Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can ions separate from an ionic compound? How?

  1. Sep 1, 2015 #1
    The atoms in an ionic compound are either positively or negatively charged. I read that sodium hydroxide can cause permanent eye damage, because 'the ions travel to the back of the eyes'. Compounds are neutral, so for this to happen the ions must separate from the compound. Is this possible? If yes, how do the ions separate? What causes them to do so when entering the eye?

    Thank you.
     
  2. jcsd
  3. Sep 1, 2015 #2

    Borek

    User Avatar

    Staff: Mentor

    Sounds nonsensical to me.

    Ions in solution do travel, but for most of the time we can safely assume solution to be electrically neutral (although, when we start to talk about smaller and smaller volumes, at some point we will deal with a volume containing a single ion).

    What NaOH does to the eye (or any tissue) is it attacks the peptides (and most other compounds present) both denaturing and hydrolyzing them, and damaging the tissue.
     
  4. Sep 2, 2015 #3
    Kinda missing the point here. I'm asking if they separate from the compound, how they do it (if yes), and what causes this separation. If they do not separate from the compound, then there's got to be some other way they in which they go through the eye and damage it. The compound itself is neutral, so NaOH is clearly not an ion.
     
  5. Sep 2, 2015 #4

    Borek

    User Avatar

    Staff: Mentor

    While NaOH is neutral in the presence of water it dissociates into ions, and that's what we are typically dealing with. Not sure what other thing you can mean by

    as it doesn't make any chemical sense to me, unless the "separation" is the dissociation.
     
  6. Sep 2, 2015 #5

    Merlin3189

    User Avatar
    Gold Member

    I don't understand why there's got to be some other way? Of course NaOH does dissociate in water, but why wouldn't a NaOH molecule (if such existed) travel through the eye in the same way as a Na+ ion? The sodium ion wouldn't be much smaller, especially if Na+ is likely to be surrounded by attached water molecules

    If someone says, "ions travel to ...", that does not preclude other things, such as neutral molecules, from doing exactly the same. Did this source suggest some mechanism which depended on the entity being charged? What transport mechanism did they suggest?
    I'd guess the transport mechanism is that the NaOH dissolves in the water of the fluid bathing the eye (and incidentally separates into its ions) and is then carried along with other solutes (and maybe suspended insoluble particles) by the movement of the fluid.

    I wonder also what you are thinking about the mechanism of "permanent eye damage"? I certainly wouldn't want to put caustic soda in my eye, but I would be worrying about its immediate effects on the front of the eye, the cornea. I would be worrying about the effect of heat generated by the NaOH dissolving or being further diluted, or about it dehydrating the tissues, had not Borek suggested other chemical reactions to fear. But by the time it reached the back of my eye, as ions or otherwise, I'd have thought maybe it would be too diluted to be dangerous, or that it would have done so much damage on the way that it wasn't worth worrying about the retina!

    Incidentally, I don't doubt for one second the mechanisms suggested by Borek, but since sodium ions (and hydroxyl ions in much lower concentration) are already present in the cells at the back of the eye, presumably it is the increased concentration and /or rise in pH which causes the damaging reaction. (I must admit that I haven't yet bothered to look this up, but I can see that I need to investigate this sort of reaction. My current understanding is limited to shifting equilibria with changes in concentration, so I need to find out about thresholds which seem to occur in biological reactions.)

    Finally, perhaps you could say where you read about this, so that we can see what they were talking about.

    Edit: PS, I forgot part of your question - about why/how NaOH ionises in water. My own guess is,
    Ionic compounds are held together by electrostatic forces. Since water molecules are polar, they are attracted to the charged ions, so ions can be pulled from the surface of the solid and become surrounded by water molecules. As water has a. high dielectric constant , this weakens the force of attraction between the oppositely charged ions and they do not immediately get pulled back together. They can diffuse through the solution so long as overall electrical neutrality is maintained by equal charges of dissolved anions and cations.
    This is only my guess at a vague, hand waving explanation. I'm sure you could find better, more detailed and quantitative explanations on the net, or from real chemists here.
     
    Last edited: Sep 2, 2015
  7. Sep 3, 2015 #6
    Do the ions separate from the compound when entering the eye? Or do they still cause damage to the, even when bonded in a neutral compound?

    Thanks for the response, although you went into far too much detail describing damage to the eye. I am focused purely on whether ions in an ionic compound can separate, or not. Positive and negative ions can cause damage, so I accept that ions damage the eye. I want to know how the ions in NaOH separate from the compound when damaging the eye, and what causes them to separate into individual ions if they do. My source is a science text book by an exam board called OCR Gateway. Do you need the exact book?
     
  8. Sep 3, 2015 #7

    Borek

    User Avatar

    Staff: Mentor

    I have a feeling you are still ignoring the dissociation, or you are using some not-so-obvious definition of what the separation is.
     
  9. Sep 3, 2015 #8
    Seperating two substances means, or should mean, being able to put them in two different bottles afterwards.
    When you say 'seperate into ions', this is wrong because something being an ion doesn't mean it is separated. Salts that dissociate into ions more often than not are mixed until you use a separation technique.

    Putting something in an eye is not a seperation technique.
     
  10. Sep 3, 2015 #9

    Merlin3189

    User Avatar
    Gold Member

    Yes, one of my faults! I get interested in a question sometimes from a different angle to the OP.
    And in this case NaOH is so obvoiusly ionised in solution, that the main interest in the question for me, is how it causes the damage.

    In the case of NaOH, certainly. As far as I understand, it is very soluble in water (more than 1kg per litre of water at 20 C) and dissociates completely into its ions when dissolved in water.
    I think this point is the crux of your question. If an ionic compound dissolves in water, then it produces ions - and see next bit.

    I gave my general take on this. You might like to look at this link, How Substances Dissolve (Just the first I found with Google, but a nice simple treatment.)

    I find this a bit strange - maybe true (or not, I don't know), but not helpful. Body fluids already have many ions in various small concentrations; Na+, K+, Ca++, HCO3-, and Cl- to mention only the most common in extracellular fluid. So ions per se are not all harmful, but the concentrations needed are fairly specific and any imbalance is likely to disrupt biochemical processes. Some ions may be harmful even in tiny concentrations, but Na+ is not one of them. OH- ions are always present in bodily fluids because they are formed from dissociation of water, but their concentration is low (around 10-7 M ) So the increase in OH- ions seems the most likely source of chemical harm from NaOH.
    I would be very surprised if there were not also a whole host of nonionic compounds/molecules which would be capable of doing damage to living tissue. Unfortunately, water is such a good polar solvent, that I am not confident any of the damaging molecules I can think of would remain completely unionised in aqueous solution. The only suggestions I can make are; nonionic surfactants, as present in some houshold detergents (though I think they are irritant rather than toxic) and carbon monoxide, which I don't think generates any ions in water, but would react with haemoglobin to reduce oxygen supply to cells and this could cause temporary and maybe permanent damage to retinal cells.
    So I think, rather than claiming ions damage the eye, you should say which specific ions.
     
  11. Sep 3, 2015 #10
    Ions do not cause damage. Some ions react while others do nothing. The reactions change our biomolecules so they lose function.

    The reason OH- in sufficiently high concentration causes damage is because it is a strong nucleophile that reacts with weak electrophiles like C=O bonds, like those in our eye tissues.
    If you do not know the chemistry of nucleophiles and electrophiles you need to understand that first to understand why OH- is corrosive.
     
  12. Sep 4, 2015 #11

    epenguin

    User Avatar
    Homework Helper
    Gold Member

    Maybe what someone was trying to say is that the infuence of acids or alkalis travels fast.
    If you frequent a lab this is fairly easy to see experimentally - I admit I've never done nor seen it that I remember.
    In a beaker of water, carefully introduce a drop of dye. Colour will spread to the rest of the water only slowly.
    Instead fill the beaker volume with some suitable colour-change pH indicator. Introduce a drop of NaOH or of acid in a corner again and you should see the colour change all over the beaker immediately. The whole volume becomes acid or alkaline fast.

    This is because the dye molecules have to find their way in the jostling crowd. But if on the other hand you have say an excess of OH- in a corner, from the H-O-H molecule next to an OH- a proton will jump from one to another, which happens very fast, so
    H-O- H-O-H becomes H-O-H -O-H
    it's like an OH- molecule has moved much faster than a molecule that size really moves.

    So if you get a droplet of alkali in one corner of the eye it will affect all watery medium in contact with it, so it could be damaging the back of the eye. The upside is when you wash it, basically put your eye under running water, the thing to do immediately, it will also wash out the inaccessible parts too. Better than water theoretically should be a proprietary buffer (substance that will absorb excess OH- or H+) (as it claimed here, just found, http://www.firstaid.cederroth.com/Global/Broschyrer/Ogonduschbrochyr_EN_USA_CAN.pdf) but the water will probably be the thing most immediately to hand and it will be better not to lose seconds.

    Maybe that is what you heard is about.
     
  13. Sep 5, 2015 #12

    Merlin3189

    User Avatar
    Gold Member

    That's interesting. I shall try it out as soon as I can find my litmus. I've just tried all the food colours in the cupboard, but none gives a decent change.

    I shall be interested to see just how fast this works. I've been trying to remember doing titrations, but it's a bit vague after all this time. I remember I used to swirl the flask after each addition, but whether the final drop changed the whole flask without a swirl, I can't remember.
     
  14. Sep 5, 2015 #13

    epenguin

    User Avatar
    Homework Helper
    Gold Member

    Remembering now my titrations, which were always with agitation you have got me worried about my explanation.
     
  15. Sep 8, 2015 #14
    Thanks for the posts everyone.

    Thanks again for the response. I heard from another person that they separate due to the water being polar. The charges in the water apparently help separate the Na+ and OH- ions from NaOH.

    Thanks, I'll look into nucleophiles and electrophiles sometime.
     
  16. Sep 8, 2015 #15

    Borek

    User Avatar

    Staff: Mentor

    Yes, that's the dissociation process. It was mentioned in this thread almost a week ago. As I explained then while NaOH is split into ions, they are not "separated", but mixed, and for most practical applications every volume of the solution contains exactly the same number of both ions (so it is electrically neutral).

    Problem is about exact definition of what the "separation" means. In the context of dissociation we speak about separation meaning ions got separated by water molecules (so they don't touch each other as they do in a solid), but they are still not separated as in "all Na+ ions in one bucket, all OH- in another bucket".
     
  17. Sep 10, 2015 #16
    I see, thanks.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook