# Water Potential

Hello,

I'm a bit confused about water potential.

Supposing I do an experiment that looks at the water potential of potato.

I use different molar concentrations of sucrose and find that at a concentration of 2.5 mol dm-3 the potato does not gain or lose any weight.

So if I plotted a graph of % change in mass with molar concentration of sucrose in water, this would be the intercept on the graph.

Pure water has a water potential of zero, and solute potential is always negative.

But how do I convert the findings from my experiment into water potential? What are the units of water potential? Do we need to know this stuff for AS Biology?

Brian
Have a look at a biology data/practical book- they have tables which help you convert things like sucrose concentration into water potential. AFAIK, there is no way of calculating the water potential directly, atleast at AS level.
thanks visesh,

I don't suppose you have a link to any such a thing online?

Thanks,
brian
brianfall
thanks visesh,

I don't suppose you have a link to any such a thing online?

Thanks,
brian

site
# Determine the approximate sucrose concentration for which there is no net change in density after tissue incubation (i.e., drop disperses, the &#936;w tissue = &#936;w solution).

# Calculate &#936;s (= &#936;w in an open system such as a solution in a beaker):

&#936;s = -miRT where

m = molality (moles/1000 g)
i = ionization constant (1.0 for sucrose)
R = gas constant (0.00831 liter MPa mol-1 K-1 )
T = Temperature in degrees K (C + 273 = K)
note: the units are MPa

NOTE: the following is the preferred SI form for the equation. The answer will be in Jm-3 which are equivalent to pressure (Pa). Divide by 106 to convert to MPa.

&#936;s = -miRT where

M = molality (1 molal = 1 x 103 mol m-3 H2O)
i = ionization constant (1 for sucrose)
R = gas constant (8.31 JK-1 mol-1 )
T = temperature (K)

I couldn't find a table, but just did a quick google search. there appears to be a calcuation you can use to convert [sucrose] to water potential.
found this;

http://www.biologycorner.com/bio3/diffusionlab.html

"The solute potential for a sucrose solution can be calculated using the following;

Solute potential = -iCRT where

i = ionization constant (1 for sucrose because it doesn't ionize water)
C = molar sucrose concentration at equilibrium
R = pressure constant (handbook value = 0.0831)
T = temperature degrees Kelvin (273 + degrees centegrade)"

so for my example this would be

-1 x 2.5 x 0.0831 x 295 = -61.286

Anyone know if this is right?

Ta,
B
I found that formula above too. You learn new things every day...

(I am quite sure that equation is not in the syllabus- they only expect you to look up the data in a book.)
thanks visesh