Since your experiment is testing the effect of different salts on the melting properties of ice. You are right to try and control the loss of salt, but also control the environment around the ice. From your post, you talk about a block of ice, so I'm guessing that you might end up with a fair amount of water that may actually cover part of the ice sample. In this case, water can actually act as insulation. The cool water at a lower temp than the surrounding air, prevents further melting and reduces the surfaced area of the ice exposed to the air. If you were going to periodically remove the water, you would have to make sure you did it often and in the same way, time frame, for each sample to ensure the conditions of the test are as close to the same for all samples, save your independent variable, the salt. So my advice is to place the ice samples in identical containers with holes in the bottom...something like a colander...so liquid water can run out. Under this, place something that can collect the runoff.
As for measurement, it would be best to use the initial mass of the ice and then the mass of the water and remaining ice. You can then make direct comparisons and calculate the percentage of ice melted, ice remaining.
For additional ideas and information, reference this project on science buddies that is very similar to the one you are conducting: http://www.sciencebuddies.org/science-f ... #procedure
You'll notice that in this experiment, you only pour off the melt at the end of the experiment, but this one uses single cubes of ice, the amount of water is unlikely to cause significant error in conditions.
If you still have questions, please post back.
I hope this helps.
"As the circle of light increases, so does the circumference of darkness around it."
~ Albert Einstein