Environmental Sampling and Analysis for Metals - Chapter 18 ppt

calibra-18.1.1 EPA-A PPROVED M ETHODS AND R EFERENCES FOR A NALYZING W ATER S AMPLES 18.1.1.1 Methods and References for Analyzing Drinking Water Methods for Chemical Analysis of Water a

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Determination of Metals in

Environmental Samples

18.1 METHODOLOGY

Methods are developed to analyze diverse media for specific parameters Each method is approved

by the Environmental Protection Agency (EPA), which specifies the procedures, instrument tion, sample preparation, analytical procedures, and quality control requirements for the analyticalwork EPA methods are differentiated according to the media (matrix) of the sample analyzed Eachlaboratory has a written guidebook that contains specific procedures used, known as standard oper-ating procedures (SOPs) SOPs should be constantly revised to include new methodologies and pro-cedural changes The SOPs are an important tool for the quality assurance/quality control (QA/QC)operation of the laboratory

calibra-18.1.1 EPA-A PPROVED M ETHODS AND R EFERENCES

FOR A NALYZING W ATER S AMPLES

18.1.1.1 Methods and References for Analyzing Drinking Water

Methods for Chemical Analysis of Water and Wastes (EPA 600/4–79–020, revised March 1983) Methods for Determining Organic Compounds in Drinking Water (EPA 600/4–88–039,

December 1988)

Standard Methods for the Examination of Water and Wastewater (APHA-AWWA-WPCF, 19th

ed., 1998) (an updated edition is issued every 5 years)

Manual for Certification of Laboratories Analyzing Drinking Water (EPA 570/9–90/008, April

1990)

CFR Part 141, Subpart C and Subpart E (monitoring and analytical requirements)

EPA 500 series (should be used for organic analyses of drinking waters and raw source waters)

18.1.1.2 Methods and References for Analyzing Surface Waters

and Wastewater Effluents

Methods for Chemical Analysis of Water and Wastes (EPA 600–4–79–020, revised in

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18.1.1.3 Methods and References for Analyzing Water Sources (Surface and

Groundwater) Pursuant to 40 CFR Part 261 (RCRA)

Test Methods for Evaluating Solid Waste (EPA SW-846, 3rd ed., 1986; rev ed., December 1987) 40 CFR, Part 261 (Methods, Appendix III, 1989)

USEPA Contract Laboratory Program Statement of Work for Inorganic Analyses (EPA SOW

Microbiological Methods for Monitoring the Environment (EPA 600/8–78–017, 1987)

40 CFR, Part 141 (Subpart C, monitoring and analytical requirements, July 1989)

40 CFR, Part 136 (Table IA, July 1989)

Methods for Measuring the Acute Toxicity of Effluent to Freshwater and Marine Organisms

18.1.2 EPA-A PPROVED M ETHODS AND R EFERENCES FOR A NALYZING

S EDIMENTS AND R ESIDUALS

18.1.2.1 Methods and References for Analyzing Soils, Sediments,

Domestic and Industrial Sludges, Solid and Hazardous Wastes

Test Methods for Evaluating Solid Waste (EPA SW-846, 3rd ed., 1986; rev ed., December

1987)

40 CFR, Part 261 (Appendix III, July 1989)

Procedures for Handling and Chemical Analysis of Sediments and Water Samples (EPA/Corps

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18.1.3.2 EPA Methods 601, 602, 624, and 625

Capillary columns may be used instead of the specified packed columns if the laboratory meets thepertinent accuracy and precision criteria and detection limit with this modification

18.1.3.3 EPA Methods 601 and 602

The photoionization detector and electrolytic conductivity detector may be used in a series if the oratory can meet the performance criteria

18.1.4 EPA C ONTRACT L ABORATORY P ROTOCOL (CLP)

This protocol was developed for the Superfund program CLP specifies a set of methods based on theexisting methodology for organic and inorganic parameters, but which are modified to incorporate

certain quality control, calibration, and deliverable requirements The data package includes a full

reporting of quality control procedures and data, making it particularly useful if litigation is a bility The results of the analyses are provided in many different formats, ranging from a sample re-port only to a full-documentation data package

possi-The CLP, as stated in the EPA statement of work (SOW), has a high level of quality assurance quirements The deliverable requirements include quality control summaries (method blank, initialcalibration verification, duplicate analysis, and matrix spike/matrix spike duplicates) and qualitycontrol data, as well as data on a diskette Consequently, CLP has become a commonly requestedmethodology and has the effect of separating larger laboratories — which have the equipment, cer-tifications, and trained personnel capable of producing data according to this protocol — from thethousands of smaller environmental laboratories which do not

re-Because EPA methods, as now written, are not interchangeable, it is very difficult for an ical laboratory to accommodate all quality control criteria for all methods Thus, the EPA’s currentintent is to create a unified method to minimize the requirement differences

analyt-18.1.5 D ETERMINATION OF S ELECTED M ETALS IN E NVIRONMENTAL S AMPLES

Table 18.1 summarizes the methods, method numbers, and references used for determination of als in environmental samples

met-18.2 ALUMINUM

Aluminum (Al) is the third most abundant element of the Earth’s crust, occurring in mineral rocksand clays Soluble, insoluble, and colloidal aluminum may appear in treated water or wastewater as

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a residual of coagulation with aluminum-containing material Filtered water from a modern, sand filtration plant should have an aluminum concentration less than 50 µg/l.

rapid-Selection of method: The FAAS, GrAAS, and ICP methods are preferred For discussion of

in-strumentation and analysis procedures, see Chapters 8, 9, and 12, respectively

18.2.1 F LAME A TOMIC A BSORPTION S PECTROSCOPY (FAAS)

Aluminum may be as much as 15% ionized in a nitrous oxide/acetylene flame Use an ionization pressor of 1000 µg/ml K as KCl (dissolve 95 g of KCl and dilute to 1000 ml) The calibration stan-dards should contain the same type of acid in the same concentration as in the sample (usually 5

sup-ml of acid per 100 sup-ml), and 2 sup-ml/100 sup-ml of KCl solution as suppressor (see above)

Parameter FL GR Other Method No Ref Method No Ref.

Note: Metals analysis by inductively coupled plasma (ICP) method is widely used according to method 6010, with reference

to R-3 Fl = flame atomic absorption technique; Gr = graphite furnace atomic absorption technique; R-1 = methods for

Chemical Analysis of Water and Wastes (EPA-600/4-79-020, Revised March 1983); R-2 = Standard Methods for the Examination of Water and Wastewater (AWWA, 18th ed., 1992); R-3 = Test Methods for Evaluating Solid Wastes (EPA SW-

846 EPA SW-846, 3rd ed., 1986).

TABLE 18.1

Methods for Determination of Metals

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18.2.1.1 Instrument Parameters

• Instrument: Aluminum hollow cathode lamp

• Fuel: Acetylene

• Oxidant: Nitrous oxide

• Type of flame: Rich fuel

• Background correction: Not required

18.2.2 G RAPHITE F URNACE A TOMIC A BSORPTION S PECTROMETRY (G R AAS)

Background correction may be required if the sample contains highly dissolved solids Chloride ionand nitrogen used as a purge gas reportedly suppress the aluminum signal; therefore, the use of halideacids and nitrogen as a purge gas should be avoided

• Drying time and temperature: 30 sec at 125°C

• Ashing time and temperature: 30 sec at 1300°C

• Atomizing time and temperature: 10 sec at 2700°C

• Purge gas: Argon

The level of antimony (Sb) present in natural waters is usually less than 10 µg/l and may be present

in higher concentrations in hot springs or waters draining mineralized areas Antimony is a regulatedcontaminant under various federal and state programs

Selection of method: The GrAAS method (Chapter 8) is the method of choice because of its sitivity Alternatively, use the FAAS method (Chapter 9) or the ICP method (Chapter 12) when highsensitivity is not required

sen-18.3.1 F LAME A TOMIC A BSORPTION S PECTROSCOPY (FAAS)

In the presence of lead (1000 mg/l), spectral interference may occur at the 217.6-nm resonance line

In this case, the 231.1-nm antimony line should be used

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High Pb concentration may cause a measurable spectral interference on the 217.6 nm-line In thiscase, a secondary wavelength or Zeeman background correction should be used See Chapter 9 forgeneral discussion of the furnace technique A soft-digestion procedure is the only recommended onefor Sb, as discussed in Sections 15.2.2 and 15.8 The addition of HCl to the digestate prevents fur-nace analysis of many metals

• Purge gas: Argon or nitrogen

• Wavelength: 217.6 nm (primary); 231.1 nm (alternate)

• Background correction: Required

Other operating parameters should be set as specified by the instrument manufacturer

18.4 ARSENIC

Severe poisoning can arise from the ingestion of arsenic trioxide (As3O2) in amounts as small as 100 mg;chronic effects may result of the accumulation of arsenic compounds in the body at low intake levels.Carcinogenic properties are also known The toxicity of arsenic depends on its chemical form The

As concentration in potable waters is usually less than 10 µg/l, but values as high as 100 µg/l havebeen reported Aqueous arsenic may result from mineral dissolution, industrial discharges, or the ap-plication of herbicides

Selection of methods: The hydride-generation atomic absorption method (Chapter 11) is themethod of choice, although the GrAAS (Chapter 9) is simpler

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18.4.1 G ASEOUS H YDRIDE A TOMIC A BSORPTION M ETHOD

This method is applicable for sample matrices that do not contain high concentrations of Cr, Cu, Hg,

Ni, Ag, Co, and Mo Instrumentation and analytical procedures are discussed in Chapter 11 The ical detection limit for this method is 0.002 mg/l

typ-18.4.2 G RAPHITE F URNACE A TOMIC A BSORPTION S PECTROMETRY (G R AAS)

Following the appropriate dissolution (acid digestion) of the sample, a representative aliquot of thedigestate is spiked with nickel nitrate solution and is placed manually or by means of an automaticsampler into a graphite furnace See Chapter 9 for details of the GrAAS technique

Elemental As and many of its compounds are volatile; therefore, samples may be subject to losses of

As during sample preparation Spike samples and standard reference materials should be processed

to determine if the chosen dissolution method is appropriate

Caution must be employed during the selection of temperature and times for the dry and char cles A nickel nitrate solution must be added to all digestates prior to analysis to minimize volatiliza-tion losses during drying and ashing

cy-Arsenic analysis may be subject to severe nonspecific absorption and light scattering caused bymatrix components during atomization Aluminum is a severe positive interferant in the analysis ofarsenic Zeeman background correction is very useful in this situation

If the analyte is not completely volatilized and removed from the furnace during atomization,memory effects will occur If this situation is detected by means of blank burns, the tube should becleaned by operating the furnace at full power at regular intervals during the analysis

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reagent-18.4.2.5 Procedure

1 Prepare samples for the analysis as described in Sections 15.6.2 and 15.6.3

2 Pipet 5 ml of digested solution into a 10-ml volumetric flask, add 1 ml of the 1% nickelnitrate solution, and dilute to 10 ml with reagent-grade water The sample is ready for in-jection into the furnace

3 The 193.7-nm wavelength line is recommended

4 A background correction system is required For other spectrophotometric parameters,follow the manufacturer’s instructions

5 Furnace parameters suggested by the manufacturer should be employed as guidelines.Because temperature-sensing mechanisms and temperature controllers can vary among in-struments or with time, the validity of the furnace parameters must be periodically con-firmed by systematically altering the furnace parameters while analyzing a standard Inthis manner, losses of analyte due to overly high temperature settings or losses in sensi-tivity due to less-than-optimum settings can be maintained Similar verification of furnaceparameters may be required for complex sample matrices

6 Calibration curves must be composed of a minimum of a blank and three standards A ibration curve should be made for every hour of continuous sample analysis

cal-7 Inject a measured microliter aliquot of sample into the furnace and atomize If the centration found is greater than the highest standard, the sample should be diluted in thesame acid matrix and reanalyzed The use of multiple injections can improve accuracy andhelp detect furnace pipeting errors

con-8 Run a check standard after every ten injections of samples Standards are run in part tomonitor the life and performance of the graphite tube Lack of reproducibility or sig-nificant change in the signal for the standard indicates that the graphite tube should

be replaced

9 Employ a minimum of one blank with a sample batch to verify any contamination

10 The standard addition method (Section 7.7.1.1.1) should be employed for the analysis ofall EPTOX extracts

11 QC requirements are listed in Chapter 13

18.5 BARIUM

Barium (Ba) stimulates the heart muscle However, a barium dose of 550 to 600 mg is consideredfatal to human beings Despite its relative abundance in nature (16th in order of rank), barium occursonly in trace amounts in water (0.7 to 900 µg/l, with a mean of 49 µg/l) Higher concentrations indrinking water often signal undesirable industrial waste pollution

Selection of method: Preferably, analyze via the FAAS (Chapter 8), GrAAS (Chapter 9), or ICP(Chapter 12) method

The FAAS technique is described in Chapter 8 A high, hollow, cathode-current setting and a narrowspectral band pass must be used, because both barium and calcium emit strongly at barium’s analyt-ical wavelength Barium undergoes significant ionization in the nitrous oxide/acetylene flame, re-sulting in a significant decrease in sensitivity All samples and standards must contain 2 ml of potas-sium chloride (KCl) ionization suppressant per 100 ml of sample (Dissolve 95 g of KCl in reagent-grade water and dilute to 1 liter.)

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Prepare calibration standards via dilutions of the stock solution at the time of analysis The bration standards should be prepared to contain the same type and concentration of acid as the sam-ples to be analyzed after digesting All calibration standards should contain 2 ml of the KCl (ioniza-tion suppressant) solution.

cali-18.5.1.1 Instrument Parameters

• Instrument: Barium hollow cathode lamp

• Wavelength: 553.6 nm

18.5.1.2 Performance Characteristics

• Optimum concentration range: 1 to 20 mg/l

• Sensitivity: 0.4 mg/l

• Detection limit: 0.1 mg/l

The use of halide acid should be avoided Because of possible chemical interaction, nitrogen shouldnot be used as a purge gas

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Background correction may be required Concentration of aluminum greater than 500 ppm may press beryllium absorbance The addition of 0.1% fluoride has been found effective in eliminatingthis interference High concentrations of magnesium and silicon cause similar problems and requirethe use of the standard additions method

sup-18.6.1.1 Instrument Parameters

• Instrument: Beryllium hollow cathode lamp

• Optimum concentration range: 0.05 to 2 mg/l

For concentrations below 0.02 mg/l, the furnace procedure is recommended

Long residence time and high concentrations of the atomized sample in the optical path of thegraphite furnace can result in severe physical and chemical interference Furnace parameters must beoptimized to minimize these effects In addition to the normal interferences experienced duringgraphite furnace analysis, beryllium analysis is subject to severe nonspecific absorption and lightscattering during atomization Simultaneous background correction is required to avoid erroneoushigh results

The above concentration values and instrument conditions are for a Perkin Elmer HGA-2100,based on the use of a 20-µl injection, continuous-flow purge gas, and nonpyrolytic graphite Smallersizes of furnace devices or those employing faster rates of atomization can be operated using loweratomization temperatures for shorter time periods than the recommended settings above

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Selection of methods: The GrAAS method (Chapter 9) is preferred The FAAS (Chapter 8) andICP (Chapter 12) methods provide acceptable precision and bias with higher concentration limits.

Nonspecific absorption and light scattering can be significant at the analyte wavelength Backgroundcorrection is required

• Type of flame: Oxidizing (lean fuel)

For concentrations of cadmium below 0.02 mg/l, the furnace procedure is recommended

In addition to the normal interferences experienced during graphite furnace analysis, cadmium sis may be subject to severe nonspecific absorption and light scattering caused by matrix componentsduring atomization Simultaneous background correction is required to avoid erroneous high results.Excess chloride may cause premature volatilization of cadmium Ammonium phosphate used as

analy-a manaly-atrix modifier minimizes this loss

Calibration standards should be prepared at the time of analysis To each of the 100-ml standardsand the sample, add 2.0 ml of 40% ammonium phosphate solution (40 g (NH4)2HPO4per 100 ml ofreagent-grade water) The calibration standards should be prepared to contain 0.5% (v/v) HNO3.Many plastic pipet tips (yellow) contain cadmium Use “cadmium-free” tips

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18.9 CALCIUM

The presence of calcium (Ca, fifth among the elements in order of abundance) in water supplies sults from the passage of water through or over deposits of limestone, dolomite, gypsum, and gyp-siferous shale Cadmium content may range from zero to several hundred milligrams per liter Smallconcentrations of calcium carbonate combat corrosion of metal pipes by laying down a protectivecoating Appreciable quantities of calcium salts, on the other hand, precipitate on heating to formharmful scale in boilers, pipes, and cooking utensils Calcium contributes to the total hardness ofwater Chemical softening treatment, reverse osmosis, electrodialysis, or ion exchange is used to re-duce calcium and associated hardness

re-Selection of method: FAAS (Chapter 8) and ICP (Chapter 12) methods are accurate means of termining calcium The EDTA (ethylene diamine tetraacetic acid) disodium salt titration method pro-vides good results for control and routine applications

de-18.9.1 F LAME A TOMIC A BSORPTION S PECTROSCOPY (FAAS)

All elements forming stable oxyanions will complex calcium and interfere unless lanthanum isadded The addition of lanthanum to prepared samples rarely presents a problem because virtually allenvironmental samples contain sufficient calcium to require dilution to obtain results in the method’slinear range Phosphates, sulfates, and aluminum, as well as high concentrations of magnesium,sodium, and potassium are interferants

Calibration standards should be prepared at the time of the analysis and should contain the sametype of acid and at the same concentrations as the preserved samples Add 1 ml of lanthanum chlo-ride solution (carefully dissolve 29 g of La2O3in 250 ml of concentrated HCl and dilute to 500 mlwith reagent-grade water) per 10 ml of standards and samples

• Instrument: Calcium hollow cathode lamp

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• Type of flame: Stoichiometric

18.9.2 Determination of Hardness by EDTA Titrimetric Method

The EDTA disodium salt forms a chelated soluble complex when added to a solution of certain metalcations If a small amount of dye such as Eriochrom black T is added to an aqueous solution contain-ing calcium and magnesium ions at a pH of 10, the solution becomes wine red If EDTA disodium salt

is added as a titrant, the calcium and magnesium will be complexed, and the solution turns from redwine to blue, marking the endpoint of the titration The sharpness of the endpoint increases with in-creasing pH Magnesium ions must be present for a satisfactory endpoint To ensure the presence of Mgions, a small amount of complexometrically neutral magnesium salt of EDTA is added to the buffer

18.9.2.1 Apparatus and Materials

• Volumetric flasks, various sizes

• Disposable transfer pipets

• Magnetic stirrer

• Teflon magnetic stirring bars

• pH paper, full range

18.9.2.2 Reagents

18.9.2.2.1 Buffer Solution

1 Solution 1:

a Weigh 1.179 g of EDTA disodium salt (Na2EDTA) and transfer to a 150-ml beaker

b Weigh 0.780 g of magnesium sulfate heptahydrate (MgSO4.7HO) or 0.644 g of sium chloride hexahydrate (MgCl2.6H2O) and transfer to the same beaker

magne-c Add deionized (DI) water to the beaker until the volume is about 100 ml and mix untilsolids are dissolved

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The buffer solution can be used for about 1 month Discard the buffer when 1 or 2 ml are added to

the sample and it fails to produce a pH of 10.0 at the titration endpoint

18.9.2.2.2 Eriochrom Black T Indicator

Weigh 0.5 g of indicator and 100 g of NaCl into a porcelain mortar, and mix well Alternatively, acoffee grinder may be used for complete mixing

18.9.2.2.3 0.02N EDTA Titrant

Dissolve 3.723 g of EDTA disodium salt in about 700 ml of DI water in a 1-liter volumetric flask anddilute to the mark with DI water Standardize against standard 0.02N CaCO3solution

18.9.2.2.4 Calcium Carbonate Standard Solution

Weigh 1.0000 g of anhydrous CaCO3(primary standard) and transfer to a 500-ml Erlenmeyer flask.Place a funnel in the flask neck and add drops of 1+1 HCl solution until all CaCO3is completely dis-solved Add 200 ml of distilled water and boil for a few minutes to expel CO2 Cool at room temper-ature Add a few drops of methyl red indicator while stirring Adjust the color, while stirring, to anintermediate orange color with 3N NH4OH or 1+1 HCl Transfer quantitatively into a volumetricflask and dilute to 1 liter with DI water Store in a polyethylene bottle

1 ml = 1 mg CaCO3

18.9.2.2.5 Reference Stock Solution, 33,333 mg/l Total Hardness, as CaCO 3

Transfer 12.4860 g of anhydrous, primary-standard calcium carbonate (CaCO3) into a 1-liter metric flask Add about 200 ml of DI water and slowly add concentrated hydrochloric acid (HCl)until calcium carbonate is completely dissolved Transfer 19.5847 g of anhydrous magnesium chlo-ride (MgCl2) into the 1-liter volumetric flask containing the calcium carbonate solution Mix well forcomplete dissolution and fill up to the 1-liter volume Mix well again The solution contains 33,333mg/l total hardness as CaCO3

volu-18.9.2.2.6 Reference (Independent) Standard, 166 mg/l Total Hardness as

CaCO 3

Pipet volumetrically 5 ml of the reference stock solution into a 1-liter volumetric flask and dilute tothe required volume with DI water This solution is the actual working reference with a value of 166mg/l total hardness as CaCO3

18.9.2.2.7 Standardization of EDTA Titrant with CaCO 3 Standard Solution

1 Pipet 10 ml of CaCO3standard solution into a 100-ml Erlenmeyer flask

2 Using a Mohr pipet, add 5 ml of buffer solution and one scoopful of Eriochrom black Tindicator Mix well Solution should be wine red

3 Rinse the buret three times with the EDTA disodium salt titrant

4 Fill buret with the EDTA titrant

5 Remove any air bubbles from the buret and bring level of titrant to 0.00 ml

6 Titrate the contents of the Erlenmeyer flask with EDTA solution until red tint disappears.The color will turn purple Continue titration slowly until the solution turns blue, which isthe endpoint Record the volume of EDTA used

7 Perform this titrant check two more times

8 Calculate the normality of the EDTA as follows:

NormalityEDTA= (0.02 × S)/V(21.7)

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0.02 = prepared normality of EDTA

S = volume of titrated CaCO3solution (ml)

V = volume of EDTA used for titration

Determine the normality with three parallel titrations The exact normality is calculated by ing the three results

averag-18.9.2.3 Procedure

1 Measure a 100-ml sample or portion diluted to 100 ml into a 250-ml Erlenmeyer flask

2 Add 5 ml of buffer solution and a scoopful of Eriochrom black T indicator and mix.Solution should be wine red

3 Rinse the buret with standardized EDTA titrant three times

4 Fill buret with standardized EDTA titrant

5 Remove air bubbles from the buret and check 0.00 level

6 Titrate sample until red tint disappears The color should turn pink Continue titrationslowly until the solution turns blue

7 If the volume of the titrant used is over 25 ml, repeat titration by using a smaller samplesize or appropriate dilution

18.9.2.4 Calculation

mg/l hardness as CaCO3= (V − B) × N × 50 × 1000/SV (18.1)

where

V = volume of titrant used for sample (ml)

B = volume of titrant used for blank (ml)

N = the determined normality of EDTA

50 = equivalent weight of CaCO3(100/2)

SV = sample volume (ml).

Use appropriate dilution factor as necessary

18.9.2.4.1 Total Hardness Calculation

mg/l hardness as CaCO3= 2.497 (Ca mg/l) + 4.118 (Mg mg/l)or

mg/l hardness as CaCO3= [(Ca, mg/l)/0.4] + [(Mg, mg/l)/0.24]

For example, calcium and magnesium have been determined by the atomic absorption technique withthe following results:

Ca = 16 mg/l

Mg = 9.6 mg/lCalculated total hardness value as CaCO3is:

(2.497 × 16) + (4.118 × 9.6) = 79.5 mg/l

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(16/0.4) + (9.6/0.24) = 80 mg/l

18.9.3 C ALCIUM D ETERMINATION BY EDTA T ITRIMETRIC M ETHOD

When EDTA is added to water containing both calcium and magnesium, it combines first with calcium.Calcium can be determined directly with EDTA when the pH is made sufficiently high so that the mag-nesium is largely precipitated as the hydroxide and an indicator is used that combines with calcium only

18.9.3.1 Apparatus and Materials

Apparatus and materials are the same as those listed for total hardness determination (Section18.9.2.1)

18.9.3.2 Reagents

18.9.3.2.1 Sodium Hydroxide, NaOH, 1N

Place a 2-liter beaker or Erlenmeyer flask on a magnetic stirrer under a laboratory hood Add about

500 ml of DI water and a magnetic stirring bar and add 40 g of NaOH slowly to the water while ring

stir-Caution: This reaction liberates heat! After complete dissolution, transfer into a 1-liter volumetric

flask and fill up to the mark with DI water Mix well Store in polyethylene bottle

18.9.3.2.2 Murexide (Ammonium Purpurate) Indicator

A ground mixture of dye powder and sodium chloride provides a stable form of the indicator Weigh0.200 g of murexide (ammonium purpurate) and 100 g NaCl, and grind the mixture to 40 to 50 mesh

in a porcelain mortar or in a coffee grinder used for this purpose

18.9.3.2.3 Standard EDTA Titrant, 0.02N

Prepare as described in Section 18.9.2.2.3

1 ml = 400.8 µg Ca

18.9.3.2.4 Reference Stock Solution, 12,500 mg/l Ca as CaCO 3

Prepare as described in Section 18.9.2.2.5 with a value of 12,5018 mg/l of Ca as CaCO3

18.9.3.2.5 Reference Standard Solution, 62.5 mg/l

Pipet 5 ml of reference stock solution (Section 18.9.3.2.4) into a 1-liter volumetric flask and dilute

to the required volume with DI water

18.9.3.3 Procedure

1 Measure 100-ml sample or smaller portion diluted to 100 ml

2 Add 2 ml of 1N NaOH solution or a volume sufficient to produce a pH of 12 to 13 Stir

3 Add a scoopful of indicator The color of the sample becomes pink

4 Titrate with standardized EDTA solution until the pink color changes to purple, which isthe endpoint Titrate immediately after adding indicator because the solution is unstableunder alkaline conditions

5 Check endpoint by adding one to two drops of titrant in excess to make certain that no ther color change occurs Facilitate endpoint recognition by preparing a color-comparison

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fur-blank containing 2 ml of 1N NaOH and a scoopful of indicator powder and sufficientEDTA titrant (0.05 to 0.10 ml) to produce an unchanging color.

18.9.3.4 Calculation

Calcium as CaCO3= (ml × N × 50 × 1000)/ml sample (18.2)where

ml = ml of ETA standard used for titration

N = exact normality of the EDTA titrant

50 = equivalent weight of CaCO3

Calcium as Ca (mg/l) = Ca as CaCO3(mg/l) × 0.4 (18.3)Magnesium may be estimated based on the difference between total hardness and calcium

as CaCO3:

Mg as CaCO3(mg/l) = total hardness as CaCO3(mg/l) − calcium as CaCO3(mg/l)

Magnesium as Mg (mg/l) = magnesium as CaCO3(mg/l) × 0.24 (18.4)

18.10 CHROMIUM

Chromium salts are used extensively in industrial processes and may enter a water supply throughwaste discharge Chromate compounds frequently are added to cooling water for corrosion control.Chromium may exist in water supplies in both the hexavalent and the trivalent states, although thetrivalent form rarely occurs in potable water

Selection of method: Use the colorimetric method for the determination of hexavalent chromium

in natural or treated water intended to be potable Use the GrAAS method for determination of lowlevels of total chromium (less than 50 mg/l) in water and wastewater Use the FAAS or ICP method

to measure concentrations up to the milligram per liter level

18.10.1 F LAME A TOMIC A BSORPTION SPECTROSCOPY (FAAS)

If the sample contains a higher level of alkali metal content than the standards, ionization ence may cause problems To avoid this interference, add potassium-chloride, ionization-suppressantsolution to standards and samples

interfer-Background correction may be required because nonspecific absorption and scattering can besignificant at the analytical wavelength Background correction with certain instruments may bedifficult at this wavelength due to low-intensity output from hydrogen or deuterium lamps Consultthe instrument manufacturer’s literature for details

• Instrument: Chromium hollow cathode lamp

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For concentration of chromium below 0.2 mg/l, the furnace procedure is recommended

Low concentrations of calcium and/or phosphate may cause interferences; at concentrations above

200 mg/l, calcium’s effect is constant and eliminates the effect of phosphate Calcium nitrate is fore added to ensure a known constant effect Nitrogen should not be used as the purge gas because

there-of possible CN band interference

Background correction may be required because nonspecific absorption and scattering can besignificant at the analytical wavelength Background correction with certain instruments may be dif-ficult at this wavelength due to low-intensity output from hydrogen or deuterium lamps Consult theinstrument manufacturer’s literature for details

Prepare calibration standards at the time of analysis These standards should be prepared to tain 0.5% (v/v) HNO3, 1 ml of 30% H2O2, and 1 ml of calcium nitrate solution (dissolve 11.8 g of cal-cium nitrate (Ca(NO3)2.4H2O), and dilute to 1 liter with reagent-grade water)

con-18.10.2.1 Instrument Parameters

• Purge gas: Argon (N should not be used!)

The above concentration values and instrument conditions are for a Perkin Elmer HGA-2100,based on the use of a 20-µl injection, continuous-flow purge gas, and nonpyrolytic graphite Smallersizes of furnace devices or those employing faster rates of atomization can be operated using loweratomization temperatures for shorter time periods than the recommended settings above

• Detection limit: 1 mg/l

18.11 HEXAVALENT CHROMIUM

18.11.1 C HELATION /E XTRACTION M ETHOD

This method is suitable for determining the concentration of dissolved hexavalent chromium, Cr(VI)

in EP toxicity-characteristic extracts, groundwaters, and domestic and industrial wastes provided that

no interfering substances are present The method is based on the chelation of hexavalent chromium

with ammonium pyrrolidine dithiocarbamate (APDC) and extraction with methyl isobutyl ketone

(MIBK) The extract is aspirated into the flame of an atomic absorption spectrophotometer

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