MadSci Network: Chemistry
Query:

Re: Silver Oxide = Silver(I)Oxide and also Silver(II)Oxide why?

Date: Wed Nov 30 11:07:55 2005
Posted By: Gareth Evans, Industrial R&D practitioner and manager ( retired )
Area of science: Chemistry
ID: 1132662760.Ch
Message:

Hello Terry

Thanks for your question. There were several parts to it so I'll deal with them in turn.

You asked about the forms of silver oxide, silver(I) oxide and silver(II) oxide and why they are written that way. Chemistry is mainly about electrons and it is vitally important to be clear about how the electrons in atoms are involved in the bonding between atoms in chemical compounds. As the discipline has evolved so has the way we describe chemicals.

Atoms of some elements, when they are bonded with other atoms, have a predictable number of their atoms involved in bonding. For example an alkali metal like sodium would be expected to donate a single electron to become a singly charged ion forming an ionic bond with, say, a halogen like chlorine. When an atom loses an electron it is said to have been oxidized. We also say that the oxidation state is 1 because one electron has been used. Many other elements though, can use various numbers of electrons depending on the nature of the other atoms with which they bond. Silver can use one, two or three electrons. We sometimes use words with conventions of spelling to indicate the oxidation state. In the case of silver and other metals we use the ending "-ous" when the fewest number of electrons is used and "-ic" for a higher number. The use of words brings some ambiguities so more rigorous descriptions are preferred. The (I) and (II) in the case of the silver oxides stand for the oxidation states. A more complete explanation of oxidation states can be found on this web-site:

http://www.psigate.ac.uk/newsite/reference/plambeck/chem1/p01027.htm

Argentous oxide is therefore also described as silver(1) oxide or Ag20 with the 2 representing the number of silver atoms combining with each oxygen atom. Argentic oxide or silver(II) oxide is also known as plain silver oxide or silver peroxide. The peroxide here indicates that the compound contains more oxygen than usual. It does not mean the presence of oxygen in the form -O-O as implied in your question, presumably by analogy with H-O-O-H or hydrogen peroxide. This ambiguity illustrates why the more formal descriptions are better. The -O-O group does not make chemical sense since oxygen uses two electrons to form two single bonds or a double bond not one or three.

While it is common for many types of atom to use more than one number of electrons to bond with ( oxidation states ) it is much less common to find an element in two oxidation states in one compound. However your question shows that you have found that silver(II) oxide is an example of this behaviour. The crystal structure indicates that half the silver is in fact in the (I) state and the other half in the (III) state forming units of 4 silver atoms combined with four oxygen atoms. So we can now add to the confusion of names by including tetrasilver tetroxide to our list of names for this compound. This page on Webelements lists silver(I,III) oxide amongst the other names, perhaps the best option.

The compound has been found to be useful as a biocide as is silver itself. It is supplied by Jonas & Company, Bensalem, Pennsylvania. More details can be found on this site.
http://www.perutechnologies.com/sipaper.html

You said in your question that the structure of tetrasilver tetroxide explains its properties for batteries. I did not found a reference to that or its use in batteries. The following site describes the electrochemistry of zinc/silver oxide batteries.
http://www.doitpoms.ac.uk/tlplib/batteries/printall.php

This site states that, "The silver oxide used is usually in the monovalent [that is in the (I) state] form (Ag2O), as it is the most stable." The following reactions take place inside the cell:

At the anode:

Zn + 2OH --> Zn(OH)2 + 2eC

At the cathode:

Ag2O + H2O +2e --> 2Ag + 2OHC

Overall:

Ag2O + H2O + Zn --> 2Ag + Zn(OH)2

The cathode is generally composed of silver(I) oxide with added graphite to improve conductivity. The anode is zinc powder mixed with a gelling agent, which is then dissolved in the alkaline electrolyte. The two are separated by a combination of layers of grafted plastic membrane, treated cellophane and non-woven absorbent fibres. The top cup (negative terminal) is made up of laminated layers of copper, tin, steel and nickel, and the bottom cup (positive terminal) is nickel-plated steel. An insulating gasket prevents contact between the two."

The supplier I mentioned above may sell silver(I) oxide. Another possible source is Johnson Matthey ( AGR Matthey in Australia ).

Incidentally, I should point out, since you come from Australia, that this last site is run by Cambridge University in the UK and the "DoITPoMS" in the address stands for the "Dissemination of IT for the Promotion of Materials Science" not what you thought !!

Best of luck with your batteries. Gareth Evans


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