BS ISO 14900:2017 pdf download – Plastics Polyols for use in the production of polyurethane – Determination of hydroxyl number
4 Principle
4.1 Method A (Acetylation): A test portion is reflllxed in a so lution of acet ic anhydr i de in pyr id ine to acetylate the hydroxyl grollps pr esen t; the reaction is cata lyzed by imidazole. The excess reagent is hydrolyzed with water and the res lllting acetic acid is titrated with standardized sodium hydroxide solution. The hydroxyl content is calcul ated from the difference in titration of the test port i on and a blank solution. CAUTION – Acetic a nhydrid e and pyridine are toxic and f1 amm a ble . In addition , ac e tic anhydride is corrosive . Take prop er pre c aution s when handling these reagents .
4.2 Method B (Phthal a tion): The hydroxyl groups in a test portion are esterified by refluxing with a solution of phthalic anhydride in pyridine; the r eaction is catalyzed by imidazole. The excess anhydride is hydrolyzed with water and the p hthalic acid formed is ti trated with standardized sodíum hydroxide solution. The hydroxyl content ís calculated from the difference in títratíon ofthe test portíon and a blank solution.
5 Interferences
5.1 Excess water will i nterfere by destroyíng part of the esterifícatíon reagen t. If the samp le contains more than 0 , 2 % water , dry the sample with a r eagent th at will n ot add acidity or basicity to the sam ple. 5.2 Pri mary and seco n da ry amin es and long-chain fa tty acids react with the reagent to f orm st ab le compo u nds th at will be included in th e res ult.
6 Reagents
Reagent-grade chemicals sha ll be used in all determinations. Un l ess otherwise indicated , it is inte n ded that all reagents conform to the specifications of I SO 6353 -1, ISO 6353-2 and ISO 6353-3, although ot h er grades may be used provided that it is first determined that the reagent is of sufficiently high purity to permit its use without lesseni ng the accuracy of the determination. Unless otherwise indicated , references to water shall be understood to mean grade 2 reagent water as defined by ISO 3696.
6.1 Acetylation re ag e nt (for method A). Mix 127 ml of acetic anhydride with 1 000 ml of dry pyridine Add 16 g of imidazole and swirl carefu lly to dissolve. Prepare t he reagent fresh daily and keep it in a dark bottle. 00 not use it if it is darker than pale yellow. NOTE Some laboratories have reported that dark , resinous solids have formed when the mixture of pyridine and esterifjcation reagent is heated. In that event , it has been found that preparing the acetylation reagent with the addition of 0 .4 % water in the pyridine w ilI correct the problem. However , care has to be taken that sufficìent reagent is present to complete the reaction quantitatively. See also 8. 1. 2.
CAUTION – Acetic anhydride and pyridine are eye , skin , and respiratory irritants. Avoid bodily contact with these reagents and use only in a well-ventilated area.
6.2 Phthalation reagent (for method B). Weigh 116 g of phthalic anhydride into a 1 I brown bottle. Add 700 ml of pyridine and shake vigorously until dissolved. Add 16 g ofimidazole and swirl carefully to dissolve. The reagent shall stand overnight before use. Avoid prolonged exposure of the reagent to moisture in the air. Discard reagent that develops a colour. In the blank titration as described in the procedure for m ethod B , exactly 25 ml of this reagent shall consume between 95 ml and 100 ml of 0 , 500 moljl sodium hydroxide
6.3 Imidazole , reagent grade or equivalent.
6.4 Hydrochloric acid , standard solution , 0 , 1 molj l. Prepare and standardize to four significant figures in accordance with good practice , using potassium acid phthalate as a primary standard. D etermine and record the temperature at which the standardization was performed. Th e concentration ofthe solution shall be corrected to the temperature at which the determination is performed , as described This solution is required only if a correction is to be applied for the presence of strong base in the sample being analysed.BS ISO 14900 pdf download.