# The Milk (Special Designation) Regulations 1988, Paragraph 3: Reagents


(1) Whenever possible, reagents of analytical quality shall be used.
(2) The buffer-substrate solution shall be prepared as follows:–
(a) Buffer solution: 3.5 g. of anhydrous sodium carbonate and 1.5 g. of sodium bicarbonate shall be dissolved in distilled or de-ionised water, and made up to one litre.
(b) Substrate: Disodium p-nitrophenyl phosphate. The solid substrate shall be kept in a refrigerator.
(c) Buffer-substrate solution: 0.15 g. of the substrate shall be placed in a 100 ml. measuring cylinder, and made up to 100 ml. with the buffer solution. The solution shall be stored in a refrigerator and protected from light. It shall give a reading of less than the standard marked 10 on the comparator disc A.P.T.W. or A.P.T.W.7 when viewed in transmitted light through a 25 mm. cell in the “all-purpose” comparator, distilled or de-ionised water being used for comparison. The solution shall not be used for more than one week.

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Source: https://www.legislation.gov.uk/uksi/1988/2204/schedule/5/part/V/paragraph/3 (the official version)
Page: https://openlegislation.uk/milk-special-designation-regulations-1988/schedule-5/part-V/paragraph-3
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.