Sample analysis: Assemble filtering apparatus and filter and begin suction. Wet filter with
a small volume of reagent-grade water to seat it. Stir sample with a magnetic stirrer at a speed to
shear larger particles, if practical, to obtain a more uniform (preferably homogeneous) particle
size. Centrifugal force may separate particles by size and density, resulting in poor precision
when point of sample withdrawal is varied. While stirring, pipet a measured volume onto the
seated glass-fiber filter. For homogeneous samples, pipet from the approximate midpoint of
container but not in vortex. Choose a point both middepth and midway between wall and vortex.
Wash filter with three successive 10-mL volumes of reagent-grade water, allowing complete
drainage between washings, and continue suction for about 3 min after filtration is complete.
Samples with high dissolved solids may require additional washings. Carefully remove filter
from filtration apparatus and transfer to an aluminum weighing dish as a support. Alternatively,
remove the crucible and filter combination from the crucible adapter if a Gooch crucible is used.
Dry for at least 1 h at 103 to 105°C in an oven, cool in a desiccator to balance temperature, and
weigh. Repeat the cycle of drying, cooling, desiccating, and weighing until a constant weight is
obtained or until the weight change is less than 4% of the previous weight or 0.5 mg, whichever
is less. Analyze at least 10% of all samples in duplicate. Duplicate determinations should agree