[mehc]

Stearic acid is soluble in ethanol. When i dissolve stearic acid in ethanol, most part of it get dissolved, but a little portion remains insoluble. What kind of impurity it can be? As commonly stearic acid can have palmitic acid, sodium salts, and ester impurities but these should also be soluble in ethanol.

Can anyone has idea  about the insoluble portion?

The stearic acid is tripple pressed.

[Archer]

Waxes maybe, what proportion of the total wont dissolve?

What concentration are you trying to achieve?

Can you fish the bits out and see if they dissolve in, say, toluene?

[mehc]

it will be about 1-2   percent.

[Archer]

What is the quoted purity?

Can you ask the supplier for a certificate of analysis?

[AlphaScent]

Can you get NMR??

[Archer]

Nice big aliphatic blob around 1.5 ppm

[Adamcp898]

TLC?

[mehc]

TLC and NMR are not possible.
Quoted purity is about 97 % fatty acids.
I cannot try toluene, as I have to determine the acid value by titration with sodium hydroxide. Sodium hydroxide will not soluble in toluene.

[Archer]

I meant try to see if the isolated impurity (by filtration) dissolves in toluene.

It sounds like waxes to me, but these should not interfere with your acid value estimation.

[orgopete]

As I recall from long long ago, fatty acids were usually GC’d as their methyl esters. I'm guessing if the supplier has indicated 97%, that may be how it was done (with an internal standard). However, that isn't what is being asked.

Re: insoluble material.
It's insoluble, collect it. I'm just guessing it may be an inorganic salt. Water soluble? Does it burn? If inorganic, follow genchem method to ID cations and anions. If organic, is it a salt? Acidify. MS.

[Archer]

There are some areas of chemistry where my knowledge is woefully inadequate, natural product chemistry is one of my few areas of experience.

As stearic acid has come from a natural source, your impurity is most likely a natural wax of some kind.

These waxes are long chain esters of long chain acids. In natural product extraction they are often removed by selective precipitation from cold alcohol.

As the insoluble matter is within the range of the quoted impurity it is likely to have come from the manufacturing process, therefore natural wax is the best place to start for identification.