Air, Water and Land Pollution
Chapter 6:
Common Operations and Wet Chemical
Methods in Environmental Laboratories
Copyright © 2010 by DBS
Contents
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Basic Operations in Environmental Laboratories
Wet Chemical Methods and Common Techniques in Environmental Analysis
Analytical Principles for Common Wet Chemical Methods
Common Operations…
Basic Operations in Environmental Laboratories
Common Operations…
Basic Operations in Environmental Laboratories
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Classical and modern techniques
‘Classical’ gravimetric or volumetric based wet chemical methods are routinely
employed in many laboratories
Still required for preparation prior to modern instrumental analysis (e.g. calibration
standards)
Common Operations…
Basic Operations in Environmental Laboratories
“A true chemist, or analytical chemist, has several characteristics. He or she has
knowledge of the methods and instruments used for analysis, and understands the
principles of the analysis. Laboratory analysts should have a chemistry background
adequate to understand and correctly apply all of the laboratory rules and to evaluate
and interpret the results of their analysis. They have to know how to plan and
organize laboratory work so the time is used efficiently. Thus, a laboratory technician
should be a skilled, well-trained chemist – in sharp contrast with the so-called
‘determinators’ who simply twist the dials of an instrument or follow ‘cookbook’
analytical procedures”
Csuros, 1997
Common Operations…
Basic Operations in Environmental Laboratories
Labware Cleaning Protocols for Trace Analysis
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Critical for quality data
Particularly true for trace analysis
Increased use of disposable labware
(e.g. pipet tips, etc.)
Lab cleaning can account for up to
one-third of an analyst’s time
In multi-use labs it is imperative not
to share glassware
Shared glassware may contain enough
residue to completely overwhelm a trace
analysis in an environmental sample
Common Operations…
Basic Operations in Environmental Laboratories
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Cleaning technique varies depending on analyte of interest
General rule of thumb is:
(i) an acid wash for trace metals
(ii) acetone wash for trace organics
Common Operations…
Basic Operations in Environmental Laboratories
Basic guidelines:
1. Soak dirty glassware in 2 % hot detergent solution (lab grade, e.g. Alconox/Liquinox)
2. For metals acid wash in 10-15 % HNO3 (except for N analysis), or 10-15 % HCl
(should not be used for metallic items)
3. For organics use wash bottles filled with pesticide-grade isopropanol, acetone or
methanol
4. Rinse in-between with hot running water detergent and acid/acetone wash, filling
each item with water, shaking and emptying at least three times. After acid/acetone
was, use reagent water to rinse at least three times.
5. Once clean, dried and stored in a location that prevents contamination from airborne
dust or other particles. If oven drying is used do not exceed 140 ºC for glass or 60 ºC
for plastic. Never oven dry burets, pipets, or volumetric flasks.
6. In biological work, glassware contaminated with blood clots or bacteria must be
sterilized with 2 % disinfectant solution or boiling water prior to cleaning. If virus or
bacteria are present, autoclaving is absolutely necessary.
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
1. Reagent Water
• Most widely used analytical solvent for preparation of analytical blanks, samples and
standards.
• Preparations: Type I, II, III and IV
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
1. Reagent Water
• Type I – highest quality, prepared by distillation, deionization or reverse osmosis
• Followed by polishing with a mixed bed deionizer and a 0.2 µm pore size membrane
filter
• Polishing removes trace organics, particulates, and bacteria
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
1. Reagent Water
• Type II – by distillation or deionization
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
1. Reagent Water
• Type III – used for glassware washing, preliminary rinsing of glassware and
feedwater for production of higher purity waters
µmho/cm = µS/cm, megohm-cm is the unit for resistivity
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
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Show that a conductivity of 0.06 µS/cm is equal to a resistivity of 16.67 megohm-cm
µmho/cm = µS/cm, megohm-cm is the unit for resistivity
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
1. Reagent Water
• Common indicator is resistivity > 10 megaohm-cm at 25 ºC / low conductivity < 0.1
µmho/cm at 25 ºC
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
2. Common Acid and Alkaline Solutions
• Commercial acids and bases commonly used in environmental laboratories
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
3. Reagent Purity
• Grade or degree of purity decided based on analytical purpose
• Highest grade for preparation of standard solutions
• ACS grade, reagent and technical grades
• Specified impurities are typically 10-1,000 ppm
Common Operations…
Basic Operations in Environmental Laboratories
Chemical Reagent Purity, Standard, and Reference Materials
4. Standard and Reference Materials
• Materials containing a known concentration of analyte
• Provide reference to determine unknown concentrations or to calibrate analytical
instruments
• Titration Standards
– Primary standard prepared from a reagent that is pure, stable, and no water of
hydration
e.g. sodium carbonate, potassium hydrogen phthalate, potassium dichromate
etc.
– Secondary standard is a reagent standardized against a primary standard
e.g. sulfuric acid, hydrochloric acid, sodium hydroxide
• Standard Reference Materials (SRM)
– Purchased from a reputable vendor or certifying organization
– EPA requires SRMs of known purity and traceability
Common Operations…
Basic Operations in Environmental Laboratories
Volumetric Glassware and Calibration
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Subject to error if used improperly
Volumetric flasks, burets, and pipets for measuring volume
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Transfer Pipets:
– Come in two main size ranges:
(a) micropipets smaller than 1 mL (1, 10, 50, 100, 500 µL)
(b) regular glass or plastic pipets (1, 2, 5, 10, 25, 50, 100 mL)
Common Operations…
Basic Operations in Environmental Laboratories
Volumetric Glassware and Calibration
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Volumetic pipets deliver fixed volumes (1, 2, 5, 10, 25, 50, 100 mL).
Volumetric pipets are allowed to drain and should never be blown out
More accurate than:
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Measuring pipets which have graduation lines, may be serological (graduations
continue to the tip, may be blow-out type) or Mohr (graduations end before tip)
Common Operations…
Basic Operations in Environmental Laboratories
Volumetric Glassware and Calibration
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Volumetic glassware is calibrated wither to contain (TC) or to deliver (TD)
Common Operations…
Basic Operations in Environmental Laboratories
Laboratory Health, Safety, and Emergency First Aid
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Minimum standard of practice for health and safety activities is detailed in
government documents OSHA 29 CFR 1910.1450
Laboratory Hygiene Plan (LHP) or Chemical Hygiene Plan (CHP)
Chemical, physical and biological hazards
Common Operations…
Basic Operations in Environmental Laboratories
Waste Handling and Disposal
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Source reduction and waste minimization
Separation of wastes
Special rules for highly toxic or reactive chemicals, biological wastes, and radioactive
chemicals
Common Operations…
Wet Chemical Methods and Common Techniques
Gravimetric and Volumetric Wet Chemical Methods
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Gravimetric – weighing with an analytical balance
– Used for the following parameters: Solids, moisture, sulfate, oil and grease
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Volumetric – measuring volumes (titration)
– Classical using indicators
– Electrochemical using potentiometry/conductivity
– Types: acid-base, oxidation-reduction, complexation, precipitation
Common Operations…
Wet Chemical Methods and Common Techniques
Commonly used wet chemical methods in environmental analysis
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Filtration
• Used to collect materials from fluid (air and water) matrices
• Gravity filtration and vacuum (suction) filtration
• Water: e.g. analysis of suspended solids in water or dissolved phase after removal of
solids
• Air: e.g. filter media connected to a vacuum sampling pump to measure particulate
matter
• Filter media come in various materials (glass fiber, membrane etc.) shapes and sizes
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Centrifugation
• Centrifuge is used to separate solids or liquid particles of different densities or sizes
by rotation at high speeds
• Ultracentrifuge > low speed centrifuge > microcentrifuge
• Precautions:
– Must be balanced
– Use correct centrifuge tubes
– Rotors must be secured before operation
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Distillation
• Required for ammonium, cyanide, sulfide, and phenol analysis
• Distillation flask, condenser, and receiving flask
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Reflux
• Used in COD analysis where organic compounds are oxidized by a strong oxidizing
reagent under acidic conditions
• Reflux apparatus allows samples to be heated for an extended time without water
loss
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Concentration
• Kuderna-Danish (K-D) evaporative concentrator or rotary evaporator (rotavap)
• Remove excess solvent from the mixture of analyte and solvent
• Concentration of analytes will be sufficiently high to be detected
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Digestion
• Used for metals, total phosphorus, and total nitrogen
• Procedure breaks down organically bound substances and converts the substance to
the analyzed form by using liquid oxidizing agents (e.g. H2SO4, HNO3, HClO3, HCl)
• If total decomposition of silicates is needed HF is used
Common Operations…
Wet Chemical Methods and Common Techniques
Common Laboratory Techniques
Extraction
• Chemicals have different solubilities in different solvents which can be used to
selectively remove a solute from a mixture
• Used primarily to concentrate trace organic samples
• Liquid-liquid (L-L) extraction using a separatory funnel for liquids
• Soxhlet extraction for solid samples
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Moisture in Solid and Biological Samples
• Moisture content of solid (soil, sediment, sludge) and biological samples is required
so that the contaminant concentration can be reported on a dry basis
• Involves weighing a portion of sample in a tarred crucible, drying to constant weight
at 104 ºC
% moisture = (wet weight – dry weight) x 100 %
wet weight
Concentration (dry weight basis) = concentration on wet basis
100 - % moisture
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Solids in Water, Wastewater, and Sludge (TS, TSS, TDS, VS)
Total Solids (TS)
• TS refers to material left behind in a vessel after evaporation and subsequent oven
drying
TS = TSS + TDS
Total Suspended Solids and Total Dissolved Solids
• TSS is the portion of solids retained by a 0.45 μm filter
• TDS is the portion that passes through the filter
• Temperature at which the residue is dried is important due to loss of volatile material,
water of crystallization, and gases
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Solids in Water, Wastewater, and Sludge (TS, TSS, TDS, VS)
Volatile Solids (VS)
• VS is the difference between dried solids and the same sample further ignited at 550
C in a muffle furnace
TVS = TS – TSash, TVSS = TSS – TSSash, TVDS = TDS – TDSash
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Ash is the residue at 550 ºC
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Acidity, Alkalinity, and Hardness of Water
1. Acidity
• Natural sources include CO2, H2PO4-, H2S, protein, fatty acids, and salt of trivalent
metals such as Fe3+ and Al3+ (e.g. Al(H2O)63+ ⇌ Al(H2O)5OH2+ + H+)
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Man-made sources include mineral acids from metallurgical industry, acid mine
drainage, acid rain and organic acid waste
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Measurement – alkaline solution (NaOH) and indicator
Mineral acidity or methyl orange acidity – red to yellow at pH 3.7
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Acidity, Alkalinity, and Hardness of Water
1. Acidity
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Measurement – alkaline solution (NaOH) and indicator
Mineral acidity or methyl orange acidity – red to yellow at pH 3.7
Total acidity or phenolphthalein acidity – colorless to pink at pH 8.3
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Acidity, Alkalinity, and Hardness of Water
1. Acidity
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To calculate acidity NaOH must be converted to the equivalent amount of CaCO3
(mg/L)
Acidity (mg CaCO3/L) = N x V x 5 x 104
Vs
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Where N = normality of base (=concentration in this case), Vs = volume of water
sample (mL), V = volume of base (mL)
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Note 1 N NaOH is 1 M wrt OH-, 2 N H2SO4 is 1 M in H+
Units
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Units are mg L-1 CaCO3
MW = 100 g/mol
CaCO3 + 2H+ ⇌ H2CO3
H+ + OH- → H2O
mg CaCO3 / L = mols OH- x 1 mols CaCO3 x 1 x 105 mg
2 mols OHmol CaCO3
Volume of sample (L)
Common Operations…
Analytical Principles for Common Wet Chemical Methods
Acidity, Alkalinity, and Hardness of Water
2. Alkalinity
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Alkalinity is the capacity of water to neutralize an acid
In natural waters alkalinity is due to H2CO3 (dissolved CO2)
In polluted waters minor amounts of NH3, salts of weak acids, and organic acids
contribute to alkalinity
Measured using titration with acid as titrant
– Phenolphthalein alkalinity pH 8.0 - 9.6 (due to OH- and CO32-); and
– Total alkalinity (methyl orange alkalinity) pH 3.1 - 4.4 (due to OH- and CO32-), and
HCO3-)
Reported as mg CaCO3/L using previous equation
Common Operations…
Wet Chemical Methods and Common Techniques
Acidity, Alkalinity, and Hardness of Water
3. Hardness
• Water hardness due to Ca2+ and Mg2+
• Adverse effects for domestic and industrial uses (scale in hot water pipes, heaters,
and boilers)
Ca2+ + 2HCO3- →CaCO3(s) + CO2(g) + H2O
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Require large amounts of soap to make foam
Hardness (mg/L CaCO3) = Σ M2+ (mg/L) x 50/EW of M2+
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Where equivalent weight (EW) of Ca2+ and Mg2+ are 20.0 and 12.2 respectively
Titration, FAAS or ICPAES/MS
Common Operations…
Wet Chemical Methods and Common Techniques
Acidity, Alkalinity, and Hardness of Water
3. Hardness
Hardness (mg/L CaCO3) = Σ M2+ (mg/L) x 50/EW of M2+
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Where equivalent weight (EW) of Ca2+ and Mg2+ are 20.0 and 12.2 respectively
e.g. water has a calcium content of 51 mg/L, what is the calcium hardness as
CaCO3?
Hardness (mg/L CaCO3)
= Σ M2+ (mg/L) x 50/EW of M2+
= 51 x 50/20.0 = 128 mg/L CaCO3
Common Operations…
Wet Chemical Methods and Common Techniques
Acidity, Alkalinity, and Hardness of Water
3. Hardness
Hardness (mg/L CaCO3) = Σ M2+ (mg/L) x 50/EW of M2+
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Where equivalent weight (EW) of Ca2+ and Mg2+ are 20.0 and 12.2 respectively
e.g. water has a calcium content of 28 mg/L and a magnesium content of 9 mg/L,
what is the calcium hardness as CaCO3?
Hardness (mg/L CaCO3) = Σ M2+ (mg/L) x 50/EW of M2+
= (28 x 50/20.0) + (9 x 50/12) = 107 mg/L CaCO3
Common Operations…
Wet Chemical Methods and Common Techniques
Oxygen Demand in Water and Wastewater (DO, BOD, COD)
1. Dissolved Oxygen
• DO originates from atmospheric O2 (21 %)
• DO in water is typically around 8 mg/L
• Amount depends on aeration, wind, water flow, algae, temperature, pressure, organic
compounds, salt content, bacteria, etc.
• Important water quality parameter
• Most common oxygen consuming pollutants are organic compounds (protein, sugar,
fatty acids) which are readily degraded by aerobic bacteria
Common Operations…
Wet Chemical Methods and Common Techniques
Oxygen Demand in Water and Wastewater (DO, BOD, COD)
1. Dissolved Oxygen
• Two analytical methods:
– Classical Winkler Method (primary)
– Polarographic membrane electrode method (secondary)
Common Operations…
Wet Chemical Methods and Common Techniques Method
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DO is fixed after sampling by reaction with Mn2+ (MnSO4) together with alkaline
iodide/azide mixture
Mn2+ + 2OH- + ½ O2 → MnO2(s) + H2O
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I- is needed for titration, N3- prevents NO2- and Fe3+ interference (production of excess
I2 from KI)
After transport to lab sample is acidified with H2SO4
(dissolves Mn4+ floc), and Mn4+ oxidizes iodine ions:
MnO2 + 2I- + 4H+ → Mn2+ + I2 + 2H2O
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I2 is then titrated with thiosulfate and starch indicator
I2 + 2S2O32- → S4O62- +2 I•
Over all reaction:
2S2O32- + 2H+ + 1/2O2 → S4O62- + H2O
Common Operations…
Wet Chemical Methods and Common Techniques Method
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Over all reaction:
2S2O32- + 2H+ + 1/2O2 → S4O62- + H2O
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1 mol of O2 is equivalent to 4 mols of thiosulfate
DO (mg/L) = N x V x 8000
Vs
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Where N and V are the normality and volume (mL) of Na2S2O3 respectively, Vs is the
sample volume
e.g. 1 mL of 0.025 M S2O32- is required to reach the blue starch end-point of a 200
mL sample, calculate the DO level (mg/L)
0.025 M S2O32- = 0.025 N S2O32DO = 0.025 N x 1 mL x 8000 / 200 mL = 1 mg/L
Example Calculation
1 mL of 0.025 M S2O32- is required to reach the blue starch end-point of a 200 mL sample.
Calculate the moles of O2 dissolved in the sample, and the mg/L DO.
1 mol O2 = 4 mols S2O32mols S2O32- x 1 mol O2
4 mols S2O32-
= mols O2
0.025 mol/L x 1 mL x 1 L / 1000 mL = 2.5 x 10-5 mols S2O322.5 x 10-5 mols S2O32- x mol O2
4 mols S2O32-
= 6.25 x 10-6 mols O2
There are 6.25 x 10-6 mols of O2 in the 200 mL sample
6.25 x 10-6 mol = 3.125 x 10-5 mol O2 / L x 32 g / mol = 1 x 10-3 g /L x 1000 mg/g = 1 mg O2 / L =
0.2 L
= 1 ppm DO
Common Operations…
Wet Chemical Methods and Common Techniques
Oxygen Demand in Water and Wastewater (DO, BOD, COD)
2. Biochemical Oxygen Demand (BOD)
• BOD refers to the amount of oxygen consumed if all the biodegradable organics in
one liter of water were oxidized by aerobic bacteria
• Standard 5-day BOD5 test – 300 mL BOD bottles are used
• Initial DO reading is made with sample saturated with DO, parallel samples are
incubated at 20 ºC in the dark for 5 days
BOD5 (mg/L) = DO0 – DO5
Common Operations…
Wet Chemical Methods and Common Techniques
Oxygen Demand in Water and Wastewater (DO, BOD, COD)
3. Chemical Oxygen Demand (COD)
• ABOD provides closest measure of the processes occurring in natural waters
• Test is time consuming
• COD test is used as a measure of the oxygen equivalent of the organic matter
content of a sample susceptible to oxidation by a strong chemical oxidant
• Results in 2 hr or less, provides good estimate of BOD
• Disadvantage is that it may oxidize materials not readily oxidized in nature
Common Operations…
Wet Chemical Methods and Common Techniques
3. Chemical Oxygen Demand (COD)
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Dichromate ion, Cr2O72- dissolved in sulfuric acid is a powerful oxidizing agent. It
is used as an oxidant to determine COD
6Fe2+ + Cr2O72- + 14H+ → 6Fe3+ + 2Cr3+ + 7 H2O
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Excess dichromate is added to achieve complete oxidation
Back titration with Fe2+ gives the desired endpoint value
COD (mg/L) = N x (V0 – V) x 8000
Vs
Where N is the normality of Fe(NH4)2(SO4)2, Vs is the volume of water sample
(mL) and V0 and V are the volumes (mL) of Fe(NH4)2(SO4)2 for the blank and
sample, respectively
Common Operations…
Wet Chemical Methods and Common Techniques
Oil and Grease in Water and Wastewater
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“Oil” represents a wide variety of substances from low to high molecular weight
hydrocarbons found in petroleum to light gasoline, heavy fuel, and lubricating oils
“Grease” represents higher molecular weight hydrocarbons and glycerides of animal
and vegetable origin
Defined by operational procedure
Partition-gravimetric method: acidification is required if fatty acids are present to
increase their solubility, hexane is used to extract the oil and grease and removed via
evaporation, final extract is weighed
Common Operations…
Wet Chemical Methods and Common Techniques
Residual Chlorine and Chloride in Drinking Water
1. Residual Chlorine
• Chlorine gas is most widely used disinfectant for water supplies
• Cl2 hydrolyzes to hypochlorous acid and hydrochloric acid
Cl2 + H2O ⇌ HOCl + HCl
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HOCl is a weak acid that dissociates into hypochlorite (OCl-)
HOCl ⇌ H+ + OCl-
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Hypochlorous acid and hypochlorite are called free chlorine residuals
Common Operations…
Wet Chemical Methods and Common Techniques
Residual Chlorine and Chloride in Drinking Water
1. Residual Chlorine
• 3 titrimetric methods and other instrumental methods for measuring residual chlorines
(a) iodometric titration – for concentrations > 0.1 mg/L
(b) iodometric back titration – primarily used for wastewaters
(c) DPD-FAS titration
Common Operations…
Wet Chemical Methods and Common Techniques
Residual Chlorine and Chloride in Drinking Water
1. Residual Chlorine
• Iodmetric Titration – free chlorine species (OCl- and HOCl) liberate iodine (I2) from
potassium iodide at pH 4 or lower
• I2 is titrated with sodium thiosulfate using a starch indicator
Cl2 + 2KI → I2 + 2KCl
I2 + starch → blue color
I2 + 2Na2S2O3 → 2NaI + Na2S2O6
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Iodometric Back Titration – liberated I2 is reacted with excess Na2S2O3, remaining
Na2S2O3 is back titrated with I2 as the titrant instead of using Na2S2O3 as the titrant
I2 + 2Na2S2O3 (excess) → 2NaI + Na2S2O6
Common Operations…
Wet Chemical Methods and Common Techniques
Residual Chlorine and Chloride in Drinking Water
1. Residual Chlorine
• DPD-FAS Titration – N,N-diethyl-p-phenylenediamine (DPD) reacts with free
chlorine residuals to produce a red color
• At pH 6.2 to 6.5 reaction is rapid and reversible
• Solution is titrated with ferrous ammonium sulfate (FAS) until disappearance of red
color
Common Operations…
Wet Chemical Methods and Common Techniques
Residual Chlorine and Chloride in Drinking Water
2. Chloride (Cl-)
• High chloride concentration may be due to seawater intrusion, evaporated salts,
discharge of human wastes
• Cl- is commonly used for tracer studies
• Argentometric (Mohr) Method – In neutral or slightly basic solution, potassium
chromate (CrO42-) indicates the end-point of a silver nitrate titration
AgNO3 + Cl- ⇌ AgCl + NO32Ag+ + CrO42- ⇌ Ag2CrO4 (orange)
Common Operations…
Wet Chemical Methods and Common Techniques
Residual Chlorine and Chloride in Drinking Water
2. Chloride (Cl-)
• Mercuric Nitrate Method – Cl- is titrated with mercuric nitrate (Hg(NO3)2). At the pH
range 2.3 to 2.8 diphenylcarbazone (DPC) indicates the end-point via the formation of
a purple complex with excess mercuric ions
Hg2+ + 2Cl- ⇌ HgCl2
Common Operations…
Wet Chemical Methods and Common Techniques
Ammonia in Wastewater
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Ammonia is produced by decomposition of organic nitrogen containing compounds
and by the hydrolysis of urea
Ammonia is titrated with H2SO4 using mixed indicator (methyl red and methylene
blue)
Sample is titrated until it turns pale lavender
2NH4+ + H2SO4 → 2H+ + (NH4)2SO4
Common Operations…
Wet Chemical Methods and Common Techniques
Cyanide in Water, Wastewater and Soil Extract
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Cyanide refers to all cyanide groups in cyanide compounds (CN-)
Used in plastics, electroplating, metallurgy, synthetic fibers, and chemicals
Titrimetric method detection limit of 1 mg/L
HCN is liberated from an acidified sample by distillation and purging with air through a
NaOH scrubbing solution
CN- in scrubbing solution is the titrated with silver nitrate to form a soluble cyanide
complex (Ag(CN)2-)
As soon as all CN- is complexed and a small amount of Ag+ has been added, excess
Ag+ combines with an indicator (p-dimethyl-aminobenzal-rhodamine, C12H12N2OS2)
which turns the solution from yellow to brownish-pnk
2CN- + Ag+ → [Ag(CN)2]-
Common Operations…
Wet Chemical Methods and Common Techniques
Sulfide in Water and Waste
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Sulfide (H2S, HS-, and S2-) present due to reducing conditions
Waste and wastewater sulfide comes from anaerobic decomposition (bacterial
reduction of sulfate)
Determined via titration – sulfide samples are preserved by addition of zinc acetate
and NaOH, ZnS precipitate is oxidized to sulfur with addition of excess iodine (I2) in
an acidic solution, remaining iodine is determined via titration with sodium thiosulfate
References
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Csuros, M. (1997) Environmental Sampling and Analysis: Lab Manual. Lewis Publishers,
Boca Raton, Fl.
HACH Company (1992) HACH Water Analysis Handbook, 2nd Edition, Loveland, CO
(Procedures available: http://www.hach.com).
US EPA (1983) Methods for Chemical Analysis of Water and Wastes. EPA/600/4-79/020.
Questions
11. 100 mL water sample was used for SS measurement. The mass of the weighing
bottle plus filter paper was 25.6257g before filtration and 25.6505 g after . What is the
TSS in mg/L?
17. If 3.0 mL of 0.02 N H2SO4 is required to titrate 200 mL of sample to
phenolphthalein end point, what is the phenolphthalein alkalinity as mg/L CaCO3? If
additional 20.0 mL of H2SO4 is needed to reach the methyl orange point, what is the
total alkalinity as mg/L CaCO3.
22. Write all chemical reactions involved in the measurement of DO using Winkler
method.
28. (a) Explain why the conversion factor in eqn. 6.13 is 8000. (b) If 200 mL sample
was used for DO measurement, it requires 8.6 mL of 0.0275 N Na2S2O3 solution.
What is the DO in mg/L?
32. Describe the difference between BOD and COD in terms of the chemicals each
method represents.