I'm synthesizing CuO at pH below neutral. In order to sort out mechanism when I see literature mostly CuO NPs are produced at higher pH by 2 step mechanism. First production of Cu(OH)2 and then conversion to CuO. In my case I don't get any evidence of Cu(OH)2 from XRD and XPS analysis of ppt.
So I wonder whether my reaction follows a different mechanism or I just don't get Cu(OH)2 in precipitate due to low pH ?
Can anyone please help sort out this problem? What can be the possible routes for synthesis of CuO at acidic pH without involving Cu(OH)2 as an intermediate?
pKa of Cu2+(aq) is around 7. At pH < 6 there is no Cu (OH)2 in aqueous solution. Read about hydrolysis of cation in water.
Cu (OH)2 is unstable state and usually water soluble which occur while you add OH- source so the yield may be CuO but the yield may be less due to low pH....
It may not be very simple to answer with partial information. As far as I know, CuO does not precipitate out directly until and the unless the hydroxide is heated in solution. What are your conditions (i.e. temperature and pH and the counter-ions)? How do you adjust the pH? You can search the "speciation diagram" of copper which shows you what species are present as a function of pH. Such diagrams are extremely useful in understanding what might be going on.
pKa of Cu2+(aq) is around 7. At pH < 6 there is no Cu (OH)2 in aqueous solution. Read about hydrolysis of cation in water.
At pH=6 the concentration of OH- will be 10-8 M. The Ksp of CuOH)2 is 3x10-19 and theoretically if the concentration of Cu2+ is a bit higher than 3x10-3 M this hydroxide will start precipitation. However, the concentrations of OH- and Cu2+ must be much higher than 10-8 and 3x10-3 M respectively to have appreciable amount of Cu(OH)2 precipitated. Therefore, Cu(OH)2 can be obtained from dilute copper(II) solutions at pH higher than 7. Cu(OH)2 can be then converted to CuO on calcination.
Have a look at the speciation diagram of copper (II) in water.
http://chemse.oxfordjournals.org/content/31/7/689/F1.expansion
At pH=6 the concentration of OH- is 10-8 M. The Ksp of Cu(OH)2 is 3x10-19 and theoretically if the concentration of Cu2+ is a bit higher than 3x10-3 this hydroxide will start precipitation. However, for having appreciable amount of Cu(OH)2 the concentrations of OH- and Cu2+ should be higher than 10-8 and 3x10-3M respectively.Since the Ka of Cu2+ is about 10-8 the pH of the dilute copper(II) solutions must be higher than 7 to have the Cu(OH)2 precipitated. This hydroxide can be converted to CuO on calcination.
an addition to a previous comment on the speciation diagram of copper (II) in water.:
recommended by IUPAC
http://www.iupac.org/publications/pac/pdf/2007/pdf/7905x0895.pdf
Below pH=7 not the Cu(OH)2 but some basic Cu-salts might be precipitated. These can be turned (hydrolysis, calcination) into Cu(OH)2 or CuO in separated steps. The conditions depends on the anion and the composition of basic salt.
Dear Researcher,
As several people said the conc of OH is extremely low at ph 6, generally all hydroxides ppt above pH8. pH 6 or below, all metals tend to dissolve (corrosion of pipes in low pH water). The obvious solution seems to ppt above pH 8 and calcify to CuO.
Tahir zaidi
Thank you so much respected all, your worthy suggestions are really helpful for me..
I think that Dr. Mohammad Kooti's answer is best one among the answers.
here is the link
Hi I think following link will help u
http://link.springer.com/article/10.1007/s10008-016-3502-2 Regards, Amarsingh Thakur, Lab of electrochemical studies, School of physical Sciences, DRF Solapur university, India Call 9172918371
- Badges
- Science topic
- Similar topics
- Chemistry
- Sulfites
- Inorganic Chemistry