ISI 28-1e Determination of Reducing Sugar, DE by Luff-Schoorl's method.

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1. Scope	The method is applicable to all starch hydrolysis products.			LT 1/9 1970 Rev.: LT 22.01.2002
2. Principle	Iodine titration of excess copper
3. Reagents	3.1 Luff-Solution: 3.1.a Citric acid: 50 g C6C8O7,H2O p.a. is dissolved in 50 ml distilled water. 3.1.b. Sodium carbonate: 143.7 g Na2CO3 p.a. is dissolved in 400-500 ml boiling distilled water.  3.1.a + 3.1.b is gently mixed after cooling of 3.1.b 3.1.c Copper sulfate: 25 g CuSO4, 5 H2O p.a. is dissolved in approx. 100 ml distilled water and transferred to the mixture of (3.1.a + 3.1.b). Dilute to 1 liter with distilled water. 3.2 Acetic acid, 0.4 N 24 ml CH3COOH p.a. up to 1 liter with distilled water. 3.3 Iodine 0.1 N (Standard titration solution) 3.4 Hydrochloric Acid 0.75 N 73 g conc. HCl up to 1 liter with distilled water 3.5 Sodium thiosulphate 0.1. (Standard titration solution) 3.6 Starch solution 2 g soluble starch is dissolved in 100 ml distilled water. Preserved with a tiny amount of HgO.			p.a. = pro analyse
Sample preparation	3.7 Sample solution: The DE of sample is estimated and the quantity consuming approx. 15 ml thiosulphate (3.5) is calculated. Weigh the sample and transfer to a 250 ml metering flask. Dilute with distilled water to mark.			a = g total weight of sample used to prepare 250 ml solution
4. Apparatus	4.1 Electrical heating plate
5. Procedure
	Transfer with pipette 25.0 ml Luff-solution (3.1) and 25.0 ml sample-solution (3.7) to a 300 ml conical flask with standard taper-ground outer joint. Add glass spheres
	Heat in 3-3½ min to boiling on electrical heater. Boil with reflux condenser in exactly 10 min. Flush condenser with a small amount of distilled water. Cool immediately in cold water.			User alarm clock
	Add 50 ml acetic acid (3.2) from graduated cylinder. Add with pipet 25.0 ml iodine 0.1 N (3.3). Add from graduated cylinder 55 ml 0.75 N HCl (3.4). Shake until all is dissolved. Titrate with thiosulphate (3.5) to copper-blue with 6-8 drops of starch solution (3.6) as indicator.
Blind	Carry out a blind with 25 ml distilled water as sample solution (3.7) - same procedure
6. Calculation	DE (average results of two samples to one decimal) on dry matter basis = (c*100)/(a*d) where B= ml 0.1 N thiosulphate used for the blind A= ml 0.1 N thiosulphate used for the sample T= (B-A)*f f= (titration) factor for the thiosulphate c= mg dextrose per 25 ml sample solution. Read from table by interpolation using the titer T a= sample g d= % sample dry matter g/100g (or Brix oBx= a = factor from table below (9. Table) b = ml test solution for titration g = gram sample dissolved to 250 ml d = % dry solids in sample
7. Notes	Alternative method: Calculation of DE from a sugar distribution obtained by HPLC.
8. Reference	Alternative method: International Standard: ISO 5377 Schoorl, N. (1929). Suiker titratie. Chemisch Weekblad 130-134 and ZLUF 57, 566 (1929)
9. Table	Titer - ml 0.1 N thiosulphate T	mg dextrose per 25.0 ml c	Difference used for interpolation	All figures relate to anhydrous dextrose.
	1	2.4	2.4
	2	4.8	2.4
	3	7.2	2.5
	4	9.7	2.5
	5	12.2	2.5
	6	14.7	2.5
	7	17.2	2.6
	8	19.8	2.6
	9	22.4	2.6
	10	25.0	2.6
	11	27.6	2.7
	12	30.3	2.7
	13	33.0	2.7
	14	35.7	2.8
	15	38.5	2.8
	16	41.3	2.9
	17	44.2	2.9
	18	47.1	2.9
	19	50.0	3.0
	20	53.0	3.0
	21	56.0	3.1
	22	59.1	3.1
	23	62.2