DETERMINATION OF CALCIUM, IRON, MAGNESIUM, MANGANESE, POTASSIUM, AND SODIUM by AAS

Reagents and Equipment

1. Lanthanum solution: Transfer 140 g of lanthanum oxide (La₂O₃, 99.997 percent pure) into a 2-L beaker. Slowly add 300 mL of concentrated hydrochloric acid, allowing time for the reaction to be completed after each addition of acid. Then, add 200 mL of distilled water. Each 4 mL of the final solution contains approximately 1 g of lanthanum.

2. Manganese stock solution: Transfer 0.3872 g of pure manganese metal into a glass beaker, add 20 mL of hot 8 M nitric acid, and gently boil the nitric acid for several minutes. After the resulting solution has cooled to room temperature, transfer the solution to a 500-mL volumetric flask and dilute to volume with distilled water. The concentration of MnO in this solution is 1,000 m g/mL.

3. Stock multiple-element standard solution: Transfer 0.8924 g of CaCO₃, 0.9435 g of NaCl, 0.7915 g of KCl. and 2.4556 of FeSO₄(NH₄)₂SO_4· 6H₂O, (all reagent-grade purity) and 0.3045 g of magnesium ribbon to a 500-mL volumetric flask. (Magnesium ribbon generally is 99 percent magnesium; therefore, the weight of the ribbon includes a 1-percent correction.) Add 50 mL of distilled water and 10 mL of concentrated hydrochloric acid. Boil the dilute acid to dissolve all the constituents. After the solution cools to room temperature, add 50.0 mL of the manganese stock solution (1,000 m g/mL of MnO), dilute to volume with distilled water, and thoroughly mix the final solution. This solution contains the equivalent of 1.00 mg/mL each of Fe₂O₃, CaO, MgO, Na₂O, and K₂O, and 0.1 mg/mL of MnO.

4. Working standard solutions: To six 250-mL volumetric flasks, add 0, 6, 12, 18, 24, and 30 mL of the standard stock solution. Then, add 1.2 g of flux mixture, 5 mL of 8 M nitric acid, and approximately 200 mL of distilled water. Agitate the nitric acid solution to dissolve the flux mixture. Then, add distilled water to make the final volume 250 mL and make the solution homogeneous by vigorous mixing. These six solutions represent a blank and 3-, 6-, 9-, 12-, and 15-percent (equivalent in the sample) standard solutions. For MnO, the same six solutions represent a blank and 0.3-, 0.6-, 0.9-, 1.2-, and 1.5-percent standard solutions.

Procedure for Calcium, Iron, Magnesium, and Manganese

1. Transfer 0.750 mL of blank, sample, and standard solutions into small vials or beakers.

2. Dilute 8 mL of lanthanum solution with 200 mL of distilled water. Add 6.5 mL of this solution to all standards, samples, and blanks.

3. Calibrate the atomic absorption spectrometer by setting the concentration scale to zero for the recommended wavelength (table 16) while the blank solution is nebulized into the flame. Then, set the concentration scale with the 6-percent working standard, and verify this setting with solutions of silicate standards. Directly measure the concentrations of calcium, iron, magnesium, and manganese in each of the samples. Most available atomic absorption spectrometers are suitable for these measurements; the optimum operating conditions for each element usually are discussed in the manual provided with the spectrometer. Importantly, the individual measurements of concentration (or, absorbance) for a sample should be "bracketed" between those of standards because the instrumental responses are usually not linear.

Procedure for Potassium and Sodium

1. Transfer 0.200 mL of blank, sample solutions, working standards, and silicate standards into a small vial or beaker.

2. Dilute 1.2 mL of lanthanum solution with 200 mL of distilled water. Add 5.0 mL of this solution to the blank and to each of the standards and samples.

3. Calibrate the atomic absorption spectrometer using the concentration mode with the 6-percent working standard, and check appropriate silicate standards for known values. Measure directly the concentration of samples.

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CONCLUSIONS

Methods based on AAS and spectrophotometry provide accurate determinations of 10 inorganic elements in coal ash. Although not as rapid as X-ray fluorescence (XRF) spectrometry, these methods furnish an approach to determining major oxides in coal ash that is both inexpensive and accurate (table 18). The agreement between our measurements and the NBS-certified concentrations for10 elements in NBS 1633a coal fly ash, demonstrated by data in table 18, is quite acceptable. Results from XRF spectrometry for SiO₂, Al₂O₃, and Fe₂O₃ in ash sample number 1 and for SiO₂ in sample number 7 (table 18) are outside the range covered by the standards used for calibration. Thus, extrapolations beyond this range could introduce error into these determinations.

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REFERENCES

Bunting, W.E., 1944, The determination of soluble silica in very low concentrations: Industrial and Engineering Chemistry (analytical ed.), v. 16, p. 612-615.

Parker, C.A., and Goddard, A.P., 1950, The reaction of aluminum ions with alizarin-3-sulfonate, with particular reference to the effect of calcium ions: Analytica Chimica Acta, v. 4, no. 5, p. 517-536.

Shapiro, L., 1975, Rapid analysis of silicate, carbonate, and phosphate rocks: U.S. Geological Survey Bulletin 1401, revised ed., 76 p.

Yoe, J.H., and Armstrong, A.R., 1947, Colorimetric determination of titanium with disodium-1,2-dihydroxybenzene-3,5-disulfonate: Analytical Chemistry, v. 19, p. 100-102.

Sodium (Na+): Perkin Elmer AAnalyst 100 Atomic Absorption Spectrometer

I) Principle

The ground state sodium atom absorbs light energy at 589.0 nm as it enters the excited state. As the number of sodium atoms in the light path increases, the amount of light absorbed also increases. By measuring the amount of light absorbed, a quantitative determination of the amount of sodium present can be made. 0.5 % lanthanum solution is added to each standard, control and sample to prevent chemical and ionization interference.

II) Preparation of Stock Standard

In a 1000 mL volumetric flask, dissolve 2.5420 g sodium chloride (NaCl) to the mark with deionized water. This standard stock solution is 1000 mg Na⁺/L.

III) Preparation of Reagents

0.5 % Lanthanum Solution

In a 1000 mL volumetric flask, dissolve 13.37 g lanthanum chloride (LaCl₃.7H₂O) to the mark with deionized water.

IV) Analysis Procedure

A) Spike each standard, control and sample 9:10 with 0.5 % Lanthanum Solution (1 part 0.5 % Lanthanum Solution and 9 parts standard, control or sample).

B) Install Na-K hollow cathode lamp. Perkin-Elmer part #N305-0204.

C) Ensure that the correct Default Conditions are entered.

1) Recall Method=2

2) Lamp Current=12

3) Slit=0.2

4) Full Height=Y(Yes)

5) Wavelength(nm)=589.0

6) Int. Time=5.0

7) Replicates=5

8) Cal=1(Nonlinear)

9) Cal=1(Hold)

10) STD1-----

11) Read Delay(sec)=3

D) Use an oxidizing (lean, blue) air-acetylene flame.

E) Calibrate with standards that bracket the sample concentration. Check the calibration curve for drift, accuracy and precision with standards and controls every 20 samples. Correlation coefficient should be greater than or equal to 0.990.

Manganese (Mn): Perkin Elmer AAnalyst 100 Atomic Absorption Spectrometer

I) Principle

The ground state manganese atom absorbs light energy at 279.5 nm as it enters the excited state.  As the number of manganese atoms in the light path increases, the amount of light absorbed also increases.  By measuring the amount of light absorbed, a quantitative determination of the amount of magnesium present can be made.  0.5 % lanthanum solution is added to each standard and sample to prevent chemical and ionization interference.

 

II) Preparation of Stock Standard

 

Stock standard of 1000 mg Mn/L is purchased commercially.  Discard on expiration date.

 

III) Preparation of Reagents

 

0.5 % Lanthanum Solution

In a 1000-mL volumetric flask, dissolve 13.37 g lanthanum chloride (LaCl₃.7H₂O) to the mark with deionized water.

 

IV) Analysis Procedure

 

A) Dilute each standard and sample 9:10 with 0.5 % Lanthanum Solution (1 part 0.5 % Lanthanum Solution and 9 parts standard or sample).

B) Install a Mn hollow cathode lamp.

C) Ensure that the correct Default Conditions are entered.

1) Recall Method=5

2) Lamp Current=20

3) Slit=0.2

4) Full Height=Y(Yes)

5) Wavelength(nm)=279.5

6) Int. Time=5.0

7) Replicates=5

8) Cal=1(Nonlinear)

9) Cal=1(Hold)

10) STD1____

11) Read Delay(sec)=3

 

D) Use an oxidizing (lean, blue)air-acetylene flame.

E) Calibrate with standards that bracket the sample concentration.  Correlation coefficient should be greater than or equal to 0.990.  Check the calibration curve for drift, accuracy and precision with standards and controls every 20 samples.

Perkin Elmer AAnalyst 100 Atomic Absorption Spectrometer: Operating Procedure

I) Principle

The Perkin Elmer AAnalyst 100 system consists of a high efficiency burner system with a Universal GemTip nebulizer and an atomic absorption spectrometer.  The burner system provides the thermal energy necessary to dissociate the chemical compounds, providing free analyte atoms so that atomic absorption occurs.  The spectrometer measures the amount of light absorbed at a specific wavelength using a hollow cathode lamp as the primary light source, a monochromator and a detector.  A deuterium arc lamp corrects for background absorbance caused by non-atomic species in the atom cloud.

II) Instrument Setup

A) Empty waste container to mark.  Add deionized water to drain tubing to ensure that water is present in the drain system float assembly.

B) Drain moisture from air compressor.

1) Unplug compressor.

 

2) Reduce compressor pressure to nearly zero by opening the pressure release valve and the drain plug located at the bottom of the tank.

 

3) Close pressure release valve and drain plug.

 

4) Plug in compressor to restart the motor.

C) Install the appropriate Hollow Cathode Lamp for the analyte to be analyzed.

D) Power AAnalyst 100 and printer ON.

E) Ensure that instrument is in AA mode.

F) Recall Method to be analyzed.

G) Ensure that the correct Default Conditions are entered.

H) Align the Hollow Cathode Lamp.

1) Press Energy.

 

2) Press Energy a second time if the bar needs to be brought on scale.

 

3) Adjust the vertical and horizontal lamp adjustment screws to obtain maximum energy.

 

I) Store Method changes in Parameter Entry, Option, Store and #.

 

J) Adjust Burner height.

 

1) Place a white sheet of paper behind the burner to confirm the location of the light beam.

 

2) Lower the burner head below the light beam with the vertical adjustment knob.

 

3) Press Cont (Continuous) to display an absorbance value.

 

4) Press A/Z to Autozero.

 

5) Raise the burner head with the vertical adjustment knob until the display indicates a slight absorbance (0.002).  Slowly lower the head until the display returns to zero. Lower the head an additional quarter turn to complete the adjustment.

 

K) Ignite flame.

 

1) Turn Fume Hood switch ON.

 

2) Open air compressor valve. Set pressure to 50 to 65 psi.

 

3) Open acetylene gas cylinder valve. Set output pressure to 12 to 14 psi.  Replace cylinder when pressure falls to 85 psi to prevent valve and tubing damage from the presence of acetone.

 

4) Press Gases On/Off. Adjust oxidant flow to 4 Units.

 

5) Press Gases On/Off. Adjust acetylene gas flow to 2 Units.

 

6) Press Flame On/Off to turn flame on.

Note: Do not directly view the lamp or flame without protective ultraviolet radiation eyewear.

 

L) Aspirate deionized water through the burner head several minutes.

 

M) Adjust Burner Position and Nebulizer.

 

1) Aspirate a standard with a signal of approximately 0.2 absorbance units.

 

2) Obtain maximum burner position absorbance by rotating the horizontal and rotational adjustment knobs.

 

3) Loosen the nebulizer locking ring by turning it clockwise.  Slowly turn the nebulizer adjustment knob to obtain maximum absorbance. Lock the knob in place with the locking ring.

Note: An element, such as Magnesium, which is at a wavelength where gases do not absorb is optimal for adjusting the Burner and Nebulizer.

 

N) Allow 30 minutes to warm-up flame and lamp.

III) Calibration Procedure

A) Calibrate with standards that bracket the sample concentrations.

 

B) Enter ------ for Std1 in the Default Conditions to obtain absorbance units for each standard.  Construct a data regression curve on a computer spreadsheet. Use standard concentrations as the X axis and absorbances as the Y axis.

 

C) Enter Standard Concentration Values in the Default Conditions to calculate an AAnalyst 100 standard curve.

 

1) Enter the concentration of the lowest standard for STD1 using significant digits.

 

2) Enter the concentrations of the other standards of the calibration curve in ascending order and the concentration of the reslope standard.

 

3) Autozero with the blank before each standard.

 

4) Aspirate Standard 1, press 0 Calibrate to clear the previous curve. Aspirate the standards in numerical order.

Press standard number and calibrate for each standard.

 

5) Press Print to print the graph and correlation coefficient.

 

6) Rerun one or all standards, if necessary.  To rerun Standard 3, aspirate standard and press 3 Calibrate.

 

7) Reslope the standard curve by pressing Reslope after aspirating the designated reslope standard.

 

D) The correlation coefficient should be greater than or equal to 0.990.

 

E) Check the calibration curve for drift, accuracy and precision with standards and controls every 20 samples.

 

IV) Analysis Procedure

 

A) Autozero with the blank before and after each standard, control and sample.

 

B) Aspirate sample and press Read. Wait until Read light goes out.  Record absorbance or concentration value.  Record the five replicate standard deviation.  Rerun the sample if the standard deviation is greater than 10% of the sample result.

 

V) Instrument Shutdown

 

A) Aspirate 5 % concentrated hydrochloric acid (HCl) for 5 minutes and deionized water for 10 minutes to clean the burner head.  Remove the capillary tube from the water.

 

B) Press Flame On/Off to turn off flame.

 

C) Close air compressor valve.

 

D) Close acetylene cylinder valve.

 

E) Press Gases On/Off three times to bleed the acetylene gas from the lines.  The cylinder pressure should drop to zero.

 

F) Power OFF the AAnalyst 100, the printer and the fume hood.

 

Magnesium (Mg 2+): Perkin Elmer AAnalyst 100 Atomic Absorption Spectrometer

I) Principle

The ground state magnesium atom absorbs light energy at 285.2 nm as it enters the excited state.  As the number of magnesium atoms in the light path increases, the amount of light absorbed also increases.  By measuring the amount of light absorbed, a quantitative determination of the amount of magnesium present can be made.  0.5 % lanthanum solution is added to each standard and sample to prevent chemical and ionization interference.

 

II) Preparation of Stock Standard

In a 1000-mL volumetric flask, dissolve 8.3632 g magnesium chloride (MgCl₂.6H₂O) to the mark with deionized water. This standard stock solution is

1000 mg Mg²⁺/L.

 

III) Preparation of Reagents

0.5 % Lanthanum Solution

In a 1000-mL volumetric flask, dissolve 13.37 g lanthanum chloride (LaCl₃.7H₂O) to the mark with deionized water.

 

IV) Analysis Procedure

A) Dilute each standard and sample 9:10 with 0.5 % Lanthanum Solution (1 part 0.5 % Lanthanum Solution and 9 parts standard or sample).

B) Install a Ca-Mg hollow cathode lamp. Perkin-Elmer part #N305-0202.

C) Ensure that the correct Default Conditions are entered.

1) Recall Method=4

2) Lamp Current=15

3) Slit=0.7

4) Full Height=Y(Yes)

5) Wavelength(nm)=285.2

6) Int. Time=5.0

7) Replicates=5

8) Cal=1(Nonlinear)

9) Cal=1(Hold)

10) STD1____

11) Read Delay(sec)=3

D) Use an oxidizing (lean, blue)air-acetylene flame.

E) Calibrate with standards that bracket the sample concentration.  Correlation coefficient should be greater than or equal to 0.990.  Check the calibration curve for drift, accuracy and precision with standards and controls every 20 samples.

Chemical composition of Fly ash by Inductively Coupled Plasma/Mass Spectroscopy (ICP/MS)

1. Powdered samples were scrapped from each of the four test materials. 100 mg of each powdered sample was weighed and poured into a Teflon screw capped jar (SavillexR).
2. 2 ml concentrated nitric acid (HNO3) was added to each jar and the jars tightly capped.
3. The jars were placed on a hot plate at a temperature of 150°C until the entire samples were dissolved.
4. The solutions were then transferred into separate 125 ml bottles, their jars rinsed thoroughly with water and transferred into appropriate bottles.
5. About 15 ml of water was then added to fill each bottle. The solutions were analyzed using Perkin Elmer Elan 5000 inductively coupled plasma/mass spectrometer (ICP/MS).

6. Using the protocol of internal standardization, about 30 parts per billion (ppb) In, Tb and Bi were added into each sample solution so as to overcome instrumental drift and matrix effect.
7. The high thermal energy and electron rich environment of the ICP resulted in the conversion of most atoms into ions.
8. A quadruple mass spectrometer permitted the detection of ions and each mass in rapid sequence, allowing signals of individual isotopes of an element to be scanned.

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Chemical composition of Fly ash by Inductively Coupled Plasma/Optical Emission Spectroscopy (ICP/OES)

1. The fly ash sample was totally digested by gently heating 0.25g of the sample in a mixture of HF/HNO3/HClO4 in a Teflon beaker on a hot plate until dry.
2. The residue was then dissolved in 5% HNO3 and topped to 15 ml with deionized water for analysis by Perkin Elmer Optima 3000 DV inductively coupled plasma/optical emission spectrometer (ICP/OES).
3. This was used to determine the major element oxides, with the exception of SiO2, and the larger suite of trace elements. 0.10 g of fly ash sample was also digested with 2.25 ml of a mixture of 9 parts HNO3 with 1 part HCl for 1 hour at 95°C in a boiling water bath and topped to 15 ml with deionized water for analysis by the ICP/OES.
4. This method was used to determine the smaller suite of trace elements.
5. The SiO2 portion was obtained by fusing 0.10 g of the fly ash sample with 1.00 g of lithium metaborate at 1000°C for 1 hour.
6. The residue was dissolved in dilute HNO3 topped to 100 ml with deionized water and then analyzed by the ICP/OES.

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