ISI 10-2e Determination of Carboxyl Groups in Starch

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1. Scope	The method is applicable to oxidised starch	LT 18. Dec. 1964 Rev. LT 15. marts 1998
2. Principle	Carboxyl groups are titrated by the Paste Titration Method
3. Apparatus	3.1 Analytical balance weighing to the nearest 0.1 mg. 3.2 Shaking boiling water bath. 3.3 Büchner funnel 55 mm
4. Reagents	4.1 Hydrochloric acid 0.1 N 4.2 Sodium hydroxide 0.1 N 4.3 Phenolphthalein: 1 g in 1 litre ethanol 90% 4.4 Silver nitrate, 0.1%
5. Procedure	Weigh 5 g (w) sample to the nearest 0.1 mg in a 100 ml beaker
Conversion	Add 25 ml hydrochloric acid (4.1) and stir for 1/2 hour.	Use magnetic stirrer
Washing	Filter on filter (3.3) or glass crucible. Flush beaker and filter with water until filtrate is free of chloride - test with silver nitrate (4.4)	Increase hydrolysis parameters if flux too low
Gelatinising	Slurry filter cake with 300 ml distilled water in beaker. Place beaker in boiling water bath (3.2) for 15 min.	Stir
Titration	Titrate with (4.2) and (4.3) while still hot. V = ml (4.2). N = normality of (4.2)	To pH 8.3
6. Calculation	Calculate carboxyl content of sample by averaging results of two samples with two decimals. COOH % = (100 N V M ) / (1000 w) - c Degree of Substitution (DS) = 0.036 x COOH%	Molecular weight of COOH = M = 45 c corrects for lipids and P - see notes
	Eventually correct to value on sample dry matter.
7. Notes	c = approx. 0.1% on dry basis for maize and wheat starch. c = 0 for wheat and maize starch defatted by Soxhlet extraction	Correction for lipids
	c = 3p for potato starch, where p = phosphorous as P %	p ~ 0.03 - 0.09%
8. Reference	ISO 11214