When studying the concentrations of substances in environmental samples, for example in soil or in plant matter, it is important to standardize how the weight is measured. Because the water content of a soil or plant sample can vary significantly, the best way of standardizing measurements is to use the "dry weight" of the sample -- the weight of the sample once all the water has evaporated off, as a proportion of the starting ("wet") weight.
Things You'll Need
- Samples (soil or plant, for example)
- Accurate scales
- Drying oven
- Small heat proof dish
- Calculator or spreadsheet
Place a small sample (for example, 1 g of soil) into a small, heat-proof dish and precisely record the weight.
Put the sample into a heated drying oven at a sufficient temperature to evaporate the moisture in the sample. Leave the sample for several hours or overnight.
Remove the sample and weigh it. The difference in weight from the original sample will be the amount of water that has been lost.
Return the sample to the oven, and remove and reweigh it an hour later. If the weight has reduced again, return to the oven for another hour and reweigh. Once the weight is constant, you know that all the moisture has evaporated.
Calculate how much moisture has been lost. For example, if a wet sample of 1.07 g weighed 0.83 g after drying, the weight of water can be calculated as follows:
1.07 g - 0.83 g = 0.24 g
Calculate this as a proportion of wet sample weight that is water:
0.24 g / 1.07 g = 0.224
Multiply the decimal by 100 to get a percentage. Thus, 22.4 percent of the wet weight is water.
Calculate the dry weight of all subsequent samples you are working with. In the example, the original material was 22.4 percent water by weight, so if you are working with a 0.75 g wet weight sample, you can calculate the dry weight like this:
0.75 g * (1 - 0.224) = 0.582 g
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