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This standard is developed in accordance with the rules given in GB/T 1.1-2009.
This standard replaces GB/T 6432-1994 Method for the determination of crude protein in feedstuffs.
In addition to a number of editorial changes, the following main technical changes have been made with respect to GB/T 6432-1994:
——The standard name is modified (see the title page);
——The application scope is modified (see Clause 1);
——The requirements for significant figures for calculation results are added (see Clause 8).
This standard was proposed by and is under the jurisdiction of SAC/TC 76 National Technical Committee on Feed Industry of Standardization Administration of China.
The previous editions of this standard are as follows:
——GB 6432-1986, GB/T 6432-1994.
Determination of crude protein in feeds - Kjeldahl method
1 Scope
This standard specifies the determination of crude protein in feeds - Kjeldahl method.
This standard is applicable to the determination of crude protein in feed materials, formula feed, concentrated feed, concentrate supplement and additive premixed feed.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 601 Chemical reagent - Preparations of reference titration solutions
GB/T 6682 Water for analytical laboratory use - Specification and test methods
GB/T 14699.1 Feeding stuffs - Sampling
GB/T 20195 Animal feeding stuffs - Preparation of test samples
3 Principles
Under the action of catalyst, the sample is digested by sulfuric acid, and the nitrogen-containing compounds are converted into ammonium sulfate, which is distilled by adding alkali to it for escape of ammonia. After the ammonia is absorbed by boric acid, the sample is titrated with hydrochloric acid standard titration solution, and the nitrogen content is measured and multiplied by 6.25 to calculate the crude protein content.
4 Reagents and materials
Unless otherwise specified, only analytical reagents are used.
4.1 Water: GB/T 6682, Grade III.
4.2 Boric acid: chemically pure.
4.3 Sodium hydroxide: chemically pure.
4.4 Sulfuric acid: chemically pure.
4.5 Ammonium sulphate.
4.6 Sucrose.
4.7 Mixed catalyst: weigh 0.4g of copper sulfate pentahydrate, 6.0g of potassium sulfate or sodium sulfate, grind and mix evenly; or buy commercial Kjeldahl catalyst tablets.
4.8 Boric acid absorption solution I: weigh 20g of boric acid, dissolve in water and dilute to 1,000mL.
4.9 Boric acid absorption solution II: 1,000mL of 1% boric acid aqueous solution, add 10mL of 0.1% bromocresol green ethanol solution, 7mL of 0.1% methyl red ethanol solution, and 0.5mL of 4% sodium hydroxide aqueous solution, mix well, and keep it for one month at room temperature (for fully automated program).
4.10 Sodium hydroxide solution: weigh 40g of sodium hydroxide, dissolve in water, cool to room temperature and dilute to 100mL with water.
4.11 Hydrochloric acid standard titration solution: c(HCI)=0.1mol/L or 0.02mol/L, which is prepared and calibrated according to the requirements of GB/T 601.
4.12 Methyl red ethanol solution: weigh 0.1g of methyl red, dissolve and dilute it to 100mL with ethanol.
4.13 Bromocresol green ethanol solution: weigh 0.5g of bromocresol green, dissolve and dilute it to 100mL with ethanol.
4.14 Mixed indicator solution: mix methyl red ethanol solution and bromocresol green ethanol solution in equal volume. Store the solution in the dark place at room temperature for a validity period of 3 months.
5 Apparatuses
5.1 Analytical balance: with a sensitivity of 0.0001g.
5.2 Boiling stove or electric stove.
5.3 Kjeldahl flask: 250mL.
5.4 Boiling tube: 250mL.
5.5 Kjeldahl distillation unit: constant direct distillation type or semi-micro steam distillation type.
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5.6 Azotometer: semi-automatic and full-automatic azotometer of various types manufactured by Kjeldahl principle.
6 Samples
Take representative feed samples according to GB/T 14699.1, and reduce the samples by the quartering method. Prepare specimens according to those specified in GB/T 20195, crush them, and make them pass through a 0.42mm sieve, mix them evenly, put them into a sealed container, and store them in dark for subsequent use.
Foreword II
1 Scope
2 Normative references
3 Principles
4 Reagents and materials
5 Apparatuses
6 Samples
7 Test steps
8 Test data processing
9 Precision
