — Determination of Kjeldahl nitrogen — Method after mineralization with selenium
KS H ISO 5378-2008(2023) Starches and derived products－Determination of nitrogen content by the Kjeldahl method－Spectrophotometric method

Standard Association of Australia （SAA）

AS ISO 8968.1:2021 Milk and milk products — Determination of nitrogen content, Part 1: Kjeldahl principle and crude protein calculation

BE-NBN

NBN T 91-255-1976 Water analysis methods - Determination of the ammoniacal and organic nitrogen - (Kjeldahl nitrogen)

CZ-CSN

CSN 56 0020-1987 General directions for the determination f nitrogen by the Kjeldahl method
CSN ISO 1871:1994 Agricultural food products-General directions for the determination of nitrogen by the Kjeldahl method

GOSTR

GOST 34454-2018 Dairy products. Determination of protein content by the Kjeldahl method

GOST

GOST 2408.2-1975 Brown coals, hard coals, anthracite, combustible shales and turf. Method for thedetermination of nitrogen content
GOST 34789-2021 Brewing products. Identification. Determination of mass concentration of total nitrogen by Kjeldahl method

PT-IPQ

NP 1986-1991 Milks Determination of protein content. Kjeldahl technique

AT-ON

ONORM L 1082-1999 Chemical analyses of soils - Determination of nitrogen according to Kjeldahl

YU-JUS

JUS B.H8.320-1988 Methods of analysis of coal and coke. Determination of nitrogen ccmtent - Makro Kjeldahl nethod

RO-ASRO

STAS SR 13348-1-1996 Tea. Analysis methods. Determination of proteic nitrogen by the Kjeldahl method. Spectrophotometric method