Noble metal

Platinum, gold, and mercury dissolve in the acid solution aqua regia, while iridium and silver do not. Palladium and silver dissolve in nitric acid. Niobium and tantalum resist all acids, including aqua regia.

Calling a metal "noble" may also be used as an adjective to describe its chemical and galvanic activity. Under this definition, metals may be ranked according to whether they are more noble or more active. This galvanic series can be used to compare one metal with another for a particular application, typically within a set of conditions (such as pH). In this context, graphite (a form of carbon) is more noble than silver.

IRIDIUM IRIDIUM-PLATINUM
IRIDIUM IRIDIUM PLATINUM
IRIDIUM-RHODIUM Palladium
IRIDIUM RHODIUM Palladium
Platinum PLATINUM-IRIDIUM
Platinum PLATINUM IRIDIUM
PLATINUM-RHODIUM Rhodium
PLATINUM RHODIUM Rhodium