Question related with destilled water

Brad Hernlem hernlem at chess.ncsu.edu
Wed Jun 16 11:45:32 EST 1993


In article <1993Jun16.105552.95153 at embl-heidelberg.de>, sensen at embl-heidelberg.de writes:

|> In article <9306151433.AA05880 at net.bio.net>, VJIMENEZ at UCRVM2.BITNET ("Victor M. Jimenez") writes:
|> > Hello,
|> >     I have a question related with the pH of destilled water for use in
|> > laboratory.
|> >     Is it supposed that destilled water has a pH of 7.0?  What happends if
|> > not?  Which would be the possible cause(s)?
|> >     I work in a plant tissue culture research laboratory, so we use this
|> > water mainly to prepare plant tissue culture media.  As we adjust de media
|> > pH to an established pH, the little bit acid water that we are using should
|> > not represent a problem.  But what happends when we use this water to prepare
|> > a buffer, for example, to calibrate a pHmeter?  One should prepare two stock
|> > solutions and then mix certain volumes of them, according to the volumes mixed,
|> > one obtains a resulting pH.  Does the pH of water affect the resulting pH of
|> > the buffer?
|> >     Thanking in advance, Victor

|> Hello Victor,
[..].
|> If you make it with an ion-exchange system, you will change your not wanted
|> ions to H+ which means that the pH of the water will go down to <6. 
[...]
|> 
|> yours Christoph

First, for truly "deionized" water both cations and anions should be replaced
by H+ and OH-, not just the cations. In that case, the pH should not change.
The problem with measuring the pH of "pure" water is that the buffering 
capacity is removed. 

To answer Victor's question, the apparent acidity of your water should not 
make any appreciable difference for what you wish to do because your buffer
salts will be present in many orders of magnitude greater concentration than
the apparent H+ concentration differences that are making your water not neutral
pH. Error in weighing out the buffer salts is much more likely to introduce
error, in my opinion.

Something else worth mentioning, if you are preparing plant tissue media like
Murashige-Skoog medium, you should be aware of two things.

1. The medium is very poorly buffered and will change pH dramatically when
plant cells or tissue are growing in it. I have experienced that suspension
cultured carrot cells will take MS medium down as low as 3.8 pH without pH
control. (The direction of pH shift will depend upon whether the cells are
using ammonia or nitrate. Generally, ammonia is utilized first and so the pH
drops)

2. The medium will change pH dramatically upon autoclaving and will also shift 
over time. This appears to be caused by precipitation of iron from the medium.
If you must autoclave your medium, try out several initial pHs and find the one
which yields your desired pH after autoclaving. Even if you filter sterilize
your medium, the pH will change with time but much more slowly.

Brad Hernlem (hernlem at chess.ncsu.edu)



More information about the Plantbio mailing list