Hi Sara

To prevent precipitation and maintain stability in your 1000 ppm stock solutions of heavy metals (Ni, Cd, Co, Cu, Pb), you can take the following steps:

1. pH Adjustment:

○ Acidify your stock solutions by adding a small amount of 1% nitric acid (HNO₃). This will help maintain an acidic pH, which reduces the likelihood of metal hydroxide precipitation.

○ Start by adding a small volume (e.g., 1 mL) of 1% HNO₃ to each liter of your stock solution.

○ Monitor the pH and adjust as needed to keep it in the acidic range (around pH 2-3).

2. Chelating Agents:

○ Consider using chelating agents to complex heavy metal ions and prevent precipitation.

○ Common chelators include EDTA (ethylenediaminetetraacetic acid) or citric acid.

○ Add a small amount of chelator to your stock solutions (follow recommended guidelines).

3. Storage Conditions:

○ Store your stock solutions in dark glass containers to minimize exposure to light.

○ Keep them in a cool, dry place to prevent degradation.

Remember to perform regular stability checks to ensure the long-term stability of your stock solutions. If you notice any signs of precipitation or instability, adjust the pH or consider adding more chelating agents.

Sources:

1: Kowalski, A. (2002). Metals Removal to Low Levels Using Chemical Precipitants. ONDEO Nalco Company. Link

Source(s)

1. Metals Removal to Low Levels Using Chemical Precipitants

2. Hydroxide Precipitation of Metals | Hoffland Environmental, Inc | (936 ...

3. Removal of Heavy Metal Ions from Water and Wastewaters by Sulfur ...

4. Heavy Metals Precipitation | Encyclopedia.com

5. Precipitation of Heavy Metals | SpringerLink

1% HNO3 should be plenty to keep elements in solution. Note that adding 1ml to the 1000ml stock solution will dilute the concentrations by 0.1% which may affect your calibrations by a non-insignificant amount. You haven't mentioned what instrumental method you are using.

Francesco Barbangelo glass containers are not recommended for long term storage as there will be (slow, low concentrations but real nonetheless) leaching of other elements that contribute to the colouring of the glass. This also goes for clear glass which contains decolouring elements which are also leached from the glass matrix. Eg. http://www.compoundchem.com/2015/03/03/coloured-glass/

Better to use acid-washed polyethylene or polypropylene containers.

Most providers of standard solutions package their products in 1-5% Nitric acid, or sometimes up to 10% HCl depending on the elements. Inorganic ventures has a detailed periodic table-based listing here: https://www.inorganicventures.com/periodic-table