Na+ and water (question).

[Singh]

---

Members don't see this ad.

Here's a question which has been bothering me for a while:

In litterature you will often read that "Sodium binds water". Is this really true, and if so, anyone care to explain the reason behind this?

I used to think sodium binded water in cells, which was the reason that cells kept their structure instead of shrinking or exploding. But I have now learnt that it's the osmosis caused by sodium which is responsible for this.

So does sodium bind water?

Thanks for any replies.

 

[MoosePilot]

Here's a question which has been bothering me for a while:

In litterature you will often read that "Sodium binds water". Is this really true, and if so, anyone care to explain the reason behind this?

I used to think sodium binded water in cells, which was the reason that cells kept their structure instead of shrinking or exploding. But I have now learnt that it's the osmosis caused by sodium which is responsible for this.

So does sodium bind water?

Thanks for any replies.

Huh?

Sodium is extremely soluble. So if you put sodium in water, it tends to dissociate into ions. If you have mostly salt and a little water, you might consider it to "bind" the water, because you'll end up with clumpy salt.

 

[emack]

Maybe it's a reference to the fact that increasing a given compartment's Na+ concentration will usually cause water retention (so osmolality stays constant). I've never heard it stated as 'sodium binds water' though-- sometimes as 'water "follows" sodium.'

😕

 

[Scottish Chap]

Maybe it's a reference to the fact that increasing a given compartment's Na+ concentration will usually cause water retention (so osmolality stays constant). I've never heard it stated as 'sodium binds water' though-- sometimes as 'water "follows" sodium.'

😕

I sense two different phenomena in your posts. 1. "Na+ binding water": Sodium does in fact have a hydration shell and it's a different size from other ions like potassium (difft. number of protons explains this). This becomes important in explaining ion slectivity as ions more through different ion channels in membranes causing a change in membrane potential. 2. "water follows" sodium". This is just osmosis. Water will move to keep the solute (in this case Na+) concentration balanced on either side of a selectively-permeable membrane. This becomes important in blood preesure and renal physiology - as more Na+ is reabsorbed, water moves with it so blood volume (and hence blood pressure) will increase.

 

[emack]

Yeah-- I figured the OP's question probably had more to do with osmosis. The last time I heard anybody cared about solvation was first year General Chemistry.

 

[Perrin]

Na has a sphere of hydration around it. The negative O portion of H2O shields the + charge on Na. Along with osmosis, this is how some secretory cells get water into whatever they are secreting.

 

[pratik7]

since Na+ is postive and water has a partial negative with lone electron pairs...there maybe some ionic interaction

 

[werd]

yeah... Na has a +1 charge and the O in water ~(-0.4); the charge attraction causes water to form a sphere around sodium ions (with O facing in)... you could call this sodium "binding" water. since water molecules are spending more time "saying hi" to sodium they are spending less time randomly moving than areas of water with fewer ions. the relative difference in random movement is one cause of what we see as osmosis... relative spread of water into areas of lower solute concentration.

 

[getunconcsious]

Yeah-- I figured the OP's question probably had more to do with osmosis. The last time I heard anybody cared about solvation was first year General Chemistry.

Ugh...I wish...we're doing this kind of boring bullcrap in MS1 Physiology right now. If the OP is also in Med Phys, I would say just make sure you understand osmosis and how the equations work. The chemistry stuff isn't so important. Also, understand how isotonicity is maintained in the 3 body fluid compartments. Sodium chemistry in and of itself (i would guess) can't be that important.

 

[Singh]

Thanks for your answers!

A special thanks to Perrin who came with an excellent explanation.

I understand things a little better now, the world suddenly looks much brighter, the flowers smell better, the water tastes better .