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2540 SOLIDS*

* Approved by Standard Methods Committee, 1997.

 

2540 B. Total Solids Dried at 103–105°C

 

1. General Discussion

 

    a. Principle: A well-mixed sample is evaporated in a weighed dish and dried to constant weight in an oven at 103 to 105°C. The increase in weight over that of the empty dish represents the total solids. The results may not represent the weight of actual dissolved and suspended solids in wastewater samples (see above).

    b. Interferences: Highly mineralized water with a significant concentration of calcium, magnesium, chloride, and/or sulfate may be hygroscopic and require prolonged drying, proper desiccation, and rapid weighing. Exclude large, floating particles or submerged agglomerates of nonhomogeneous materials from the sample if it is determined that their inclusion is not desired in the final result. Disperse visible floating oil and grease with a blender before withdrawing a sample portion for analysis. Because excessive residue in the dish may form a water-trapping crust, limit sample to no more than 200 mg residue (see Section 2540A.2).

 

2. Apparatus

 

    a. Evaporating dishes: Dishes of 100-mL capacity made of one of the following materials:

    1) Porcelain, 90-mm diam.

    2) Platinum—Generally satisfactory for all purposes.

    3) High-silica glass.*

    b. Muffle furnace for operation at 550°C.

    c. Steam bath.

    d. Desiccator, provided with a desiccant containing a color indicator of moisture concentration or an instrumental indicator.

    e. Drying oven, for operation at 103 to 105°C.

    f. Analytical balance, capable of weighing to 0.1 mg.

    g. Magnetic stirrer with TFE stirring bar.

    h. Wide-bore pipets.†

    i. Graduated cylinder.

    j. Low-form beaker.‡

______________

* Vycor, product of Corning Glass Works, Corning, NY, or equivalent.

† Kimble Nos. 37005 or 37034B, or equivalent.

‡ Class B or better.

 

 

3. Procedure

 

    a. Preparation of evaporating dish: If volatile solids are to be measured ignite clean evaporating dish at 550°C for 1 h in a muffle furnace. If only total solids are to be measured, heat clean dish to 103 to 105°C for 1 h. Store and cool dish in desiccator until needed. Weigh immediately before use.

    b. Sample analysis: Choose a sample volume that will yield a residue between 2.5 and 200 mg. Pipet a measured volume of well-mixed sample, during mixing, to a preweighed dish. For homogeneous samples, pipet from the approximate midpoint of the container but not in the vortex. Choose a point both middepth and midway between wall and vortex. Evaporate to dryness on a steam bath or in a drying oven. Stir sample with a magnetic stirrer during transfer. If necessary, add successive sample portions to the same dish after evaporation. When evaporating in a drying oven, lower temperature to approximately 2°C below boiling to prevent splattering. Dry evaporated sample for at least 1 h in an oven at 103 to 105°C, cool dish in desiccator to balance temperature, and weigh. Repeat cycle of drying, cooling, desiccating, and weighing until a constant weight is obtained, or until weight change is less than 4% of previous weight or 0.5 mg, whichever is less. When weighing dried sample, be alert to change in weight due to air exposure and/or sample degradation. Analyze at least 10% of all samples in duplicate. Duplicate determinations should agree within 5% of their average weight.

 

4. Calculation

 

                                                                            (AB) X 1000

                                        mg total solids/L =                                      

                                                                        sample volume, mL

                                       

where:

A = weight of dried residue + dish, mg, and

B = weight of dish, mg.

5. Precision

 

Single-laboratory duplicate analyses of 41 samples of water and wastewater were made with a standard deviation of differences of 6.0 mg/L.

 

 

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6. Bibliography

 

SYMONS, G.E. & B. MOREY. 1941. The effect of drying time on the determination of solids in sewage and sewage sludges. Sewage Works J. 13:936.

 

 

©Standard Methods for the Examination of Water and Wastewater. 20th Ed. American Public Health Association, American Water Works Association, Water Environment Federation.