141.75
mg KOH/g
4.297
% OH
395.77
g/eq
2.53
eq/mol
10
mL
140.25
mg KOH/g
141.75
mg KOH/g
4.297
% OH
395.77
g/eq
2.53
eq/mol
10
mL
140.25
mg KOH/g
The Hydroxyl Value (HV) calculator determines the number of milligrams of potassium hydroxide equivalent to the hydroxyl groups in one gram of sample. This measurement quantifies the free hydroxyl (-OH) content in polyols, fatty alcohols, waxes, resins, and other organic materials. The hydroxyl value is critically important in the polyurethane industry, where it determines the stoichiometric ratio of polyol to isocyanate needed for crosslinking. It is equally vital in the coatings industry for formulating alkyd resins and polyester coatings, and in the food industry for characterizing mono- and diglycerides. The method involves acetylation of hydroxyl groups with acetic anhydride, followed by titration of the excess reagent. The acid value correction accounts for free carboxylic acid groups that also consume the base titrant.
The standard method (AOCS Cd 13-60 / ASTM E222) involves two steps:
1. Acetylation: Hydroxyl groups react with acetic anhydride in pyridine:
$$R\text{-}OH + (CH_3CO)_2O \rightarrow R\text{-}OOCCH_3 + CH_3COOH$$
2. Back-titration: Excess acetic anhydride is hydrolyzed, and the total acetic acid is titrated with KOH.
The hydroxyl value is:
$$HV = \frac{(V_{blank} - V_{sample}) \times N \times 56.1}{W} + AV$$
where the first term represents the KOH equivalent of the hydroxyl groups (by difference between blank and sample titrations), and AV is added because free acids in the sample also react with the acetylating reagent.
The equivalent weight (grams per OH equivalent) is:
$$EW = \frac{56100}{HV}$$
The functionality (number of OH groups per molecule) is:
$$f = \frac{MW}{EW}$$
Typical hydroxyl values: polyester polyols for coatings 30-150 mg KOH/g, polyether polyols for foams 28-800 mg KOH/g, castor oil 155-170, glycerol ~1830 (theoretical), mono/diglycerides 100-200. In polyurethane formulation, the hydroxyl value determines the amount of isocyanate needed: NCO/OH ratio = (isocyanate eq. wt × HV) / (56100 × desired ratio). The equivalent weight is the molecular weight per OH group — lower values mean more reactive sites per gram. The functionality estimate assumes a reference molecular weight (set at ~1000 g/mol as default); for accurate functionality, use the actual molecular weight of the polyol.
Inputs
Results
HV = (7.8 × 0.5 × 56.1)/2.0 + 1.5 = 110.4 mg KOH/g. Equivalent weight ~508 suggests a difunctional polyol of MW ~1000.
Inputs
Results
HV = (11.5 × 0.5 × 56.1)/2.0 + 2.0 = 163.8. Functionality ~2.9 reflects castor oil's naturally trifunctional ricinoleic acid content.
Hydroxyl value is the number of milligrams of KOH equivalent to the hydroxyl content in one gram of sample. It measures the free -OH groups available for chemical reactions such as esterification, urethane formation, or crosslinking in polymer chemistry.
During acetylation, free carboxylic acids in the sample also react with the acetylating reagent, consuming some acetic anhydride. The acid value correction compensates for this, ensuring that only hydroxyl groups are counted in the final result.
In polyurethane chemistry, isocyanate (-NCO) groups react with hydroxyl (-OH) groups to form urethane linkages. The hydroxyl value of the polyol determines the exact amount of isocyanate needed for complete reaction (stoichiometric balance), directly affecting the polymer's properties.
Equivalent weight is the mass of sample containing one equivalent (one mole) of hydroxyl groups. It equals 56100/HV (since 56.1 g/mol KOH × 1000 = 56100). Lower equivalent weight means more OH groups per gram, indicating higher reactivity or shorter chain length.
Functionality (f) is the average number of OH groups per molecule: f = MW/EW. You need to know the molecular weight (from GPC, vapor pressure osmometry, or other methods) and the equivalent weight (from HV). Polyether diols have f ≈ 2, triols have f ≈ 3.
The acetylation reagent is typically acetic anhydride in pyridine. Pyridine serves as both solvent and catalyst for the acetylation reaction. Some newer methods use phthalic anhydride in pyridine or imidazole-catalyzed methods to avoid the toxicity of pyridine.
The standard HV method measures total hydroxyl content without distinguishing primary from secondary OH groups. However, under carefully controlled conditions (lower temperature, shorter time), primary OH groups react faster. Some specialized catalytic methods can differentiate them.
The theoretical HV of glycerol (MW 92.09, 3 OH groups) is 3 × 56100/92.09 = 1827 mg KOH/g. Experimental values are typically 1820-1830, confirming the three hydroxyl groups per molecule.
Water contains an OH group and reacts with acetic anhydride just like sample hydroxyls. If the sample contains moisture, the measured HV will be artificially high. Samples should be dried before analysis, or a Karl Fischer water content correction should be applied.
The NCO:OH ratio (isocyanate index) determines the polymer properties. Ratio = 1.0 gives a fully reacted stoichiometric balance. Ratios of 1.05-1.10 (5-10% excess NCO) are common in practice to ensure complete OH reaction. Higher ratios give harder, more crosslinked polymers.
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