As we add NaOH to AgNO3 at approximately what pH will it form a hydroxide of silver? What is its and fate stability?
Silver(I) cation (as each metallic element cations in aqueous solution) is in the form of aqua complexes, which can give a proton - behaving like a weak acid:
Ag (+) • H2O + H2O = AgOH + H3O (+)
Thus, the silver(I) cation in aqueous solution is a weak acid. The pH of the aqueous solution of silver(I) salts can be calculated from the formula to a pH of weak acids, that is:
pH = 0.5 • pKa - 0.5 • log ca
pKa = 11.7 - exponent of acid dissociation constant for Ag(+),
ca - concentration of Ag(+).
For ca = 0.01 mol / l, pH = 6.85
The precipitation of sparingly soluble AgOH may already occur in solutions of pH greater than that calculated by about one order of magnitude, therefore at a pH of about 7-8.
My information is from the French edition of the Gaston Charlot book:
G. Charlot, Les Reactions Chimiques en solution: L'analyse qualitative minérale, 6 ª. Edición, Paris, Ed. Toray-Masson, 1969
But similar information can be found in most good textbooks of analytical chemistry.
I attached the graph of solubility AgOH versus pH.
Dear Kruti
Silver cation (Ag+) as a Bronsted acid has a exponent of acid dissociation constant - pKa =11.7.
This means that for a typical aqueous solution of silver ions, with the concentration of about 0.001-0.1 mol/l, pH at which the silver cation dissociates to form the proton (i.e. at which in the the solution will be an excess of OH- ions, which can cause precipitation of AgOH precipitate) indicatively has the value in the range 6.5-7.5.
Dear Kruti,
Additionally, the newly formed hydroxide,yields the corresponding brown silver oxide. Biedermann and Sillén calculated the solubility of Ag2O in base of the salt concentration as:
1/2 AgO + 1/2 H2O = Ag+ + OH-, log Ks = - 7.71
See "The hydrolysis of cations" C.F. Baes, Jr. and R. E. Mesmer, John Wiley & Sons, New York, 1976, p.274-276.
The Ag(I) cation is a weak bronsted acid and, hence, the hydroxide precipitates at relatively high pH (see Figure 1 in the attachment). This hydroxide does not solve in OH- excess. On the contrary, due to the noble character of the element, it easily and spontaneously yields the brown oxide Ag2O, which is thermodinamically more stable.
On the other hand, in your question you refer to the addition of NaOH to a AgNO3 solution. Kindly note that this precipitate is easily soluble in nitric acid or NH3 solution.
Regards,
Eduardo
Thank you so much Andrzej Szymanski, Horacio Moya, Nanda Gunawardhana and Eduardo M. Cuerda-Correa for your valuable inputs.
@ Andrzej Szymanski: could you suggest me a related research paper or web link to understand better.
@ Horacio Moya: i am unable to get this paper could you suggest me a link to the same.
@ Nanda Gunawardhana: The paper is very useful.
@ duardo M. Cuerda-Correa: The figure in the attachment is useful, could you suggest me the link to the same as a reference.
Thank you all once again.
Dear Kruti,
The figure is published in a well-known and classic textbook of Analytical Cehmistry. Unfortunately, the author is Spainsh and I seriously doubt that an English version of the book is available. Anyhow, the complete reference of this book is:
F. Burriel Martí, F. Lucena Conde, S. Arribas Jimeno and J. Hernández Méndez. Química Analítica Cualitativa. 13ª Edición. Editorial Paraninfo, 1989.
Best regards,
Eduardo
Silver(I) cation (as each metallic element cations in aqueous solution) is in the form of aqua complexes, which can give a proton - behaving like a weak acid:
Ag (+) • H2O + H2O = AgOH + H3O (+)
Thus, the silver(I) cation in aqueous solution is a weak acid. The pH of the aqueous solution of silver(I) salts can be calculated from the formula to a pH of weak acids, that is:
pH = 0.5 • pKa - 0.5 • log ca
pKa = 11.7 - exponent of acid dissociation constant for Ag(+),
ca - concentration of Ag(+).
For ca = 0.01 mol / l, pH = 6.85
The precipitation of sparingly soluble AgOH may already occur in solutions of pH greater than that calculated by about one order of magnitude, therefore at a pH of about 7-8.
My information is from the French edition of the Gaston Charlot book:
G. Charlot, Les Reactions Chimiques en solution: L'analyse qualitative minérale, 6 ª. Edición, Paris, Ed. Toray-Masson, 1969
But similar information can be found in most good textbooks of analytical chemistry.
I attached the graph of solubility AgOH versus pH.
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