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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Donaldson Tan on August 24, 2004, 04:36:54 AM

Title: 24 AUG 2004: General Chemistry
Post by: Donaldson Tan on August 24, 2004, 04:36:54 AM

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

Title: Re:24 AUG 2004: General Chemistry
Post by: Mitch on August 24, 2004, 11:48:42 PM

ppt?

Title: Re:24 AUG 2004: General Chemistry
Post by: Donaldson Tan on August 25, 2004, 04:45:54 AM

Quote from: Mitch on August 24, 2004, 11:48:42 PM

ppt?

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

Title: Re:24 AUG 2004: General Chemistry
Post by: Mitch on August 25, 2004, 07:12:24 PM

Oh, I was thinking parts per trillion.

Title: Re:24 AUG 2004: General Chemistry
Post by: Limpet Chicken on August 25, 2004, 08:12:08 PM

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 ;D

Title: Re:24 AUG 2004: General Chemistry
Post by: Donaldson Tan on August 25, 2004, 10:43:00 PM

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.

Title: Re:24 AUG 2004: General Chemistry
Post by: AWK on August 26, 2004, 02:13:18 AM

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

Title: Re:24 AUG 2004: General Chemistry
Post by: Donaldson Tan on August 26, 2004, 02:46:12 AM

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

 :arms:

Title: Re:24 AUG 2004: General Chemistry
Post by: AWK on August 26, 2004, 06:23:05 AM

I didn't know dye chemistry is so interesting

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

Title: Re:24 AUG 2004: General Chemistry
Post by: Donaldson Tan on August 26, 2004, 09:58:15 PM

 :bangin:

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

Title: Re:24 AUG 2004: General Chemistry
Post by: AWK on August 27, 2004, 02:58:33 AM

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.