STAV 2008 Unit 4 Question 5 - Please Help

[Scarlett]

Can someone please help me with this question?

"Blood refrigerated at 4 degrees C was found to have a pH of 7.0. From this information, blood at this temperature is
        Acidic  /  Basic  /  Neutral (circle your response)

    Explain your response."

The answer is acidic, but why? If the reaction where water is self-ionised is reversed, then wouldn't there still be equal concentrations of the hydronium ion and hydroxide ion, making it neutral?

[bec]

I got this question wrong too...You just have to watch out with the temperature. Neutral pH is only 7 at 25^oC...So if the pH is 7 but the solution isn't at that 25^oC, it means that it can't be neutral.

If the reaction where water is self-ionised is reversed, then wouldn't there still be equal concentrations of the hydronium ion and hydroxide ion, making it neutral?

Yes, that's right...but the thing is, blood isn't just pure water. If it was, then it wouldn't have pH 7 at 4 degrees, it would have a lower pH due to the increase in [H₃O⁺].

Hope that helps!

[We Want Peace.]

Just think this:
At 25, the pH of pure water = 7
Since the self- ionisation of water is endothermic
As the temperature goes down to 4 degrees, [H3O+] will decrease.
->  pH of H2O > 7

Next compare this to the pH of the blood
 At 4℃pH of blood is still 7
 
that means it has got H3O+ ions when it is at 25℃

Therefore it appears to be acidic

( Above is my way of understanding. ...not sure if it is right or not.)

[Mao]

I got this question wrong too...You just have to watch out with the temperature. Neutral pH is only 7 at 25^oC...So if the pH is 7 but the solution isn't at that 25^oC, it means that it can't be neutral.

If the reaction where water is self-ionised is reversed, then wouldn't there still be equal concentrations of the hydronium ion and hydroxide ion, making it neutral?

Yes, that's right...but the thing is, blood isn't just pure water. If it was, then it wouldn't have pH 7 at 4 degrees, it would have a lower pH due to the increase in [H₃O⁺].

Hope that helps!

almost but not quite, self ionisation of water is endothermic, hence at 4 degrees, there will be less hydronium/hydroxide, i.e. pH of neutral water is greater than 7.

hence a pH of 7 at 4 degrees is acidic.

[Scarlett]

Thanks everyone! It makes sense now.