How to adjust osmolarity of patch pipette filling solution ???

r norman rsn_ at
Tue Aug 3 10:57:55 EST 2004

On 3 Aug 2004 07:42:17 -0700, tehgabriel at (tomte) wrote:

>How may one adjust the osmolarity of the solution used to fill patch
>electrodes? Though i am using a receipe that seems to be quite
>conventional i always obtain a solution with low osmolarity compared
>to the values i found in literature. I use a standard recipe (see
>below) for which i would expect an osmolarity of 280 +/- 5 mOsm. But
>what i get after checking osmolarity is a value of 255 mOsm. Now i
>could increase osm. by simply adding more k+-gluconate. but to reach a
>final level of 285 mOsm i would have to add ~ 25 mM k-gluconate,
>thereby increasing the final [k+] of the solution to ~160 mM!!!
>Is there a better way to increase osmolarity without affecting [K+]
>Or is it a kind of 'common knowledge' that a adjusting osmolarity is
>accompanied by an alterarion of e.g. [K+]?
>Any comment would be appreciated!
>Here is the recipe i  am using (all in mM):
>K - Gluconate	115
>KCl	                20
>MgCl2	               2
>HEPES	             10
>Mg - ATP	     2
>Na2 - ATP	     2
>Tris - GTP	      0,3

Miracle of miracles, an actual real science question on this

Measured osmolarity is never what you expect because you never have
perfect solutions that really obey van't Hoff's Law.  K-gluconate does
not completely dissociate (nor does KCl, for that matter).   Your
measured value of 255 seems sufficiently close to 280.

Do you remember "activity coefficients" from GChem or PChem? They tend
to be seriously below 1.0 for "concentrated" solutions.
Unfortunately, all biological solutions are "concentrated" as far as a
physical chemist is concerned.

If your recipe is a standard one and your cells are happy, then don't
worry about the actual measured osmolarity.  Just for fun, try making
very carefully a 0.150 M solution of KCl and see if it turns out to be
exactly 300 mOsM.

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