Where the
world gathers for metal finishing
Q&As since 1989


-----

Temperature at which silver becomes silver oxide?

The conversion of silver oxide to silver or silver to silver oxide by annealing. I want to know if TGA of a sample done then at what temperature it will convert especially if it is nanoparticles.

Vidyanand Singh
- New Delhi, India
2003

---

2003

This is elementary thermochemistry, which someone having the use of a TGA is expected to know. Especially since the silver-silver oxide equilibrium temperature is a function of oxygen pressure, which is easily varied within a TGA. I will outline the method. First, for the equilibrium 2 Ag(s) + ï O2(g) = Ag2O(s) you need the thermodynamic data for the equation Delta G = Delta H ¯ T x Delta S. The necessary data are in books such as the CRC Handbook of Chemistry & Physics [adv: on Amazon & AbeBooks & eBay] of Chemistry and Physics, Metallurgical Thermochemistry, etc. With this data, for oxygen at 1 bar (0.986 atm), you will see that Delta G < 0 at room temperature; hence, Ag2O is stable. Solve for T at which Delta G = 0. Above this temperature, in 1 bar oxygen, the Ag2O will lose weight (decompose to Ag).

Second, you need the effect of P(O2) upon the Ag-AgO equilibrium. With activities of Ag and Ag2O both = 1, Delta G = -RT ln [1/P(O2)**(1/2)] Since you already have Delta G = Delta H ¯ T x Delta S, you can substitute a value for P(O2) and solve for T. You will find that lowering P(O2) lowers the equilibrium temperature. I did a quickie calculation and found that lowering P(O2) from 1 to 0.21 atm lowers the equilibrium temperature by 41 K (or C).

Notes: 1) This method assumes Delta H and Delta S are constants (independent of T). You can correct for variation with T if you look up Cp data. 2) Correct for reduced buoyancy upon heating by running an inert sample (easy if weight data is logged on computer). 3) There is some solubility of oxygen in silver at Ag2O decomposition temperatures, so the actual activity of silver is very slightly less than 1. 4) There is also a particle size effect upon the thermodynamics, but probably negligible for your purposes. 5) Don't run the experiment in vacuum; the Ag2O would make a mess inside your TGA when heated (explosive decomposition, i.e., "popcorn effect").

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.