Acetyl number

Acetylation

To determine the acetyl number of a sample, first it is treated with acetic anhydride. This acetylation reaction attaches acetyl groups to all the free hydroxyl groups in the sample.

Purification

The reaction mixture is washed several times with warm salt water in a separatory funnel to remove excess acid and other water-soluble impurities. The oil layer is then dried using a drying agent and filtered to remove any remaining moisture and solid particles.

Saponification

Next, the purified acetylated sample is heated with a measured amount of potassium hydroxide. The potassium hydroxide reacts with the acetyl groups, forming potassium acetate and consuming part of the base.

Titration

Afterward, the mixture is cooled, and phenolphthalein is added as an indicator, making the solution appear pink. The solution is titrated with standardized hydrochloric acid until the pink color disappears. The volume of hydrochloric acid added is then used to calculate the amount of KOH remaining by the neutralization reaction: KOH + HCl → KCl + H₂O.^{[better source needed]}

Equations

${\displaystyle {\text{SV}}={\frac {(B-V)\times N\times 56.11}{W}}}$

In the above formula, SV stands for saponification value, B = the volume of HCl required for the blank titration in mL, V is the volume of HCl required for the sample titration in mL, N is the exact normality of the HCl titrant in Eq/L, and W is the mass of the sample portion. The number 56.11 comes from the equivalent weight of potassium hydroxide in g/Eq, scaled to mg/mmol.

By running this calculation for both saponifying the original and acetylated substance separately, two distinct variables are created: S, the saponification value of the original, unacetylated sample, and A, the saponification value of the acetylated sample.

${\displaystyle {\text{Acetyl Number}}={\frac {A-S}{1-(0.00075\times S)}}}$

The formula above, the André–Cook formula, calculates the acetyl number from the saponification values of the original and acetylated samples.