Nicoleta wanted to know how long a fairly average standard solution would remain chemically stable — for example, 6 ppm lead in HNO3. This is a topic that Inorganic Ventures has been studying for over a decade.
The stability of each element is dependent upon the chemical form (for inorganics, it's the oxidation state in many cases), the matrix, the container, storage conditions, and the concentration. In the case of your example (lead), lets assume the +2 oxidation state (most common) and that the matrix in high purity water acidified with high purity nitric acid. Lets also assume that the container is LDPE that it has been acid-leached and that the solution is being stored in a 15-25 °C temperature controlled area. Under these conditions, the chemical stability of Pb at the ppm level would last for years and years. We have been doing studies for over a decade and have not seen Pb under the above conditions drop out of solution.
Another form of instability is transpiration of the container, i.e. evaporation of water around the cap when closed and left on the shelf. This effect is dependent upon the type and geometry of the container. For a 125 mL LDPE bottle (the geometry used by Inorganic Ventures), the transpiration shelf-life is 21 months. Our Interactive Periodic Table contains stability information for all of the metals and many of the non-metals. In addition, our Tech Center features several articles about shelf-life and transpiration stability.
Thank you for contacting Inorganic Ventures and for your question.