[Donaldson Tan]

Suggest an explanation for the appearance of a blue precipitate when ethanoic acid is added slowly to a deep dark blue solution.

[Mitch]

ppt?

[Donaldson Tan]

ppt?

ppt is abbreviation for 'precipitate' - something that I was taught in school

[Mitch]

Oh, I was thinking parts per trillion.

[Limpet Chicken]

Dark blue suggests to me, either copper aspirinate, hydrated copper (II) sulfate, or a tetrammine copper (II) complex salt.
 
Maybe dry hydrated copper (II) sulfate, and acetylsalicylic acid mixed, then acetic acid (aq) added to form dark blue copper aspirinate while liberating H2SO4.

Tetrammine copper solution+an alkali chlorate/perchlorate salt+H2O2-->tetramminecopper (II) perchlorate  on addition of acid to liberate chloric/perchloric acid (dilute) from the chlorate/perchlorate?

In my experience, I have prepared tetramminecopper (II) perchlorate, and the way I prepared it, it crystallises as a dark turquoise-to-blue precipitate that deflagrates slowly when ignited, roughly as fast as a drop of glycerine on powdered KMnO4 ( I have heard that the chlorate is a friction sensitive primary)

I only have experience with the perchlorate, and it looked like you described, however, I think the product was at least slightly impure, hmm, I think I will start a thread on tetrammine complexes when I'm slightly less stoned, and have sufficient inclination LOL

[Donaldson Tan]

I was thinking along the line of a solution of tetra amine copper(II) complex being converted to copper(II) hydroxide upon addition of ethanoic acid. tetra amine copper(II) is a soluble dark blue complex whereas copper(II) hydroxide is the light blue precipitate.

Cu²⁺ (aq) + 4NH₃ (aq) <-> Cu(NH₃)₄²⁺ (aq)
Addition of ethanoic acid removes aq. ammonia, thus favour the backward reaction of the above chemical equilibrium, reducing the amount of complex present and increasing the amount of copper(II) ions present, such that the ionic product of Cu(OH)₂ exceeds its solubility product constant, thus precipitation of Copper(II) hydroxide occurs.

[AWK]

Any tetraammine copper complex, when reacts with acetic acid can form insoluble blue pigment called blue verdigris (basic copper acetate - Cu(CH₃COO)₂•CuO•6H₂O

Maybe dry hydrated copper (II) sulfate, and acetylsalicylic acid mixed, then acetic acid (aq) added to form dark blue copper aspirinate while liberating H2SO4

Acetic acid cannot liberate  H₂SO₄ !

subscript corrected

[Donaldson Tan]

Wow.. I didn't know dye chemistry is so interesting

 :arms:

[AWK]

I didn't know dye chemistry is so interesting

In contrast to dye a pigment has no affinity to a substrate, and is insoluble

[Donaldson Tan]

 :bangin:

so a dye and pigment differs in terms of solubility.. i always thought these two terms are inter-changeable.

[AWK]

so a dye and pigment differs in terms of solubility..

Also in affinity to substrate. It reacts with, say, fiber, at least through hydrogen bonds.
Transparent materials colored with dye are still transparent, the same materials colored with pigment are opaque.