Chemistry Practical for B. Sc. Part II) - Dr Deepak Pant (read e book txt) 📗

of the analyte. Physical gravimetry is the most common type used in environmental engineering. It involves the physical separation and classification of matter in environmental samples based on volatility and particle size (e.g., total suspended solids). With thermogravimetry, samples are heated and changes in sample mass are recorded. Volatile solids analysis is an important example of this type of gravimetric analysis. As the name implies, precipitative gravimetry relies on the chemical precipitation of an analyte. Its most important application in the environmental field is with the analysis of sulfite. Electrodeposition involves the electrochemical reduction of metal ions at a cathode, and simultaneous deposition of the ions on the cathode.
Common Procedures in Gravimetric Analysis
a. Drying to a Constant Weight
All solids have a certain affinity for water, and may absorb moisture from the laboratory air. Reagents that readily pick up water are termed hygroscopic. Those that absorb so much water that they will dissolve in it and form a concentrated solution are called deliquescent (e.g., sodium hydroxide, trichloroacetic acid). These types of substances will continually increase in weight while exposed to the air. For this reason, many types of laboratory procedures require that a sample be dried to a constant, reproducible weight (i.e., absorbed moisture removed to some standard, low level). This is especially important for the gravimetric methods. Generally, the sample is dried in a 103 C to 110 C oven for about 1 hour and allowed to cool to room temperature in a desiccator. It is then weighed, and heated again for about 30 minutes. The sample is cooled and weighed a second time. The procedure is repeated until successive weighings agree to within 0.3 mg.
b. Description and Use of the Analytical Balance
The analytical balance is the most accurate and precise instrument in an environmental laboratory. Objects of up to 100 grams may be weighed to 6 significant figures. Volumetric glassware is accurate to no more than 4 significant figures, and the accuracy of complex analytical methods rarely justifies more than 2 significant figures. Analytical balances are generally used for gravimetric analyses, and for the preparation of standard solutions.
Summary of Gravimetric Methods for Environmental Analysis
Some gravimetric methods are in generally using for the analysis of waters and wastewaters.
Type Analyte Pretreatment
Physical Total Solids Evaporation
Suspended Solids Filtration
Dissolved Solids Filtration + Evaporation
Oil & Grease extraction with C2Cl3F3 + distillation of solvent
Surfactants extraction into ethylacetate + evaporation
Thermal Volatile Solids Evaporation + 550`C for 15 min
Volatile Suspended Solids Filtration + 550`C for 15 min
Precipitative Mg with Diammonium hydrogen phosphate and final pyrolysis
Na with zinc uranyl acetate
Silica precipitation/ ignition/ volatilization (with HF)
SO4 with Ba

PHYSICAL GRAVIMETRY
1. Total, Dissolved and Suspended Solids
a. Definitions
Total solids (TS) is generally defined as all matter in a water or wastewater sample that is not water. Because solids are not a specific chemical compound, but rather a diverse collection of dissolved and particulate matter, their concentration cannot be determined in an unambiguous way. Instead, they must be defined by the procedure used to estimated their concentration. Total solids may be differentiated according to size into total dissolved solids (TDS) and total suspended solids (TSS). Once again, this is an operational distinction, whereby all solids passing through filter paper of a certain pore size (e.g., 1.5 microns, Whatman #934AH) are called dissolved, and those retained are termed suspended.
b. Significance to Environmental Engineering
Most of the impurities in potable waters are in the dissolved state, principally as inorganic salts. Thus, the parameters, "total solids" and especially "total dissolved solids" are of primary importance here. Waters containing high concentrations of inorganic salts are not suitable as sources of drinking water, because such materials are often difficult to remove during treatment. Finished drinking waters containing more than 1000 mg/L TDS are generally considered unacceptable. Waters of this type may also be unsuitable for agricultural purposes due to the harmful effects of high ionic concentrations on plants. In most natural waters, the TDS (total dissolved solids) concentration correlates well with total hardness (i.e., [Ca] + [Mg]). This is useful in assessing the corrosivity of a water and the need for softening.
The total suspended solids (TSS) content of natural waters is of interest for the purpose of assessing particle bed load and transport. High concentrations of suspended matter may be detrimental to aquatic life. In theory, TSS could be used for assessing particle removals during water treatment. However, nearly always the concentration of colloidal particles in water is measured as turbidity since this latter technique is faster and more precise.
Some Typical Solids Concentrations
Source Concentration (mg/L)
Low Avg High
NATURAL WATERS
Fresh TDS 20 120 1,000
Brines TDS 5,000 300,000
DOMESTIC WASTEWATER
Raw TDS 350 600 900
VDS 165 285 600
TSS 100 200 350
VSS 75 135 215
Secondary Effluent TSS 10 30 60
Activated Sludge Mixed
Liquor (conventional) TSS 1,500 3,000
Activated Sludge Mixed
Liquor (extended aeration) TSS 3,000 6,000
Primary Sludge TSS 20,000 70,000
Secondary Sludge TSS 5,000 12,000
STORM WATER TSS 5 300 3,000

Procedures
Total Solids (Total Residue). Total solids is determined by the final weight of a dried sample (minus tare) divided by the original sample volume. Evaporating dishes of platinum, vycor or porcelain may be used. Platinum is preferred, because it is more inert than the other two, and can be heated to a constant weight more easily. However, platinum is very expensive, so porcelain is often used. Porcelain is difficult to bring to a constant weight, and its use should be avoided. Space permitting, evaporating dishes should be stored in a desiccator so as to avoid the collection of dust and absorption of moisture while not in use. The precision of this method has been estimated to be  4 mg or 5%. However, settled wastewater may give better precision, on the order of  1 mg.
1. Preheat a 100 mL evaporating dish at 550 50 C for 1 hour, cool in a in a drying oven or in the open air (protected from dust) for 15-20 minutes, bring to room temperature in a desiccator, and weigh. Repeat until a constant weight is achieved.
2. Measure 75 mL of sample or a volume sufficient to yield 200 mg TS, whichever is less. Add this to the preweighed dish and evaporate to dryness in a drying oven set at 98 C. Alternatively, a steam bath may be used.
3. Dry for an additional hour at 103-105 C.
4. Cool in a desiccator and weigh.
Dissolved Solids (Filtrable Residue). Dissolved solids may be determined directly by analysis of the filtered sample for total solids, or indirectly by determining the suspended solids and subtracting this value from the total solids. When using the direct method, final drying may be conducted at one of two temperatures.

1. Analyze the filtrate in accordance with the total solids procedure.
2. Final drying (1 hour period) may be conducted at either 103-105 C or 180 2 C.
Suspended Solids (Non-filtrable Residue). Suspended solids is measured directly by drying and weighing the solids retained during filtration.
1. Dry this filter at 103-105 C for 1 hour, and cool in a desiccator.
2. Weigh the filter, then pass a water sample of sufficient volume to yield 50-200 mg suspended solids through it. Smaller volumes will result in reduced accuracy.
3. Dry for at least one hour at 103-105 C.
4. Cool in a desiccator and weigh.
C. THERMOGRAVIMETRY AND COMBUSTION ANALYSIS
Thermogravimetry and combustion analysis involve the heating of a sample to 500 C or more with the oxidation and/or volatilization of some of the sample constituents. Either the change of sample weight is determined (thermogravimetry), or the combustion gases are trapped and weighed (combustion analysis). With thermogravimetric methods, it is especially important to return the sample to room temperature before weighing. Otherwise the differences in temperature will create convection currents around the balance pan, which will severely disrupt method accuracy. A steady increase in apparent weight while the sample is on the pan indicates a problem of this type. Large vessels and samples will require longer cooling times to dissipate their excess heat.
Volatile Solids and Fixed Solids
Fixed solids are those that remain as residue after ignition at 550 C for 15 minutes. The weight of material lost is called the volatile solids. Thus the total operational definition for volatile solids would be: all matter lost upon ignition at 550 C for 15 minutes, but not lost upon drying at 103-105 C for 1 hour. The portion lost upon ignition is generally assumed to be equivalent to the organic fraction. The portion remaining is considered the inorganic fraction. For waters of moderate to high hardness, most of this is calcium carbonate which decomposes only at temperatures exceeding 800 C. When igniting a filter with suspended matter, one must be especially careful of the temperature; above 600 C glass fiber filters begin to melt and can loose a significant amount of weight in 15 minutes.
Combining the fractionations resulting from ignition and filtration, one arrives at a total of 9 separate categories: total solids (TS), fixed solids, volatile solids, total dissolved solids (TDS), fixed dissolved solids, volatile dissolved solids, total suspended solids (TSS), fixed suspended solids, and volatile suspended solids (VSS). In practice, only four of these (TS, TDS, TSS, and VSS) are commonly used. When comparing fixed solids with inorganic content, one would expect positive bias from incomplete oxidation of organic matter, and negative bias from decomposition of certain inorganics. Ammonium salts may be lost during low temperature drying or upon ignition. Most others are stable under the conditions used for volatile solids determination with the exception of magnesium carbonate. Volatile solids may be effected by these as well as loss of recalcitrant waters of crystallation (positive bias), and previous losses of organic matter to volatilization during low-temperature drying (negative bias). A modest interlaboratory study found an average standard deviation of 11 mg/L on a sample of 170 mg/L volatile solids.
MgCO3 ------------> MgO + CO2 ¬
Ammonium compounds (often present in sludge in the form of ammonium bicarbonate) may be lost during low temperature drying and therefore should not introduce a bias in volatile solids
NH4HCO3 ---------> NH3 ¬ + H2O ¬ + CO2 ¬
1. Dry and weigh a vessel containing the solids to be analyzed. For volatile and fixed suspended solids analysis, the filter (with residue) prepared for suspended solids analysis and dried to a constant weight may be used. For volatile and fixed total (or dissolved) solids, the evaporating dish (with residue) prepared for total (or dissolved) solids analysis dried to a constant weight should be used.
2. Ignite the sample and vessel in a preheated muffle furnace set at 550 50 C for 15-20 min (water & wastewater) or 1 hour (sludge, sediment & soil).
3. Cool for 15 minutes in the open air in an area protected from dust.
4. Place vessel in a desiccator for final cooling to room temperature and weigh. Due to the approximate nature of this test samples are not generally re-heated and dried to a constant weight.
D. PRECIPITATIVE GRAVIMETRIC ANALYSIS
Precipitative gravimetric analysis requires that the substance to be weighed be readily removed by filtration. In order for a non-filtrable precipitate to form, it must be supersaturated with respect to its solubility product constant. However, if it is too far above the saturation limit, crystal nucleation may occur at a rate faster than crystal growth (the addition of molecules to a crystal nucleus, eventually forming a non-filtrable crystal). When this occurs, numerous tiny micro-crystals are formed rather than a few large ones. In the extreme case, micro-crystals may behave as colloids and pass through a fibrous filter. To avoid this, precipitating solutions may be heated. Because the solubility of most salts increases with increasing temperature, this treatment will lower the relative degree of supersaturation and slow the rate of nucleation. Also, one might add the precipitant slowly with rapid mixing to avoid the occurrence of locally high concentrations.
It is very important that the precipitate be pure and have the correct stoichiometry. Coprecipitation occurs when an unwanted ion or