Most research articles and protocols mention using CaCl2. 2H20 in the preparation of ACSF. Is there a specific reason why we use CaCl2.2H20 in ACSF preparation rather than just using CaCl2? The same scenario seems to exist for MgSO4 which is used as MgSO4.7H2O.
The hydrated salts are more rapidly dissolved than non-hydrated salts
They simply crystallize including the water. It takes some effort to extract the water and why would you want to do that? Plus and as already mentioned, they dissolve more rapidly.
The hydratation is a very rapid process. This even can affect the precision of sampling, when scaling.
Thank you all for your responses!
Elmira Anderzhanova's response gives rise to another question here. Considering the hygroscopic nature of CaCl2 (and CaCl2.2H2o), how can we be precise about the final concentration of CaCl2 in our buffers, over time? Is this difference negligible?
I am asking this question since I am currently working on High K model of Epilepsy in acute rat brain slices, and we find changes in Ca concentration from 1.5mM to 1.2mM seems to change the pattern of seizure significantly (a similar results has been reported by Heinemann et al n early 90's).
Paul Manis- Thank you for your response. As you had mentioned, my primary concern is about synaptic transmission dynamics being affected. An old bottle of CaCl2 with unknown hydration levels, regardless of whether its anhydrous form of CaCl2 or the dihydrate form, should affect the final concentration of Ca in the ACSF.
Is there a way around this issue? When do we decide the CaCl2 under use is old?
Paul Manis- I completely accept the fact that preparing Stock solutions with a fresh bottle of CaCl2 is definitely the easiest/best option and i should probably prepare weekly stocks to be safe.
However, would it be scientifically acceptable if i prepare CaCl2 by dissolving in water and measuring the weight (Archimedes principle). Say for example, I weigh 50ml of water which weighs 50gms. I then add a few scoops of CaCl2 till the solution level reaches 55ml. Ideally this should weigh 55gms but since CaCl2 is added it might weight 2-3 gms more, which would be pure CaCl2. I am sure weighing CaCl2 this way could be much more complicated than buying a fresh batch of CaCl2, but do you think this would be a way around the hydration/hydration over time?
Also, I think MgSO4 is non-hygroscopic. Could you please reconfirm?
What we have usually done is to prepare concentrated aCSF (10 times the final solution) and freeze aliquots that are then used to make up fresh solution. This can be done once or twice a year. If there is a need to vary CaCl2 concentrations for example you can freeze CaCl2 stock solution separately. This further elimnates vaiability and contamination problems.
Filip Bergquist- Thank you very much for your suggestion. I am having the same concern as mentioned by Paul Manis.
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Paul Manis- With the volume displacement method i think the key word is "enough accuracy" as the error might be equal/more than what an old CaCl2 stock would give.
As suggested earlier, I think a new CaCl2 stock should be the easiest way around for me.
Thank you all for your responses. Your suggestions are much appreciated.
Because the anhydrous form will absorb water from the atmosphere so that you will not obtain thr correct Ca2+ concentration.
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