Hydroxyl number

The hydroxyl number (OHN), also called acetyl number , is a measure of the content of hydroxyl groups in organic materials, e.g. B. in resins , paints , polyesterols , fats and solvents . The hydroxyl number correlates with the OH content, which indicates the amount of hydroxyl groups in another unit.

The hydroxyl number indicates the amount of potassium hydroxide in milligrams , which is equivalent to the amount of acetic acid bound during acetylation of one gram of substance .

execution

In order to determine the hydroxyl number experimentally, a corresponding mass of the material to be examined (e.g. hard fat) is precisely weighed. Then an acetylation mixture of acetic anhydride and anhydrous pyridine is added in excess (ratio 1: 3) and heated for one hour in a water bath. One hydroxyl group (of the fat) reacts with acetic anhydride to form the acetylated hydroxyl group and one molecule of acetic acid each. All unused acetic anhydride molecules are converted into two acetic acid molecules in the next step with water. This is followed by neutralization with ethanolic KOH solution and phenolphthalein .

The same is repeated without the analyte ( blank value ), then each molecule of acetic anhydride is converted into two molecules of acetic acid - correspondingly more KOH is required to neutralize the acetic acid that is formed. In the main experiment with the analyte, exactly one less acetic acid molecule is produced for each hydroxyl group present. The OHZ can be determined from this relationship. The relationship is important: n (OH groups) = n (acetic acid, which is less produced) = n (KOH, which is less used for acetic acid in the main experiment).

Calculation according to DIN

${\displaystyle \mathrm {OHZ={\frac {M(KOH)\cdot (Blindversuch-Hauptversuch)\cdot c\cdot t}{m(Einwaage)}}+SZ} }$

Here is

M (KOH): molar mass of potassium hydroxide [56.106 g / mol]

Blind test: Potassium hydroxide consumption in the blind test [ml]

Main experiment: Potassium hydroxide consumption in the main experiment [ml]

c: concentration of the potassium hydroxide measure solution

t: titer

m (initial weight): sample weight [g]

SZ: acid number [mg KOH / g sample]

Other common units and information

Hydroxyl group content

In addition to specifying the hydroxyl number, the so-called OH content can also be specified. The OH content indicates the mass fraction of OH groups in the entire sample.

${\displaystyle OH-Gehalt={\frac {m(OH-Gruppen)}{m(Gesamt)}},[Prozent]}$

A hydroxyl group here always has a weight of 17 g per mol.

The conversion of the OH content into the hydroxyl number is possible using the following conversion.

${\displaystyle OHZ={\frac {OH-Gehalt[\%]\times 56100[mg(KOH)/mol]}{17[g/mol]\times 100[\%]}}}$

Changing the relationship results in the calculation of the OH content from the OH number.

${\displaystyle OH-Gehalt[\%]={\frac {OHZ[mg(KOH)/g]\times 17[g/mol]}{56100[mg(KOH)/g]}}\times 100[\%]}$

Conversion in terms of solids or dissolution

In some cases, the OH number and OH content are related to the solid body of a solution and in some cases to the entire solution in the delivery form. The OH number and the OH content of the solid are always higher than solutions in an inert solvent. A conversion is possible as follows.

${\displaystyle OHZ[Anloesung]=OHZ[Festkoerper]\times Konzentration[\%]}$

Hydroxy equivalent weight

In addition to specifying the hydroxyl number or OH content, the hydroxyl equivalent weight can also be specified. The hydroxy equivalent weight indicates how many grams of resin or sample contain one mole of hydroxyl groups. The hydroxy equivalent weight can be calculated according to the following equation.

${\displaystyle OH-Aequivalentgewicht[{\frac {g}{mol}}]={\frac {56100[{\frac {mg(KOH)}{mol}}]}{OHZ[{\frac {mg(KOH)}{g}}]}}}$

Calculation of the molar mass of polycondensates

For hydroxyl-terminal polyesters and other condensates, the number average molecular weight (Mn) can be calculated using the following relationship

${\displaystyle \mathrm {Mn=1000mg/g\cdot {\frac {z\cdot 56.106g/Mol}{OHZ[mg/g]}}} }$

Where z is the number of OH groups in the macromolecule. For a linear, to (alcohol) terminal polyester, for example: z = 2.

Individual evidence

1. ↑ Metrohm working instructions (PDF; 511 kB)
2. ↑ ^{a b} Mettler working instructions

literature

The implementation and evaluation is described in DIN 53240. A classic regulation from an industrial laboratory in 1940 (PDF; 314 kB).