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2540 SOLIDS*

* Approved by Standard Methods Committee, 1997.

 

2540 C. Total Dissolved Solids Dried at 180C

 

1. General Discussion

 

    a. Principle: A well-mixed sample is filtered through a standard glass-fiber filter, and the filtrate is evaporated to dryness in a weighed dish and dried to constant weight at 180C. The increase in dish weight represents the total dissolved solids. This procedure may be used for drying at other temperatures.

    The results may not agree with the theoretical value for solids calculated from chemical analysis of sample (see above). Approximate methods for correlating chemical analysis with dissolved solids are available.1 The filtrate from the total suspended solids determination (Section 2540D) may be used for determination of total dissolved solids.

    b. Interferences: See 2540A.2 and 2540B.1. Highly mineralized waters with a considerable calcium, magnesium, chloride, and/or sulfate content may be hygroscopic and require prolonged drying, proper desiccation, and rapid weighing. Samples high in bicarbonate require careful and possibly prolonged drying at 180C to insure complete conversion of bicarbonate to carbonate. Because excessive residue in the dish may form a water-trapping crust, limit sample to no more than 200 mg residue.

 

2. Apparatus

 

    Apparatus listed in 2540B.2a-h is required, and in addition:

    a. Glass-fiber filter disks* without organic binder.

    b. Filtration apparatus: One of the following, suitable for the filter disk selected:

    1) Membrane filter funnel.

    2) Gooch crucible, 25-mL to 40-mL capacity, with Gooch crucible adapter.

    3) Filtration apparatus with reservoir and coarse (40- to 60- μm) fritted disk as filter support.

    c. Suction flask, of sufficient capacity for sample size selected.

    d. Drying oven, for operation at 180 2C.

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* Whatman grade 934AH; Gelman type A/E; Millipore type AP40; E-D Scientific Specialties grade 161; Environmental Express Pro Weigh; or other products that give demonstrably equivalent results. Practical filter diameters are 2.2 to 12.5 cm.;

Gelman No. 4201 or equivalent.

 

 

3. Procedure

 

    a. Preparation of glass-fiber filter disk: If pre-prepared glass fiber filter disks are used, eliminate this step. Insert disk with wrinkled side up into filtration apparatus. Apply vacuum and wash disk with three successive 20-mL volumes of reagent-grade water. Continue suction to remove all traces of water. Discard washings.

    b. Preparation of evaporating dish: If volatile solids are to be measured, ignite cleaned evaporating dish at 550C for 1 h in a muffle furnace. If only total dissolved solids are to be measured, heat clean dish to 180 2C for 1 h in an oven. Store in desiccator until needed. Weigh immediately before use.

    c. Selection of filter and sample sizes: Choose sample volume to yield between 2.5 and 200 mg dried residue. If more than 10 min are required to complete filtration, increase filter size or decrease sample volume.

    d. Sample analysis: Stir sample with a magnetic stirrer and pipet a measured volume onto a glass-fiber filter with applied vacuum. Wash 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. Transfer total filtrate (with washings) to a weighed evaporating dish and evaporate to dryness on a steam bath or in a drying oven. If necessary, add successive portions to the same dish after evaporation. Dry evaporated sample for at least 1 h in an oven at 180 2C, cool in a desiccator to balance temperature, and weigh. Repeat drying 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. Analyze at least 10% of all samples in duplicate. Duplicate determinations should agree within 5% of their average weight. If volatile solids are to be determined, follow procedure in 2540E.

 

4. Calculation

 

                                                                        (A B) X 1000

                        mg total dissolved solids/L =                                     

                                                                        sample volume, mL

 

where:   

 

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

    B = weight of dish, mg.

 

5. Precision

 

    Single-laboratory analyses of 77 samples of a known of 293 mg/L were made with a standard deviation of differences of 21.20 mg/L.

 

6. Reference

  1. SOKOLOFF, V.P. 1933. Water of crystallization in total solids of water analysis. Ind. Eng. Chem., Anal. Ed. 5:336.

 

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7. Bibliography

 

HOWARD, C.S. 1933. Determination of total dissolved solids in water analysis. Ind. Eng. Chem., Anal. Ed. 5:4.

 

U.S. GEOLOGICAL SURVEY. 1974. Methods for Collection and Analysis of Water Samples for Dissolved Minerals and Gases. Techniques of Water-Resources Investigations, Book 5, Chap. A1. U.S. Geological Surv., Washington, D.C.

 

 

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