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Chemical Analyses: Water: Total Dissolved Solids
Total dissolved solids (TDS), also referred to as filterable residue,
represents the portion of the sample (water, wastewater, effluent)
that passes through a filter of a particular size. Generally, a
pore size of 0.45 mm is considered to be adequate to differentiate
between microscopic particulate and dissolved material. The final
result, after evaporation and drying to constant weight at 180�C,
represents the total dissolved solids.
Total dissolved solids can be used for a variety of applications
at mine sites:
- Determination of mine runoff, receiving water, and mine waste
water quality.
- General check of other water quality parameters. For example,
total dissolved solids should approximate the sum of all dissolved
cations and anions analyzed in a water sample. Any suspect samples
can be reanalyzed; and
- Calibration of another parameter. For example, sulfate concentration
can be calibrated to TDS allowing reduction in frequency of analysis
of either parameter.
Total dissolved solids results are limited to providing a measure
of the total water soluble fraction and do not reveal the quantity
or type of individual contaminants in the sample (MEND,
2001).
The suggested method for measuring TDS is outlined in Standard
Methods, Section 2540C (Standard
Methods, 1998). See below for a brief description of the Standard
Methods procedure for TDS.
TDS method description
In preparation for the analysis, a clean evaporating dish is heated
in an oven at 180�C for one hour. After cooling in a dessicator,
the dish is weighed on a four or five decimal balance immediately
before use. The filter paper to be used is prepared by washing with
several portions of distilled or deionized water on a vacuum filtration
unit. Most commonly, glass fiber filter discs, such as Whatman 934
AH, Gelman type A/E, Millipore AP 40 and others are used for the
filtration of the sample. A known volume of sample is filtered through
the prepared filter. The filtrate is collected in a clean flask
and the filter is washed with three successive 10 mL portions of
reagent grade water which are added to the filtrate. The total volume
of filtrate and washings is transferred into the preweighed evaporation
dish and placed on a steam bath for evaporation. Ideally, sufficient
sample is processed to obtain 10 to 200 mg of residue. Analysis
is completed by heating the evaporating dish for at least one hour
in an oven at 180�C. The dish is then cooled in a dessicator and
weighed. The cycle of drying, cooling and desiccating is repeated
until a constant weight is obtained. The total dissolved solids
are reported as the increase in weight of the dish per unit volume
of sample, mg/L. For field measurements, a combination TDS/conductivity
meter can provide an estimate of TDS concentration.
Chemical
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