Make fair tissue comparisons
Use pieces with comparable dimensions, source tissue and exposure time. Keep temperature and solution volume comparable. Tissue variability can affect the result, so independent pieces are more informative than repeatedly weighing one piece.
Measure what you intend
Blot surface liquid consistently before final weighing; otherwise external solution can add to the mass recorded. Blotting should remove adhering liquid without deliberately squeezing water from the tissue. Record both the initial and final masses.
Calculate signed percentage change
Use (final − initial)/initial × 100. Percentage change allows comparison of pieces with different initial masses. Keep the initial mass as the denominator. A final mass below the initial mass gives a negative result.
Read the zero-change point
Plot mean percentage change against solution concentration. A zero-change estimate locates a solution at which there was no net mass change under those conditions. To express the result as water potential, use the supplied concentration-to-water-potential relationship or calibration; concentration alone is not a water-potential unit.
Limit the conclusion
The zero crossing is an estimate influenced by measurement uncertainty, sampling and the fitted line or curve. It does not mean water molecules stop moving. Osmosis continues in both directions, with no net change when the relevant water potentials balance.
Worked data example
A tissue piece changes from 2.00 g to 1.80 g. Mean changes at 0.20 and 0.40 mol dm⁻³ are +4% and −4%. Assume a straight line locally.
Mass change = (1.80 − 2.00)/2.00 × 100 = −10%. The local zero crossing lies halfway between the two concentrations: 0.30 mol dm⁻³. Convert that concentration to water potential only if a suitable relationship is supplied.
Does a zero net mass change show that water has stopped crossing cell membranes?
No. Water molecules still move in both directions; the measurements indicate no net change under the experimental conditions.