AS ISO 8968.1 pdf download

AS ISO 8968.1 pdf download.Milk and milk products — Determination of nitrogen content
1  Scope
This International Standard specifies a method for the determination of the nitrogen content and crude protein calculation of milk and milk products by the Kjeldahl principle, using traditional and block digestion methods. The methods are applicable to: — liquid cow’s (whole, partially skimmed or skimmed milk), goat’s and sheep’s whole milk; — hard, semi-hard and processed cheese; — dried milk and dried milk products (including milk-based infant formulae, milk protein concentrate, whey protein concentrate, casein and caseinate). The methods are not applicable to samples containing ammonium caseinate. NOTE  Inaccurate crude protein results will be obtained if non-milk sources of nitrogen are present in the products specified in this International Standard.
4  Principle
A test portion is digested with a mixture of concentrated sulfuric acid and potassium sulfate. Using copper sulfate (II) as a catalyst to thereby convert any organic nitrogen present to ammonium sulfate. The function of the potassium sulfate is to elevate the boiling point of the sulfuric acid and to provide a stronger oxidizing mixture for digestion. Excess sodium hydroxide is added to the cooled digest to liberate ammonia. The liberated ammonia is steam distilled into the excess boric acid solution and titration with hydrochloric acid standard volumetric solution is carried out. The nitrogen content is calculated from the amount of ammonia produced.
5  Reagents
Use only reagents of recognized analytical grade, unless otherwise specified, and distilled or demineralized water or water of equivalent purity. NOTE  The solutions specified in this procedure might be different than those required for the operation of automated titrators. An effort is made to address those, but it is the responsibility of the operator to follow the directions of the equipment manufacturer. 5.1 Potassium sulfate (K 2 SO 4 ), nitrogen free. 5.2 Copper (II) sulfate pentahydrate solution, c(CuSO 4 .5H 2 O) = 5,0 g/100 ml. Dissolve 5,0 g of copper(II) sulfate pentahydrate in water in a 100 ml one-mark volumetric flask. Dilute to the mark with water and mix. 5.3 Sulfuric acid (H 2 SO 4 ), with a mass fraction of between 95 % and 98 %, nitrogen-free (approximately ρ 20 = 1,84 g/ml). 5.4 Sodium hydroxide (NaOH) solution, nitrogen-free, containing 50 g of sodium hydroxide per 100 g. With automated distillation systems, other mass fractions of sodium hydroxide may be used, provided an excess of sodium hydroxide is dispensed to the distillation mixture; for example, a mass fraction of 40 % sodium hydroxide solution may be used instead of a mass fraction of 50 %, where plugging of the automated flow system is a problem.5.5 Indicator solution Dissolve 0,1 g of methyl red in 95 % (volume fraction) ethanol in a 50 ml one-mark volumetric flask (6.16). Dilute to 50 ml with ethanol and mix. Dissolve 0,5 g of bromocresol green in 95 % (volume fraction) ethanol in a 250 ml one-mark volumetric flask (6.16). Dilute to 250 ml with ethanol and mix. Mix one part of the methyl red solution with five parts of the bromocresol green solution or combine and mix all of both solutions. 5.6 Boric acid solution, c(H 3 BO 3 ) = 40,0 g/l. Dissolve 40,0 g of boric acid (H 3 BO 3 ) in 1 l of hot water in a 1 000 ml one-mark volumetric flask (6.16). Allow the flask and its contents to cool to 20 °C. Adjust to the mark with water, add 3 ml of the indicator solution (5.5) and mix. Store the solution, which will be light orange in colour, in a borosilicate glass bottle. Protect the solution from light and sources of ammonia fume during storage. With automated distillation systems, other boric acid concentrations may be used after validating accordingly. If using the electronic pH end point titration, the addition of the indicator solution to the boric acid solution may be omitted. On the other hand, the change in colour may also be used as a check on proper titration procedures.