How to determine the hydroxyl value
发布时间：2024-07-04
The determination of hydroxyl value is an important means of production control for polyol factory, and is the basis of formula calculation for polyurethane products factory. Generally, the hydroxyl content is often expressed in terms of hydroxyl value. The definition of hydroxyl value is: per gram of sample containing hydroxyl acylation, the consumption of acid equivalent to the mass of KOH (mg), can be expressed as mgKOH/g. The determination principle is based on the acylation method (also known as esterification method), that is, the hydroxyl group in the sample reacts with the anhydride quantitatively to generate ester and acid. After the excess anhydride is hydrolyzed into acid, it is titrated with the base standard solution. With different acylating agents, there are a variety of acylation methods, such as acetylation, phthaloylation (also known as phthaloacylation) and benzoylation. Acetylation reaction is faster, but the reagent is more volatile. And it will be interfered by low carbon aldehydes. Phthaloylation method is not interfered by aldehydes and phenols, and the reagent is not volatile, but the acylation reaction rate is slow. The pyromellitic acid dianhydride is used as acylating agent in the pyromellitic acylation method, and the reagent is not volatile, the reaction rate is fast, and it is not interfered by aldehydes and phenolic compounds. In order to speed up the acylation reaction rate and shorten the determination time to achieve rapid determination, acylation catalysts such as perchloric acid, p-toluenesulfonic acid, imidazole compounds and boron trifluoride diethyl ether complexes are often used, which is very practical in production. Phthalene two formylation method for rapid determination of hydroxyl value: This method is suitable for the determination of polyether polyol hydroxyl (primary and secondary hydroxyl) and polyester polyhydric alcohol containing some secondary hydroxyl. When determining the hydroxyl group of polyether polyol, the sampling amount is based on the amount of (561/estimated hydroxyl value), and the reference is taken. The reaction is as follows.
(1) Preparation of Acylating Agent Weigh 42g of phthalic anhydride dissolved in 300mL of dried pyridine, completely dissolved and stored in a brown bottle and placed in a dryer for later use.
(2) Selection of acylation catalyst Weigh 6g of imidazole into the prepared acylating agent.
(3) The analysis procedure accurately weighed a certain amount of sample on the analysis balance, put it into an acylation bottle with a grinding mouth and installed with a net flow condenser, accurately added 25mL of acylating agent with a pipette, dissolved the sample and placed in a constant temperature water bath for acylation reaction for 20~25min, removed from the water bath, cooled to room temperature, and carefully added 20mL 1:1 pyridine distilled water solution from the upper edge of the condenser tube, to hydrolyze the remaining anhydride, shake well, add 3-5 drops of phenolphthalein indicator, titrate with 0.8 or lmol/L KOH standard solution to the pink, and the end point will not change for 15s, and do the blank test in the same way. The hydroxyl value is calculated by the following formula, and the allowable error is less than 0.5mg KOH/g.
Hydroxyl value = (volume of KOH solution consumed by blank-volume of KOH solution consumed by sample) × KOH concentration × 56.1/sampling amount (g)
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