# Water Potential

Bres O'La la la Breslin at St-Davids-coll.ac.uk
Wed Jun 26 09:51:56 EST 1996

```I am a Biology teacher at St David's Sixth Form College and I have come
across a fundamental problem in the determination of water potential.For
over 20 yrs I have been using a Psi value of -2482kpa for 1.0M Sucrose
solution at 20 degrees centigrade.  Simpkins and Williams textbook
Advanced Biology gives a similar value -2270kpa. Hendrickse Laboratory
biology gives an equation  (see attachment)

Where z = molarity of the sucrose solution and 273 + y = temperature in
degrees Absolute/Kelvin. 1 = pressure in atmospheres, or 101.32kPa may be
used instead. All references, so far, seem more or less compatible but  MV
Roberts Biology Students Manual p36 gives a table to plot a graph etc with
a Psi value of -3510 kPa for 1M sucrose soln at, presumably,  room
temperature ie 293 degrees K.
Green Stout and Taylor another Advanced Level textbook volume 2 p478 uses
exactly the same set of figures. Obviously the two Psi values are totally
incompatible and I would very much appreciate an authoritative opinion on
this matter as to which is correct.

Cheers

Mick Roberts
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