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The partial specific volume has units of milliliters per gram (ml/g). In this sense it is simply the inverse of the density. however, partial specific volume is more strictly defined as the volume taken up upon mixing into a solution. So a protein may have different partial specific volumes for different buffers. Some substances (like ammonium chloride in water) may even have a negative partial specific volume because instead of taking up volume upon mixing, they actually reduce the volume of the solution by condensing water around themselves.