Kjeldahl method
Encyclopedia
The Kjeldahl method or Kjeldahl digestion (t͡ʃɛldɑːl) in analytical chemistry
is a method for the quantitative determination of nitrogen
in chemical substance
s developed by Johan Kjeldahl
in 1883.
, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as ammonium sulfate
. In this step potassium sulfate
is added to increase the boiling point
of the medium (from 337°F to 373°F / 169°C to 189°C). Chemical decomposition
of the sample is complete when the medium has become clear and colorless (initially very dark).
The solution is then distilled with sodium hydroxide (added in small quantities) which converts the ammonium salt to ammonia
. The amount of ammonia present (hence the amount of nitrogen present in the sample) is determined by back titration. The end of the condenser is dipped into a solution of boric acid
. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate
solution with a methyl orange
pH indicator
.
Nowadays, the Kjeldahl method is largely automated and makes use of specific catalysts (mercury oxide
or copper sulfate) to speed up the decomposition.
and the 2008 Chinese milk powder scandal
, when melamine
, a nitrogen-rich chemical, was added to raw materials to fake high protein contents. Also, different correction factors are needed for different proteins to account for different amino acid sequences. Additional disadvantages, such as the need to use concentrated sulfuric acid at high temperature and the relatively long testing time (an hour or more), compare unfavorably with the Dumas method
for measuring crude protein content.
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
is a method for the quantitative determination of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
in chemical substance
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
s developed by Johan Kjeldahl
Johan Kjeldahl
Johan Gustav Christoffer Thorsager Kjeldahl , was a Danish chemist who developed a method for determining the amount of nitrogen in certain organic compounds using a laboratory technique which was named the Kjeldahl method after him....
in 1883.
Method
The method consists of heating a substance with sulfuric acidSulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
. In this step potassium sulfate
Potassium sulfate
Potassium sulfate is a non-flammable white crystalline salt which is soluble in water...
is added to increase the boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of the medium (from 337°F to 373°F / 169°C to 189°C). Chemical decomposition
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
of the sample is complete when the medium has become clear and colorless (initially very dark).
The solution is then distilled with sodium hydroxide (added in small quantities) which converts the ammonium salt to ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
. The amount of ammonia present (hence the amount of nitrogen present in the sample) is determined by back titration. The end of the condenser is dipped into a solution of boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
solution with a methyl orange
Methyl orange
Methyl orange is a pH indicator frequently used in titrations.It is often used in titrations because of its clear and distinct colour change. Because it changes colour at the pH of a mid-strength acid, it is usually used in titrations for acids...
pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
.
- Degradation: Sample + H2SO4 → (NH4)2SO4(aq) + CO2(g) + SO2(g) + H2O(g)
- Liberation of ammonia: (NH4)2SO4(aq) + 2NaOH → Na2SO4(aq) + 2H2O(l) + 2NH3(g)
- Capture of ammonia: B(OH)3 + H2O + NH3 → NH4+ + B(OH)4–
- Back-titration: B(OH)3 + H2O + Na2CO3 → NaHCO3(aq) + NaB(OH)4(aq) + CO2(g) + H2O
Nowadays, the Kjeldahl method is largely automated and makes use of specific catalysts (mercury oxide
Mercury oxide
Mercury oxide can refer to:* Mercury oxide , Hg2O* Mercury oxide , HgO...
or copper sulfate) to speed up the decomposition.
Applications
The Kjeldahl method's universality, precision and reproducibility have made it the internationally-recognized method for estimating the protein content in foods and it is the standard method against which all other methods are judged. It does not, however, give a measure of true protein content, as it measures nonprotein nitrogen in addition to the nitrogen in proteins. This is evidenced by the 2007 pet food incident2007 pet food recalls
The 2007 pet food recalls comprise the contamination and wide recall of many brands of cat and dog foods beginning in March 2007, and the ensuing developments involving the human food supply. The recalls in North America, Europe, and South Africa came in response to reports of renal failure in pets...
and the 2008 Chinese milk powder scandal
2008 Chinese milk scandal
The 2008 Chinese milk scandal was a food safety incident in the People's Republic of China, involving milk and infant formula, and other food materials and components, adulterated with melamine....
, when melamine
Melamine
Melamine is an organic base and a trimer of cyanamide, with a 1,3,5-triazine skeleton. Like cyanamide, it contains 66% nitrogen by mass and, if mixed with resins, has fire retardant properties due to its release of nitrogen gas when burned or charred, and has several other industrial uses....
, a nitrogen-rich chemical, was added to raw materials to fake high protein contents. Also, different correction factors are needed for different proteins to account for different amino acid sequences. Additional disadvantages, such as the need to use concentrated sulfuric acid at high temperature and the relatively long testing time (an hour or more), compare unfavorably with the Dumas method
Dumas method
The Dumas method in analytical chemistry is a method for the quantitative determination of nitrogen in chemical substances based on a method first described by Jean-Baptiste Dumas over a century and a half ago....
for measuring crude protein content.
Conversion factors
Conversion factors for common foods range from 6.38 for dairy and 6.25 for meat, eggs, corn (maize) and sorghum to 5.83 for most grains, 5.70 for wheat flour and 5.46 for peanuts.See also
- Total Kjeldahl nitrogenTotal Kjeldahl NitrogenTotal Kjeldahl Nitrogen or TKN is the sum of organic nitrogen, ammonia , and ammonium in the chemical analysis of soil, water, or wastewater...
- Dumas methodDumas methodThe Dumas method in analytical chemistry is a method for the quantitative determination of nitrogen in chemical substances based on a method first described by Jean-Baptiste Dumas over a century and a half ago....
, another nitrogen analysis method - Devarda's alloyDevarda's alloyDevarda's alloy , is an alloy of aluminium , copper and zinc ....
, a powerful reducing agent for nitrate analysis