Finding the isotonic point

When plotting the weight change on the y-axis against the percentage of salt dissolved in water from the x-axis, the isotonic point can be found.

If the weight increases, water has moved into the potato cells. Therefore, the solution must have had higher water potential and therefore hypotonic. If the weight decreases, water has moved out of the potato cells. Therefore, the cells must have had the higher water potential and the solution was hypertonic. If the weight did not change, the solutions had equal water potential and the salt solution was isotonic.

Figure 1: Graph with the weight change plotted against the salt concentration. The isoelectric point is the point where the regression line crosses the x-axis.

In the graph, you can see this as the point where the line crosses the x-axis. At this point, there is a 0% weight change.