Concentration of Picral

[firkelloy]

If I am adding 4 grams of picric acid to 100ml of ethanol, and the specific gravity of picric acid is 1.77, wouldn't the concentration of the resulting be 2.26%?

All the handbooks and manuals I have found call this a 4% picral solution.

What am I missing?

 

[unclesyd]

I don't think you are missing anything as most instruction for the reagent will say "dissolve 4 grams picric acid in 100 ml ethonol". This is good enough for the etchant.
You might want to add 1 ml conc HCl if the picral doesn't give a good etch.
Don't make any more of the Picral than necessary and dispose of the remainder if it's not going to be used in short time. Picric acid can be explosive under certain conditions.

 

[firkelloy]

Is it a 4% picral solution or a 2.26% picral solution?

 

[TVP]

The ratio can be expressed as a percent by mass (also called weight percent) or a percent by volume. The ratio should really be called 4 w/v % meaning 4 g / 100 mL.

 

[SMF1964]

One of the reasons I majored in metallurgy was to avoid taking too many chemistry or math courses. Correct me if I'm wrong: Technically: The density of ethanol is 0.791 g/cc at 20 C, so 100ml would be 79.1 grams. Adding 4 g of Picric to that amount would produce (roughly) a 5 weight percent solution of picral. Using a volume calculation, you end up with your 2.3 volume percent. Now, since no one in their right mind works with DRY picric acid (too noisy), how do you take into account the added weight of the moisture present in the picric powder that is present to prevent the material from going ka-boom?

Since metallurgists like to keep things simple, 4 grams of picric powder (with moisture to prevent it from going ka-boom) is added to 100 ml of ethanol (with a density rounded off to 1) to make a 4% solution of picral. Whether it is really 5 wt%, 4 w/v % or just 2.26 volume % is immaterial in this process - it still works to etch the carbides present in my steel and it is quiet.