ACKNOWLEDGMENT
The authors thank Edward Ruiz and Mrs. Vivian Zoller for programming assistance, and R. G. Ridley, Mass Spectrometry Data Centre at Aldermaston, and H. G. Boettger, Jet Propulsion Laboratory, for their collaboration in compiling the spectrum collection. Robert C. Murphy kindly donated the sample of Green River Shale extract discussed in this paper.
RECEIVED for review October 23, 1970. Accepted January 28, 1971. This work was supported by a National Institutes of Health Research Grant (No. RR00317 from the Division of Research Resources), a National Institutes of Health Training Grant (No. G M 01523), and a National Aeronautics and Space Administration Research Grant (No. NGR 22-009-005).
Ultra-Pure Water Preparation and Quality R. C. Hughes, P. C. Murau, and Gordon Gundersen Philips Laboratories, A Division of North American Philips Corporation, 345 Scarborough Road, Briarcliff Manor, N . Y. 10510 The analysis of ultra-pure water by quiescent evaporation in Teflon (Du Pont) followed by spectrography of the residue isdescribed. The preparation of ultra-pure water by distillation and by deionization is reviewed. The product attained by deionization in metal apparatus is shown to be subject to significant contamination by metallic constituents, presumably present in nonionized form. This contamination may not be revealed by measurement of conductivity. The necessity for a specific analysis for contaminants i s indicated. The desirability of constructing apparatus from inert materials (fused quartz, polyethylene, Teflon) is emphasized. Water having as metallic contaminants no more than approximately 1 ppb each of Ca, Mg, AI, and Si may be prepared by appropriate techniques of distillation or ion exchange.
THEPREPARATION, evaluation, and storage of pure water is exhaustively reviewed by Gmelins Handbuch (1, 2) in which the literature coverage appears to be complete through 1949 and partial through 1960. The earliest requirement for ultra-pure water was in electrochemical research ; attainment of lowest conductivity and neutrality (pH 7) were of primary concern. The classical methods of distillation were developed to meet these needs. Pharmaceutical uses, for injection, imposed additional special requirements of freedom from pathogens and pyrogens. These requirements were easily met by distillation but not by simple deionization. Biochemical research on the influence of trace elements upon both plant and animal life required lowest attainable levels of specific elements (principally metallic) in water, and stimulated new investigations into both methods of attaining water low in trace metallics, and methods of analysis for these. These requirements were met initially by distillation and more recently by deionization. The preparation and evaluation of ultra-pure water for biochemical research is comprehensively reviewed by Thiers (3). Ultra-pure water for nuclear reactors ( 4 ) , low in dissolved solids, and specially low in oxygen and (1) “Grnelins Handbuch der Anorganischen Chemie,” 8th ed., “Sauerstoff,” Part 5, Verlag Chemie C.m.b.H., Weinheim, 1963, pp 1191-1207, (2) Ibid.,Part 6, 1964, pp 173551740, (3) R. E. Thiers, in “Methods of Biochemical Analysis,” Vol. V, D. Click, Ed.. Interscience, New York, N. Y . , 1957, pp 273335. (4) F. N. Alquist, “Symposium on High Purity Water Corrosion,” ASTM Special Technical Publication No. 179, American Society for Testing Materials, Philadelphia, Pa., 1956, pp 1-7.
silica, is obtained by refined methods of deionization ( 5 ) . The newest stimulus to research on ultra-pure water was provided by the advent of the semiconductor industry (6). Approximately in parallel, the importance of ultra-pure water for the processing of other electronic materials and devices was recognized (7). Deionization is the almost universally employed method for these purposes. It has been an almost universal practice to assess the quality of ultra-pure water by measurement of its specific conductivity. The conductivity of intrinsic water, calculated to be mhojcm at 25 “C ( 8 ) , will not be significantly 0.0548 X altered by ionized impurities at the fractional ppb level, nor is it affected by the presence of un-ionized impurities. Consequently, conductivity does not provide a fully adequate assessment of purity. In view of the inadequacy of the conductivity measurement, and the prevailing absence of other valid assessment in much of the available literature, it appears warranted to reexamine the quality of water obtained by best current techniques of distillation and deionization. PROCESSES FOR PREPARING ULTRA-PURE WATER
Distillation. Ideally, distillation should give a perfect separation of water from nonvolatile solids; however, in practice, two effects cause contamination of the distillate with unrectified liquid. A film of water wetting all internal surfaces of the still may flow toward the condenser under the combined influence of capillarity and the vapor stream. This creeping of unrectified liquid from the still pot into the condenser can be prevented b y providing a heated region between the pot and condenser (9), most conveniently obtained by a small resistance heater. Entrainment is the major factor which prevents the perfect separation of a volatile substance from nonvolatile solids during distillation. Rising bubbles of vapor break through the surface of the liquid with considerable violence, throwing a fog of droplets of colloidal dimen(5) H . W. Huntley and S. Untermyer, ibid.,pp 8-18. (6) D. E. Koontz and M. V. Sullivan, “Symposium on Cleaning of Electronic Device Components and Materials,” ASTM Special Technical Publication No. 246, American Society for Testing Materials, Philadelphia, Pa., 1959, pp 183-194. (7) P. P. Prichett, ibid.,pp 205-213. (8) A . Iverson, J. Phys. Chem., 68, 515-521 (1964). (9) P. A. Thiessen and K. Herrmann, 2. Elektrochem., 43, 66-69 (1937). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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sions (IO) into the vapor space above the liquid surface. These droplets will be carried into the condenser in varying degree depending upon still design and its operating conditions. A wide assortment of spray traps interposed between the boiler and condenser, and other means of minimizing entrainment, have been suggested. Thiers (3) recommends a spray trap which provides abrupt reversal in direction of vapor flow. Ballentine (11)has employed a 5-foot Vigreux column as a spray trap; he obtains a purification factor of lo7 (no more than 1 part in l o 7 parts carried over by entrainment). Weber (12) employed a 4.5-cm diameter by 50 cm long vapor tube, thermally lagged, and rising a t a n angle of 5-10’ between the boiler and condenser. Under slow distillation (400 mlihr), entrained droplets fall to the lower surface of this tube and flow back t o the boiler. I n theory (and probably in practice), this is a n excellent method of minimizing entrainment. Powers (13) used a n unlagged tube of similar large size, but vertically disposed. Entrainment is obviated by slow distillation without boiling (IO); attainment of a practical rate of distillation requires a short vapor path of large cross section; as has been described for a n all polyethylene still initially designed for the distillation of hydrofluoric acid (14) and subsequently adapted for water distillation (15). An all-quartz still available from Quartz et Silice (16) utilizes nonebullient distillation. Kolarow (17) has employed a n electrostatic field t o precipitate entrained droplets, with apparently favorable results. Ion Exchange. Kunin and Myers (18) described the extensive history of ion exchange, and indicated that the method has been useful for the purification of water since 1935, consequent upon the discovery of synthetic ion exchange resins by Adams and Holmes (19). However, attainment of completely deionized (“conductivity”) water awaited the development of strong cation resins and the technique of “mixed bed” or “monobed” deionization in which cation and anion exchangers are mixed together in a single column unit. These developments were described by Kunin and Myers (18), Kunin and McGarvey (20, 21), and Reents and Kahler (22), in 1949-51. These works emphasize that deionization removes only the ionized impurities and that suspended and un-ionized dissolved solids may remain in the effluent. Freezing. The desalination of sea water by freezing is well known (23). However, the removal of trace contaminants for the production of ultra-pure water has been but little
investigated. Shapiro (24) has shown that dissolved organic matter is strongly concentrated in the liquid phase during the freezing of water, and recommends the method for the concentration and isolation of organics from water and for the production of water low in organic content. Gross (25) has shown that the distribution coefficients (concentration in the solid at the solid-liquid interface + concentration in the liquid at the interface) for several trace inorganics range from 10-5 to Malo and Baker (26) have found, under practical working conditions (freezing rate, stirring rate, and fraction frozen), that trace cations (Ca, Cu, Fe, Pb) can be recovered in the liquid phase a t 60-90% efficiency by a single stage of freezing. A two-step cascade process was shown t o recover 90 of the lead initially present in 2000 ml of water in a final liquid volume of about 75 ml. Dufaure de Citres (27) described the triple freezing of water, with rejection of lIa as liquid each time, with the result that the starting material, previously purified by distillation followed by deionization, was reduced in boron content t o 0.02 ppb and in lithium and beryllium to 0.0002 ppb. Zone refining (by freezing) has been considered for the purification of water; however, Pfann (28) rejected the method as excessively costly in power. In view of the effectiveness, economy, and convenience of distillation and deionization, freezing does not appear to be a n attractive alternative for the production of ultra-pure water from natural water supplies. However, it is quite possible that freezing can be effective in further refinement of water already purified to the ultimate attainable quality by other means. The low temperature involved would minimize the pickup of impurities from the container. Storage of the product in the form of ice as suggested by Freeman and Kuehner (29) should minimize subsequent contamination. Electrophoresis. Electrophoretic treatment has been extensively investigated for the desalination of sea water. Haller and Duecker (30) have investigated the applicability of the method for obtaining ultra-low conductivity water. The water is repeatedly recirculated through a n electric field of 1000 volts/cm maintained between cation and anion selective membranes. A terminal conductivity of 0.0589 X mho/cm a t 25 “C is reached, approaching very closely their calculated theoretical value. This method is therefore quite suitable for the production of small volumes of water of exceptionally low conductivity. MATERIALS FOR APPARATUS CONSTRUCTION
(10) D. Balerew and N. Kolarow, Z . A m i . Chem., 107, 30-32 (1936). (11) R. Ballentine, ANAL.CHEM., 26, 549-550 (1954). (12) H. H. Weber, Z . Nafurforsch.,4b, 124-125 (1949). (13) R. W. Powers, Electrochem. Tech., 2 , 163-166 (1964). (14) P. P. Coppola and R. C. Hughes, ANAL.CHEM.,24,768 (1952). (1 5) R. Mavrodineanu and H. Boiteux, “Flame Spectroscopy,” John Wiley & Sons, Inc., New York, N. Y., 1965, pp 165-166. (16) Leaflet No. S1-5, Quartz et Silice, 8 Rue D’Anjou, Paris 8e,
France.
(17) N. Kolarow, Oesterr. Chem. Zfg.50, 180 (1949). (18) R. Kunin and R. J. Myers, “Ion Exchange Resins,” John Wiley & Sons, Inc., New York, N. Y., 1950. (19) B. A. Adams and E. L. Holmes, J . Soc. Chem. Ind., 54, 1-6T (1935). (20) R. Kunin and F. X. McGarvey, Ind. Eng. Chem., 41, 1265 (1949). (21) Ibid., 43, 734-740 (1951). (22) A. C. Reents and F. H. Kahler, ibid.,pp 730-734. (23) J. C. Orcutt, in “Fractional Solidificaiion,” M. Zief and W. R. Wilcox, Ed., Marcel Dekker, Inc., New York, N. Y., 1967, pp 441-459. 692
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The quality of water produced by a purification process depends critically upon the ability of the materials from which the apparatus is constructed to resist dissolution in the water. Tin. Tin and tinned copper have, until recently, been favored materials for the construction of stills intended for laboratory supplies. The condenser is most frequently of pure (“block”) tin. The product from such stills (“ordinary”
(24) J. Shapiro, ANAL.CHEM.;39, 280 (1967); Sciellce, 133, 2063-2064 (1961). (25) G. W. Gross, Adcan. Chern. Ser., 73, 27-97 (1968). (26) B. A, Malo and R. A. Baker, ibid., pp 149-163. (27) Dufaure de Citres, J., Bull. Soc. Clzim. Fr. 1969, 1072. (28) W. G. Pfann, “Zone Melting,” 2nd ed., John Wiley & Sons, Inc., New York, N. Y., 1966, p 157. (29) D. H. Freeman and E. C. Kuehner, paper presented at the
symposium on Trace Characterization, Chemical and Physical, National Bureau of Standards, Washington, D. C.. Oct. 3-7, 1966. (30) W. Haller and H. C. Duecker, J . Res. Nor. Bur. Stand., Sect. A , 64,527-530 (1960).
distilled water) is contaminated at the fractional part per million level with elements such as copper, lead, and zinc (31). Glass. Kohlrausch and Heydweiller (32) obtained water having a conductivity of 0.062 x 10-6 mho/cm after 36 consecutive vacuum distillations in glass. Under ordinary conditions of laboratory distillation, the best modern resistant glasses (borosilicate) will contaminate the product by dissolution in it. Boron, for example, has been detected at the level of 20 ppb (33). Fused Quartz. Kendall(34) has described the use of fused quartz for the construction of stills. Weiland (35) obtained mho/cm by diswater of conductivity of 0.05-0.07 X tillation in quartz. There have been many subsequent descriptions of stills constructed wholly or partially from fused quartz. This is the presently favored material for small laboratory stills. Aluminum. Churchill (36) has recommended aluminum for the construction of stills. Wagner and Wyma (37) have extensively investigated the performance of aluminum for piping, fittings, and storage vessels for handling high-purity water in stream generation. The rate of corrosion of aluminum in high-purity distilled water, aerated or deaerated, is shown to be less than 0.2 mil/year. Storage of deionized water in aluminum tanks results in the pickup of no more than 10-20 parts per billion of aluminum. Stainless Steel. Beck et a/. (38) have evaluated type 304 stainless steel (in comparison with tin) for the construction of high purity water systems. They found from 2-18 ppb of metals in distillate from tin, tin-lined, and 304 stainless steel apparatus. Polyethylene. Coppola and Hughes (14) have described an all-polyethylene still originally intended for the distillation of hydrofluoric acid and subsequently applied to water (15). Jordan and Wyer have employed polyethylene for the condenser (39) of a still. Teflon. Vasilevskaya er al. (40) have referred to a Teflon still for the redistillation of hydrofluoric, and other acids. Though no reference is found to the use of Teflon for the construction of water stills, it is obvious that the material would be well suited, and its use for the condenser would be particularly advantageous. Other Plastics. Plastics such as nylon, polyvinyl chloride, and Lucite are useful in deionization systems, in which heat resistance is not a factor. Of the foregoing materials, fused quartz, polyethylene, and Teflon are apparently the most suitable constructional materials for final stages of ultra-pure water systems. Other materials which are more readily available in desired sizes and (31) G. F. Liebig, A. P. Vanselow, and H. P. Chapman, Soil Sci., 55, 371-376 (1943). (32) F. Kohlrausch and A. Heydweiller, Wied. Ann., 53, 209-235 (1894); Z . Physik. Chem., 14, 317-330 (1894). (33) R. Cliquet, J. Guilbert, and H. Penau, J . Phurm. Chim.,18, 321-333 (1933). (34) J. Kendall, J . Amer. Chem. Soc., 38, 2460-2466 (1916). (35) H. J. Weiland, ibid., 40, 131-150 (1918). (36) H. V. Churchill, IND.ENG. CHEM.,ANAL. ED., 5, 264-266 (1933). (37) R. H. Wagner and B. H. Wyma, Proc. Amer. Power Conf., 27, 825-834 (1965). (38) W. D. Beck, J . R. Irwin, and P. D. Miller, Muter. Prot., 6, 50-53 (1967). (39) J. D. Jordan and G. D. Wyer, Chemist-Analyst, 48, 39-41 (1959). (40) L. S. Vasilevskaya, V. P. Muravenko, and A. I. Kondrashina, Zh. Analit. Kim., 20, 540-546 (1965) (Available in English translation from Consultants’ Bureau.)
form, and more easily fabricated, such as stainless steel and aluminum, are advantageously employed in all but final stage purification. METHODS FOR THE ANALYSIS OF ULTRA-PURE WATER
The voluminous literature on water analysis has been reviewed periodically in ANALYTICAL CHEMISTRY, annually from 1949 to 1953 and biennially thereafter. The most recent of these reviews appeared in the April 1971 issue. Though the vast majority of the publications refer to the analysis of natural waters, many of the described methods are sufficiently sensitive for the analysis of ultra-pure water. In general, sensitive methods of water analysis involve the concentration of impurities by conventional means such as evaporation, solvent extraction with organic reagents, ion exchange, coprecipitation with a gathering agent, etc., followed by a determination by sensitive methods such as emission spectrography, flame photometry, atomic absorption, neutron activation, or polarography. Hume (41) has recently reviewed the subject, indicating the general applicability of emission spectrographic, atomic absorption, colorimetric, polarographic, and neutron activation techniques. Colorimetric methods employing inorganic, or more frequently organic, reagents are extensively applicable to the analysis of water. Of these, solvent extraction into an organic phase by use of an organic reagent usually gives greatest sensitivity. A number of such colorimetric methods are referenced in Gmelins Handbuch (1). The standard reference works on trace analysis ( 4 2 , 43) serve as an adequate guide to these methods. A highly useful and often employed method for the determination, as a group, of the most commonly present heavy metals in pure water, as described by Stout and Arnon ( 4 4 , is extraction with dithizone in chloroform. This technique permits the determination of less than 1 ppb total of Cu, Zn, Pb, Ni, Co, Hg, Cd, T1, and Bi. McHargue and Offutt (45) have employed this method, after preliminary concentration of the sample by evaporation, to assess the quality of distilled water. Pohl(46) has described the extraction of trace metals from pure water with mixed organic reagents (8-hydroxyquinoline and diphenyldithiocarbazone [dithizone] in chloroform) followed by spectrochemical analysis of the extract. A sensitivity of about 1 pg/l. (1 ppb) is attained. He (47, 48) has also described the polarographic analysis of reactor water to the ppb level through the use of ethylenediaminetetraacetic acid (EDTA). Pohl points out that the usual control of water purity by conductivity fails by an order of magnitude to provide the needed sensitivity and that oxide corrosion products are not detected by the measurement. Mitchell (49) analyzed water by evaporating 1 to 5 liters (41) D. N. Hume, Adcan. Chem. Ser. 67, 30-44 (1967). (42) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, N. Y., 1963. (43) G. H. Morrison and H. Freiser, “Solvent Extraction in Analytical Chemistry,” John Wiley & Sons, Inc., New York, N. y., 1957. (44) P. R. Stout and D. I. Arnon, Amer. J . Bot., 26, 144-149 (1939). (45) J. S. McHargue and E. B. Offutt, IND. ENG. CHEM.. ANAL ED.,12, 157-159(1940). (46) F. A. Pohl. Z . Anal. Chem.. 139.241-249 (1953). (47) F. A. Pohl; Microchim. Acta, 1963, 855-863. (48) F. A. Pohl, Z . Anal. Chem., 197, 193-199 (1963). (49) R. L. Mitchell, in “Trace Analysis,” J. H. Yoe and H. J. Koch, Jr., Ed., John Wiley & Sons, Inc., New York, N. Y . , 1957, pp 398-412. ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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to dryness, taking up the residue in hydrochloric acid, and after the addition of AI, Fe, and Cd, precipitating the trace element content with a mixture of 8-hydroxyquinoline, tannic acid, and thioanilide. The precipitate was ashed and the residue spectrographed. The foregoing very incomplete discussion will serve t o indicate the nature and variety of methods which are available for the analysis of ultra-pure water. RECENT REVIEIVS ASD INVESTIGATIONS OF THE PREPARATION OF ULTRA-PURE WATER
Recent significant investigations of ultra-pure water are reviewed hers; tht referenced publications provide a n adequate exposition of best current technology. Powers (13) recently described a simplified method for the preparation of high-purity water by distillation. He employed a 12-liter borosilicate glass still pot, a fused silica condenser connected to the still pot through a 18-in. long by 1 3,’4-in.diameter vapor tube disposed vertically, and a throat heater at the top of the vapor tube. Thc output was recei\,ed through a silica still head containing a conductiLity cell. [The original paper should be consulted for further constructional details.] A major finding of Powers’ work was that bubbling purified oxygen through the still charge held at 98-99 ’C for at least 16 and preferably 24 hours prior to distillation, followed by distillation in a stream of argon gives a product having a conductivity of only about 1.3 times the intrinsic value, when ordinary distilled water is employed as feed. Deionized water as feed gave a less favorable result (conductivity 1.7-2 X intrinsic value). Powers also found that efluent qualitj suffered when alkaline permanganate was added to the still pot, because of carry-over by entrainment. The work of Kunin and associates. previously referenced, showed that monobed deionization with strong cation and anion exzhangers can give water highly freed of ionized substances, including silica and carbon dioxide, and of 10-14 by sensitive colorimegohm-cm resistivity. Healy er 01. (%I), metric analysis, found that monobed deionized water (ordinary distilled water fetd) was lower in content of Zn, Pb, Cu, Fe, and A1 than the product obtained by redistillation of the same feed water in borosilicate glass. Thiers ( 3 ) ,employing single pass monobed deionization of the tap water, found by sensitive spectrographic analysis that the product was superior with respect to metallic contaminants to distilled water. Jennings and Knight (51) have described a continuous water washing system for semiconductor devices in which the wattr is recirculated through the exchange column. Their entire apparatus was constructed of plastic (polythene and PVC) t o avoid contamination of the water. An effluent measuring 20 X 106 ohm-cm was obtained. Simon and Calmon ( 5 2 ) have discussed the production of ultra-pure water (by deionization) in a comprehensive manner, including the treatment of raw water supplies by chlorination, precipitation, and sand filtration-all operations normal t o conventional “city” water purification-followed by preliminary roughing purification by passage through actikated carbon and single bed columns of strong cation resin and strong anion resin. This fairly pure water was then supplied to a recirculating loop system of mixed bed, rough filtration, (50) G. M. Healy, J. F. Morgan, and R. C. Parker, J . Biol. Chem., 198,305-312 (1952). (51) V. J. Jennings and R. D. Knight, J . Sci. Instrum., 36,432-434 (1959). (52) G. P. Simon and C. Calmon, SolidState Technol., 11 (2), 21-30 (1968). 694
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and ultraviolet sterilization. Water drawn from this loop was further purified by a mixed bed column, ultrafilter, and sterilizer, a t the point of use. This description is pertinent to good central systems applicable to a sizeable semiconductor plant. An effluent of 15-18 X lo6 ohm-cm resistivity was produced. I t was pointed out that all surfaces in contact with the ultra-pure water should be insoluble and noncontaminating, for which purpose plastics (PVC and PVCD) are close to ideal. Iverson (8) has emphasized the certainty of contamination of ultra-pure water by metals employed in the construction of purifying systems, recommending ceramics and glass (except soft glass) and plastics (particularly Teflon) as suitable materials of construction. He described a n all-glass system with flexible joints of Teflon. This system involved the following processes : ion exchange on nuclear grade monobed resins, organic removal on treated activated carbon, sterilization by ultraviolet light, and filtration. This system gave a n effluent measuring about 18 X lo6 ohm-cm a t 25 “C, closely approximating the calculated value for intrinsic water. Koontz and Sullivan (6) discussed the preparation and use of high-purity intrinsic water for electron device processing, recommending the sequence of steps: distillation (to remove the bulk of contaminants), filtration through activated charcoal for removal of organics, ion exchange (monobed), filtration (sub-micron, for removal of bacteria and other particulate matter), and sterilization (by heating or ultraviolet light). They described a continuously recirculating washing unit in which the rinse water is returned to the activated charcoal stage. Distilled water feed was supplied to the recirculating washer from a separate distillation unit. A specific conductance of 0.05 X 10-6 mho/cm (at unspecified temperature) was attained in the recirculating water. Prichett (7) described pure water systems suitable for large capacity. He discussed the preliminary treatment of raw water by conventional water works processes (chlorination, coagulation, settling, filtration, and active carbon treatment), followed by. deionization. Pritchett recommends the use of “polishing” mixed bed deionizers a t the point of use. Schafer (53) has described laboratory-scale systems for the complete deionization of water, with extensive reference t o equipment commercially available in Germany. Systems employing a single monobed column, as well as those employing separate cation and anion columns followed by a monobed polisher, were described. Extensive details were given concerning the operation and regeneration of deionization systems. Evaluation of effluent quality, in comparison with distilled water, was made only with respect t o conductivity. Holt, Lux, and Valberg ( 5 4 ) have described a system for the preparation of metal-free water for biological research. This system consists of a commercial metal still, followed by two monobed deionization columns in series. Beyond the still, all piping and fittings are of plastic (polyethylene, vinyl, and nylon). Storage tanks are polyethylene. No submicron fiItration is employed. Purity is monitored with a conductivity meter. The quality of the output from this system was evaluated by sensitive spectrographic analysis, for which a 2-liter sample acidified with HCI was evaporated down t o about 10 ml in Teflon under a dust cover. The concentrated sample was then taken t o dryness in platinum, the residue (53) W. Schafer, G1as.-1ristrum.-Tecli., 9, 351-362 (1965). (54) J. M. Holt, W. Lux, and L. S. Valberg, Can. J. Biochent. Physiol., 41, 2029-2034 (1963).
dissolved in 2N HC1 containing Mo as internal standard. Portions (0.1 ml) of the solution were dried on plicene-coated electrodes and spectrographed. Results of the spectrographic analysis showed the water to contain, in ppb: Mg, 1.0; Ca, 2.0; Fe, 1.0; Cu, 0.3; Cr, 0.05; Ni, 1.0. Al, Zn, Sn, As, Au, B, Be, Bi, Co, Cd, K, Hg, Mn, Na, P, Pb, Pd, Ru, and Sb were sought but not detected. It was noted that the product from this system had a resistivity of approximately 10 x 106 ohm-cm, but that this degraded fairly rapidly on storage, because of diffusion of COa through the walls of the polyethylene storage vessel. Mottershead (55) has described a 2000 gal/day deionization system for electronics plants. Special attention was given to the problem of removing colloidal iron-silica present in the feed water (city water derived from surface sources) at approximately the part per million level. This colloidal ironsilica matter is not removed by conventional deionization. It is retained (largely) by a membrane filter, but causes very rapid clogging of the filter. Mottershead describes the measurement of such colloids by a determination of the rate at which flow diminishes when a small (10-ml) sample is driven under constant pressure through a small area membrane filter of 0.45 mp pore size. A dimensionless unit termed silting index, as a measure of colloid content, is given by tio
- 2ts
Silting Index = ___ tl
where, tl is the time for the first ml to flow through the filter, t5 is time for flow of the first 5 ml, and t10 for the flow of 10 ml. Mottershead’s preliminary treatment process consists of the following stages : filtration through a leaf-type filter press, cation removal on Amberlite IR 200, anion removal on Amberlite I R 911. The effluent is of about ‘/z X l o 6 ohm-cm resistivity, is higher than the feed water in colloidal iron-silica, and contains 1-5 ppm of ionized impurities. It is then passed through a special resin (Amberlite xE238) of large pore size (average = 70,000 A) to remove the colloidal iron-silica. Final purification is effected in a monobed column of strong cation and strong anion resins (Amberlites I R C 200 and IRA 900), giving an effluent of 15-20 X loGohm-cm resistivity, but still containing colloidal matter amenable to removal by submicron filtration, accompanied by fairly rapid clogging of the filter. Perrin (56) discussed the preparation of water for use in the cosmetic and pharmaceutical industries, emphasizing ion exchange. He indicates that sterile, pyrogen-free water can be produced on a small scale by deionization if the feed water is sterilized by halogenation, the resin bed is presterilized by gaseous formaldehyde, the effluent subjected to submicron filtration and passage through (sterile) charcoal, and is further subjected to final sterilization by an ultraviolet lamp. The more commonly employed industrial processes for the production of sterile, ion-free, apyrogenic water, according to Perrin, involves at least a final stage of distillation. Zak (57) has comprehensively reviewed the literature on the preparation of very pure water, covering the subjects of exclusion of atmospheric gaseous contaminants, materials of construction of stills, the distillation process, deionization, commercially available equipment for both distillation and deionization, the analysis of pure water, and quality attained as reported by several other investigators. ( 5 5 ) T. Mottershead, Filtr. Separ. 1969,263-266, May/June. ( 5 6 ) J. H. Perrin, Amer. Perfum. Comet., 83, 25-30 (1968).
( 5 7 ) F. Zak, Chem. Listy, 63, 1033-1044 (1968).
Vasilevskaya et al. (40) have extensively investigated the analysis of water (and hydrofluoric, nitric, and hydrochloric acids). Their procedure is to evaporate a large sample to dryness in the presence of 20 mg of carbon powder, add 2 mg of sodium chloride, and to spectrograph the mixture. As a major point of their work, the necessity of carrying out the evaporation in a plastic enclosure supplied with filtered air is shown. The use of Teflon for the evaporation vessel is shown to give a lesser contamination than is obtained with fused quartz or platinum. Distillation of acids in Teflon is also shown to provide a superior product to that attained by distillation in quartz or platinum. Deionized water is shown to become significantly contaminated by Al, Fe, Ca, Mg, Mn, Cu, and Cr during 30 days’ storage in Teflon or polyethylene; accordingly, the use of freshly deionized water is recommended for particularly clean work. Adams and Lunney (58) reported data on the purity of water obtained by several sequences of distillation (in glass) and deionization. Best results were obtained by the sequence:
distillation-deionization-distillation-deionization. EXPERIMENTAL EVALUATION OF QUALITY OF ULTRA-PURE WATER
Water of good quality was available from several systems, including a two-stage, all-metal still, fused quartz two-stage still, and a variety of deionization systems. Samples taken from these various sources were analyzed for spectrographically detectable impurities by evaporating a large volume (1 or 2 liters) of the water to near dryness, taking up the residue in nitric acid, transferring to treated graphite electrodes, drying, and spectrographing. Comparison spectra obtained from standard solutions added to electrodes and similarly spectrographed are employed for visual comparison. The sample is evaporated without boiling from a 2-liter Teflon beaker placed in a laminar flow hood and heated by an infrared lamp. The residue is taken up in 2 successive 10-drop portions of 1-1 nitric acid ( 2 x redistilled in quartz), and transferred, several drops at the time, to a l/&-ich diameter graphite electrode which has previously beencoated with a solution of polyethylene. Transfer and intermittent drying is continued until the entire washing has been transferred to the electrode. The sample electrode is arced against a pure counter electrode at 4400 volts ac and with an exposure time of 60 seconds. Spectra are taken on a Baird-Atomic 3-meter spectrograph and recorded on Spectrum Analysis No. 1 plates. Plates are examined on a comparator, and evaluated by comparison with plates made with standards containing known additions of the various elements for which analyses is made. (Reproducibility of the basic procedure is such that refined techniques employing internal standards, densitometry, and working curves are not warranted.) Results obtained from the analyses are listed in Table I , accompanied by some comparison results taken from the literature. (Results for sodium were obtained on a separate sample by flame photometry.) Best quality of water, either quartz distilled or deionized, is indicated to contain (in ppb) the order of: Ca, 0.5, or less; Al, 1, or less; Cu, 0 to 1 ; Mg, 0.1, or less; S O z , 1 to 10, and no other metallic constituents at levels detectable by the method employed. The actual presence of these constituents at the indicated levels is even questionable, since the detected (58) P. B. Adams and J. K. Lunney, Cernm. Bull., 49, 543-548
(1970). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
695
Table I. Metal Content of Ultra-Pure Water Prepared by Various Methods (in parts per billion) Philips Laboratories analytical resultsa Comparison results from the literature
-I-
-
2
.P B 2 m R
P
La
0%
3'
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or;m
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0
-wB Ca A1
%E
b
'R
855
$E!-
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- 4 -
50.0 10.0 1.0 50.0
0.03 0.1
0.5 -
-
8.0 0.01 50.0
0.01
-
B
1.0 0.1 10.0 0.01
Si
50.0
Ag
cu
Mg
Mn Cr Pb Ni Fe Zn
Sn 5.0 1.0 Na Ti a -, Sought, not detected;
+
-
-
-
1.0
0.05
10.0 10.0 0.7 >10.0
0.1 0.1 0.1
0.01
0.1 0.1 10.0
0.1
-
--
-
5.0 0.1 0.1
0.1
0.1
0.5
1 .o
10.0
4.0
10.0
>10.0 >10.0 4.0
-
0.1 0.5
-
-
+, detected, not determined quantitatively; x, not sought.
quantities may easily have been derived from the air, electrodes, or containers, during the analytical procedure. The failure of the conductivity determination to disclose the presence of a total contamination level of approximately 100 ppb in the worst sample of deionized water is clearly seen. (All deionized samples were of 15-18 X lo6 ohm-cm resistivity at room temperature.) It must be assumed that the detected impurities were present in particulate or colloidal form, not appreciably ionized. This is the condition to be expected for elements such as AI, Cu, Mn, Cr, Pb, Ni, Fe, Zn, and Sn. In the presence of a preponderance of these elements as oxides, hydrous oxides, or hydroxides, it is not unlikely that normally ionizable species such as Ca, Mg, and Na are absorbed upon or reacted with them and consequently not appreciably ionized. Thus, a measurement of con-
696
0.05 >10.0 3 .O
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
i
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0
3 .g0 q cka
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