Hi all,
I've recently found that my calcium phosphate based HEK293T transfections have been unsuccessful due to the lack of formation of the calcium/DNA precipitate.
After a little testing, I found that the source of the problem was some "old" calcium chloride. When I tried with fresh CaCl2, the precipitate formed rapidly. This has left me wondering - what is the shelf life of a CaCl2 solution stored at 4C?
Many thanks,
Daniel
The expected shelf life depends on the concentration: the bigger the concentration the longer the period, e.g. 1 M solutions should last at least 1 y
Thanks for your answer Paul. I was a little surprised as I was working with a 3 month old, 0.5M solution, so I'd have expected it to be fine! Perhaps the storage conditions haven't been consistent in this case.
For such high concentrations Daniel, the most common effect of storage is evaporation of water, i.e. the concentration increases over time, Paul.
Calcium chloride is very hygroscopic and deliquescent. It thus may have to be conveniently dried before weighted. The aq. salt solution may either uptake or evaporate water if its container is left open for some time, depending on solution concentration, temperature, and on the contacting atmosphere relative humidity. The wet salt or its aq. solution are prone to react with atmospheric CO2, accordingly: CaCl2 + H2O ↔ CaO + 2HCl; CaO + CO2 → CaCO3↓. You may want to check the pH of the calcium chloride solution, as it tends to decrease upon such carbonation. The slightly carbonated salt can be purified by recrystallization from absolute ethanol.
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