1970
Anal. Chem. 1984, 56, 1970-1973
Registry No. KI, 7681-11-0; H2S04,7664-93-9; Pt, 7440-06-4.
LITERATURE CITED (1) Ibl, N.; Dossenbach, 0. I n “Comprehensive Treatise of Electrochemistry”: Yeager, E., Bockris, J. O’M., Conway, B. E., Sarangapanl, S., Eds.; Plenum Press: New York, 1983;Vol. 6, pp 139-140,
223-227. (2) Kolthoff, I. M.; Lingane, J. J. ”Polarography”; Intersclence: New York. 1975:Vol. 1: DD 50-53. 61. (3) Albery, W. J.; Bruckbnsteln, S . J . Nectroanal. Chem. 1983, 144, 105.
(4) Flanagan, J. B.; Marcoux, L. J . Phys. Chem. 1974, 78, 718. (5) Malachesky, P. A. Anal. Chem. 1060, 4 7 , 1493. (6) Hitchman, M. L.; Albery, W. J. Nectrochim. Acta 1972, 17, 787. (7) Pratt, K. W.; Johnson, D. C. Nectrochim. Acta 1982, 2 7 , 1013. (8) MacDonald, D. D. “Translent Techniques In Electrochemlstry”; Plenum Press: New York, 1977;pp 32-33. (9) Falkenhagen, H., Kelbg, G., Schmutzer, E., Eds. “Landolt-Bornsteins Zahlenwerke und Funktlonen”, 6th ed.;Springer-Verlag: Berlin, 1960; Vol 2,part 7, pp 257-259.
(10) Oesterling, T. D.; Olson, C. L. Anal. Chem. 1967, 39, 1546. (1 1) Delahay, P. “New Instrumental Methods in Electrochemistry”: Interscience: New York, 1954;pp 61-62,67-70. (I2) MacDonald, D. D.“Transient Technlques In Electrochemistry”; Plenum Press: New York, 1977;p 20. (13) Delahay, P. ”New Instrumental Methods in Electrochemistry”: Interscience: New York, 1954;p 224. (14) MacDonald, D. . “Translent Technlques in Electrochemistry”; Plenum Press: New York, 1977;pp 77-78.
RECEIVED for review March 15,1984. Accepted May 7,1984. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
High-Temperature and High-pressure Decomposition and Comprehensive Analysis of Steel by Direct Current Plasma Atomic Emission Spectrometry Lancelot A. Fernando Allegheny Ludlum Steel Corporation, Research Center, Brackenridge, Pennsylvania 15014
Complete decomposltlon of steel samples Is achieved In a Teflon (Du Pont) container-metal bomb apparatus (Parr) using a HNOS-HCI-HF acids system at 180 OC. This procedure was found to be free of voiatillratlon losses in addition to being adaptable for routlne, rapid analyses. The resulting salt-free matrlx provides a favorable environment for concentration measurements by direct current plasma atomic emission spectrometry, ellmlnating difficulties found wlth the matrlx resulting from the more conventional fusion method. Major and minor element concentratlons have been determined by using the same sample solution, and results are presented for Ai, 6, Ti, Mn, Si, Mo, Cr, Ni, P, and Sn. Also, results from fusion and simple acld dissolution are compared with the bomb results.
The ability to determine trace levels of elements in steel is becoming increasingly important with the realization that some trace elements have a dramatic effect on the properties of steel. Analytical techniques commonly used for trace analysis of steel, such as wet chemical methods, atomic absorption, and the more recent plasma emission spectrometry, require the sample to be in solution. The conventional sample decomposition methods are simple acid dissolution and the use of fused-salt media. Both of these approaches do, sometimes, present difficulties. Simple acid dissolution would be inadequate for samples containing compounds which are attacked slowly, if at all, by the common liquid reagents. The analysis of trace aluminum in steel is a good example of such a problem. ASTM (I)recommends a sodium hydrogen sulfate fusion for this analysis. In general, however, several disadvantages attend the use of fluxes (2). These include the possibility of incomplete
digestion as well as the undesirably high salt content of the resulting solution, which may cause difficulties in subsequent analysis steps. An alternate decomposition procedure was sought that would give complete aluminum digestion and, at the same time, be simple and rapid. Preferably, the procedure should be adaptable to a comprehensive analytical scheme. The high-pressure acid digestion bomb (Parr) method was investigated in this regard. The apparatus consists of a cup and cover made of Teflon (registered Trademark of Du Pont) contained in a stainless steel bomb. These sturdy digestion bombs enable digestion to be carried out at elevated pressures and temperatures without contamination and volatilization losses. Coal, soils, and foodstuffs (3-5) are some of the materials which have been digested by using the bomb method. Materials which evolve a large volume of gas when reacted with acids are not usually decomposed in this fashion. In the current procedure this difficulty was overcome by simply carrying out the initial part of the digestion in an open vessel. Only after the reaction subsided was the reaction mixture subjected to high pressure and temperature. The solutions resulting from the Parr bomb digestion of steel samples were analyzed for several elements by direct current plasma atomic emission spectrometry (DCP-AES). In addition to the Parr bomb procedure, an investigation of the fusion method (1) is also presented here. Analytical results from the bomb method, the fusion method, and simple acid dissolution are compared.
EXPERIMENTAL SECTION Apparatus and Reagents. The acid digestion bomb used is the 23-mL No. 4749 bomb manufactured by Parr Instrument Co., Moline, IL. All concentration measurements were made with the Spectrallletrics, Inc., Spectraspan I11 instrument. This consists of
0003-2700/84/0356-1970$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
5t-
1071
Table I. Analysis of NBS-SRM 179 method acid diss., DCP-AES fusion, DCP-AES Parr bomb. DCP-AES
-
no. of A1 concn re1 std repetitions (ppm in solid) dev, %
5 29 5
17
28 28.6
18 14 2
Table 11. Analysis of Standard Reference Material (% in Solid) RV6554 BCS 452 NBS 179 std cert cert element X, nm value found value found value found 0.1
0.2
0.3
COPPER Concentration (
0.4 0%
B cu Mn P Ti Si Cr Ni Mo
0.5
in Solid 1
Figure 1. Emission intensity vs. concentration of Cu for the Cu 326.8 nm line. The broken line indicates the 0% to 0.2% straight line
calibration. a three-electrode direct current plasma source coupled to a high-resolution echelle grating spectrometer, and a preprogrammed computer for data collection and manipulation. All acids used were of reagent grade. Fisher-certified loo0 ppm atomic absorption reference standards were used to prepare the calibration standards. Spex Hi Pure iron was used on the blank for all elements except aluminum. For aluminum, the blank solution was prepared using GAF Corp. HP-226 iron powder. The samples analyzed include the British Chemicals Standard 452 (BCS 452), the NBS Standard Reference Material 179 (NBS 179), an in-house standard (RV6554), and a series of production steel samples. All samples, other than BCS 452, have nominal silicon concentration of 3%; BCS 452 is a mild steel. Sample Decomposition Procedure. One gram of sample is weighed into a 100-mL plastic beaker and 2 mL of concentrated nitric acid is slowly added, followed by 4 mL of concentrated hydrochloric acid. The reaction mixture is warmed, 2 mL of hydrofluoric acid is added, and the mixture is warmed again until the reaction subsides. The above acid additions should be done carefully to prevent spattering or spilling over. The sample is transferred to the Teflon cup using a few milliliters of water, making sure the final volume is not more than two-thirds the volume of the cup. The closed bomb is heated at 180 "C for at least 1 h in the oven. The bomb is cooled gradually-first air cooling it for about 5 min and then running cold water around it. After cooling, the sample solution is made up to 100 mL with deionized water. Calibration Curves. As part of the standard Spectraspan operation, a straight-line calibration curve is derived from the measured intensity values of a high and low standard solution. The calibration blank is prepared by dissolving 1g of iron in the same acid mixture as the sample. Heating in the bomb is not necessary. The high standards were prepared by adding an appropriate volume of lo00 ppm Fisher Reference Standard solution to the blank for all elements except silicon. The Si High Standard is prepared by dissolving British Chemical Standards No. 317 (1 g/100 mL) which has a certified value of 3.49% Si. The high calibration standard values for the other elements are as follows: B, 100 ppm; Cr, 500 ppm; Mo, 300 ppm; Ni, 500 ppm; P, 1000 ppm; Sn, 500 ppm; Ti, 100 ppm. For all of the above elements, the low calibration standard value was set at zero. For aluminum, the high standard was set at 107 ppm and the low standard was set at 7 pprn to compensate for the aluminum content in the blank. TWO different calibration curves were used for copper analyses, depending on the concentration of copper in the sample. NBS 179 was analyzed with a calibration of 0% low standard and 0.1% high standard. All other samples were analyzed with a high calibration of 0.5% Cu and a low standard of 0.2% Cu. This is because the copper calibration curve deviates from linearity at the higher concentrations (Figure 1). The firsborder correlation coefficient degrades from 0.9997 for a 0% to 0.2% calibration to 0.9984 for a 0% to 0.5% calibration. Hence, to determine copper
a
249.7 0.0031 0.0032a 324.8 0.22 403.1 213.6 0.033 334.9 0.020 251.6 357.9 0.022 352.5 0.050 317.0 0.03
0.23
0.056 0.058 0.094 0.092 0.033 0.006 0.006 0.019 3.19 3.22 0.023 0.022 0.023 0.047 0.033 0.014 0.013
Average of eight determinations. All other vaues are the aver-
age of two determinations.
above 0.2%, the calibration standards were set close to the unknown concentrations so as to approximate very closely the actual curve. Similarly,for the determination of manganese, a calibration range of 0.01% to 0.1% was used to analyze NBS 179, while all other samples were analyzed with a 0% to 0.05% calibration m e . It should be noted that the calibration procedure described above directly gives the analyte concentration in the solid. That is, the solution concentrations measured are more dilute by a factor of 100.
RESULTS AND DISCUSSION Results for the analysis of standard reference materials can be used to compare the accuracy and precision of the different methods. Considering first experimentation on aluminum determination, Table I gives the results for the different analysis methods for NBS 179 Standard. The certified value for the aluminum concentration of NBS 179 is 0.0028%. Simple acid dissolution, using a mixture of HC1, HN03, HC104, and H F results in a low aluminum value for this standard (Table I), clearly indicating that simple acid treatment is not sufficient for complete sample dissolution. The fusion method, which was being used for routine analysis in our laboratory, gave accurate results for NBS 179 (Table I). However, the relative standard deviation (RSD), calculated for fusions routinely carried out in our laboratory (Table I), is very high. Possible sources of this imprecision, as well as the possibility of incomplete digestion at high aluminum concentrations, were investigated. Briefly, the fusion procedure followed in our laboratory is as follows: The sample is first reacted with dilute H N 0 3 and HCl and then filtered through a 0.22-pm Millipore filter to separate soluble and insoluble aluminum. The filtrate contains the acid-soluble aluminum and is made up to volume with 2 mL of H F and water. The residue from burning off the filter disk is reacted with H F and then fused with a mixture of potassium pyrosulfate and potassium bisulfate, commercially known as fused KHSO4. The resulting melt is dissolved in dilute HCl and contains the acid-insoluble aluminum. These two dissolved solutions resulting from treatment of the acid-soluble and acid-insoluble fractions are analyzed separately by DCP-AES. An experiment in which the aluminum concentration in solution was kept constant while the concentration of the flux, fused KHS04, was varied (Figure 2), indicates an interference on the 396.2-nm aluminum atom line. Also, the concentration
1072
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
Table 111. Analysis of Routine Samples Following Simple Acid Dissolutions (Lab) and Parr Bomb Digestion (Parr) X 249.7
nm
X 334.9 nm
B,PPm
X 324.8 nm % cu
Ti, ppm
X 251.6 nm % Si
X 403.1 nm % Mn
X 326.2 nm % Sn
sample
lab
Parr
lab
Parr
lab
Parr
lab
Parr
lab
Parr
lab
Parr
81-5538 81-5539 81-5544 81-5545 81-5547 81-5548 81-5553 81-5559
9 8 11
12 11 13 12 14 10
19 19 21 21 20 27
19 19 22 22 19 28
0.32 0.32 0.32 0.32 0.35 0.34
0.36 0.36 0.39 0.41 0.40
3.19 3.21
3.27 3.34
0.031 0.030 0.031 0.033 0.035 0.032
0.034 0.034 0.037 0.037 0.038 0.038
0.035 0.036 0.036 0.036 0.040 0.036 0.035 0.045
0.044 0.040 0.043 0.043 0.049 0.043 0.043 0.053
11 10 8
n
5.-
h
C
0
8
k
W
W
Ba
a
H
P
!i
lot 1
P
2
3
4
5
6
Heating time , hrs. Figure 3. Recovery of AI
5
0
KHSO,
10
q ~ ~ o d Conc. ),
Figure 2. Effect of variation of fused KHS04 concentration on AI emission intensity. The horizontal axis is in mL of 5 % fused KHS04 in 100 mL of sample solutlon.
of K, in solutions obtained after the fusion step, was determined. The RSD for five determinations was 2%. This variation would, in turn, be a source of imprecision in the aluminum determinations. Also, it has been reported in the literature that aluminum emission is enhanced due to the iron matrix (6). Hence, any residual iron in the insoluble fraction of the samples will result in an erroneously high aluminum concentration value. Since the amount of residual iron would be varying, this would be another factor contributing to the imprecision of the method. Repeated analysis of a solution resulting from the fusion, Le., the acid-insoluble portion, gave aluminum concentrations with a RSD of 6%. In this experiment, the same calibration standards were used to obtain five independent calibration curves for the five repetitive analyses of the same sample. Hence, a major part of the imprecision is traced to signal instability at the analyte wavelength due to the solution matrix. From the above discussion, it can be seen that the poor precision of the fusion method is due to a combination of matrix-related factors. A recent publication (7) reviews existing models for analyte emission enhancement in the three-electrode dc plasma and discusses a t length the effects of easily ionized elements present in sample matrices. Evidence for incomplete digestion at a high aluminum concentration is given by the results of the following experiment: The fusion procedure was carried out with a 10-g sample of NBS 179. The final solution was filtered and was found to contain some residue. This residue was analyzed by scanning electron microscopy (SEM); the major constituent was found to be silicon. However, the NBS 179 residue also had 9 % Al, and this indicates that the fusion procedure does not digest all the aluminum.
as a function of bomb heating tlme.
Results for the Parr bomb method given in Table I show accuracy and precision. In this method, the fiial solution does not contain any extraneous substances; also, the number of procedural steps is very much less as compared to the fusion procedure. In order to minimize the time needed for the Parr bomb method, the recovery of aluminum was studied as a function of the time the bomb was heated. Results are shown in Figure 3, and it is apparent that a minimum of 1 h is needed for complete recovery. In the second part of the project, data were gathered to confirm the applicability of this method for multielement analyses. Table I1 gives resulb from the analyses of standard reference materials for a number of elements routinely determined in our laboratory. It is seen that all the elements determined give concentration values which are in good agreement with the standard values. Of special significance are boron and silicon. In the presence of HF, they form volatile compounds; hence, generally intense heating of the reaction mixture has to be avoided when boron and silicon are to be determined with HF digestion. The Parr bomb, being a closed system, prevents such volatilization losses. Table I11 is a comparison of Parr bomb digestion results, with analysis results routinely obtained in our laboratory. The routine digestion technique used is simple acid dissolution. The sample is warmed with 10 mL of 1:4 HN03 and, when the reaction subsides, 2 mL of HF is added. Loss of boron and silicon is minimized as no intense heat is applied. The results in Table I11 indicate that the Parr bomb gives more complete digestion. As a result of this effort, the Parr bomb method is currently being used as a general digestion procedure for routine analyses of steel in our laboratory.
ACKNOWLEDGMENT I duly acknowledge the development work on the fusion method by Margaret A. McMahon and the contributions made by Carol Lowe, C. C. Gabrielli, W. D. Heavner, and James A. Salsgiver for this project. I thank S. D. Washko and B. R. Jack for SEM analyses.
Anal. Chem. 1984, 56, 1973-1975
LITERATURE CITED (1) "1980 Annual Book of ASTM Standards", Part 12; ASTM: Philadelphla, PA, 1960; Method E350-80. (2) Bernas, Bedrich Anal. Chem. 1968, 40. 1682-1686. (3) Harstein, A. M.; Freedman. R. W.; Platter, D. W. Anal. Chem. 1973, 45. 611414. (4) Sihnetziei, C. C.; Nava, D. F. Earth Panef. Sci. Lett. 1971, 1 1 , 345-350.
1973
(5) Nelson, G.; Smith, D. L. Proc, SOC.Anal. Chem. 1972 (Aug), 168. (6) Savolainen, A. M. Ph.D. Thesis, Department of Chemistry, Providence College, Provldence, R I , 1980, (7) Miller, Myran H.; Eastwood, DeLyle; Hendrick, Martha S. Spectrochlm. Acta, Part 8 1984, 398, 13-56.
RECEIVED for review March 29,1984. Accepted May 14,1984.
CORRESPONDENCE Residual Ozone Determination by Flow Injection Analysis Sir: For the past 80 years free chlorine has been utilized for the disinfection of wastewater and drinking water in the United States. Recently, the concerns about the chlorinated byproducts produced during the chlorination process (1-4) have renewed interests in alternatives to free chlorine disinfection. Ozonation has been shown (5,6)to be a cost-effective disinfectant which exhibits the additional capability of removing taste, odor, and color-producing compounds. Before any disinfectant can replace free chlorine as the standard method, several important criteria must be met. They include: easy generation of the disinfecting species, production of few, if any, undesirable byproducts, and an easily measured residual (7). Currently, there are a minimum of eight commonly used methods for the determination of residual ozone. A recent report (8)indicates that the determination of residual ozone by the indigo method appears to be the method of choice. Bader and Hoigne' (9) have described a manual method and an automated method (10) for ozone determination based on indigo in the ranges of 0.014.1 mg/L, 0.05 to 0.5 mg/L, and over 0.3 mg/L. In their study the concentration of ozone was related to the difference in absorbance between the sample and the indigo reagent by the equation
A 100 fb v
[Os]= - where A is the difference in absorbance between a blank and a sample, f = 0.42 cm-' per 1mg/L of ozone and is the sensitivity of the determination, b is the cell path length in cm, and loo/ V is a dilution factor, where V is the volume in mL of the sampled used. per 100 mL final volume. Here we describe an automated system for ozone determination based upon the indigo method which incorporates the advantages of the flow injection analysis (FIA) technique (11). We report a comparison of this system to the manual method in terms of detection limits, linear working range, sampling frequency, and interferences.
EXPERIMENTAL SECTION Apparatus. A diagram of the FIA manifold used in this study is presented in Figure 1. The indigo reagent was pumped through 0.5-mm Teflon tubing with a Tecator, Inc., Model 5020 flow injection analyzer at a 1mL/min flow rate. The aqueous ozone sample volume was 100 rL. The decolorization of the indigo reagent was measured at 600 nm by using an Isco V4 UV-VIS spectrometer fitted with a 0.5 cm path length flow cell. Manual UV measurements of ozone concentration were performed on a Hewlett-Packard Model 8450A spectrophotometer at 259 nm using a single stoppered 5.00 cm path length quartz cuvette. For the calculation of ozone concentrations a molar absorptivity of 3300 M-l cm-I (12) was used. (It should be noted that the molar absorptivity of 2900 M-' is not considered to be 0003-2700/84/0356-1973$01.50/0
a "true value", rather as the value which corresponds with the conventional iodide method. Recently, however, Hart (12) has published a molar absorptivity of 3300 M-' cm-' at 260 nm for aqueous ozone which we believe to be the preferred value (S).) Reagents. All chemicals used in this study were analytical grade. Water from a Model D3600 Barnstead Nanopure double still system with UV attachment was used to prepare all solutions. An indigo reagent stock solution was prepared by adding 770 mg of potassium indigo trisulfonate (Riedel-deHaen AG, Hannover, BRD) to a 1-L volumetric flask containing approximately 500 mL of water and 1 mL of concentrated phosphoric acid, stirring, and diluting to 1L. Reagent solutions, 77 mg/L and 15 mg/L, were prepared by adding 100 and 20 mL, respectively,of the stock solution to a 1-L volumetric flask containing 10 g of sodium dihydrogen phosphate and 7 mL of phosphoric acid and diluting to the mark with water. Mn(VI1) standards were prepared by appropriate dilution of a 1000 ppm stock solution of potassium permanganate. Chlorine standard solutions were prepared by diluting a 6 mg/L stock solution which was made by bubbling C12through 1 L of chlorine-demand-free water (13). The chlorine content was determined by the DPD/ferrous titrimetric method (14).The titer of the chlorine stock solution was checked in triplicate before and after completion of the FIA testing and was found to differ by less than 3%. Ozone Samples. Ozone was generated by passing oxygen gas through an Ozone Research and Equipment Corp. (OREC, Phoenix, AZ) Model 03V9-0 ozonator. Aqueous ozone samples were prepared by bubbling the ozone into a 3-L contractor filled with the prepared water. Procedure, Due to the volatility and rapid decomposition of ozone in solution the preparation of standard ozone solutions cannot be readily performed (8, 15, 16). For this reason comparative techniques must be used to verify the relative accuracy of a new method of determining ozone. In this study direct ultraviolet detection was chosen as the comparative method. Samples of the ozonated water were removed from the contactor through a sampling stopcock directly into a 5.00 cm path length quartz cuvette which was immediately placed into the UV-VIS spectrophotometer for direct UV measurement. triplicate FIA samples were removed from the cuvette by the peristaltic sampling pump of the FIA instrument for introduction into the indigo reagent carrier stream. Triplicate blank solutions were injected after every nine sample injections to correct for any base line fluctuation.
RESULTS AND DISCUSSION In commercial water purification facilities the measurement range of residual ozone is typically between 0.05 and 5 mg/L. Two concentrations of indigo reagent were investigated for detection limits, sensitivity, and linear ranges in this region. The single-channel FIA manifold, shown in Figure 1, had a dispersion coefficient of 1.8 and an injection frequency of 120 injections/h. The results are summarized in Table I. 0 1984 American Chemlcal Society
