[ken3009]

i test water samples for heavy metals and have to prepare 1ppm lead standard solution. does this need to be freshly prepared from the standard lead solution or can it be kept for longer like one year?

[Stepan]

Shelf life depends on How often you use it and type of storage bottle. For 100 mL  1 ppm lead in 1-5% HNO3/water stored in PE or Glass bottle with PE or PTFE lid, I would expect shelf life to be about 3-6 months. You may want to keep old standards and re-analyse them from time to time. then you will be able to establish shelf life acceptable for your test.