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Determination of Total Solutes in Synfuel Wastewaters Sir: The measurement of total residue, which is defined as the mass remaining when a sample is dried at 103-105 "C, is performed frequently on wastewaters as an indication of total dissolved solutes (1, 2). To the plant operator this measurement provides a simple indication of the performance of the pollution control plant operation, and to the analytical chemist it provides a check on the completeness of the analyses. Unfortunately, this measurement is not appropriate for wastewaters from many synfuel processes which contain major proportions of volatile materials such as carbonate, bicarbonate, ammonia, sflide, and organic compounds. These materials may be partially or entirely lost during the drying step, resulting in low and irreproducible results. An alternative method is clearly desirable and is the subject of this paper. One alternative method considered in this study, as well as by other investigators, is lyophilization (freeze-drying), whereby the sample remains cold throughout the drying process, only reaching room temperature (at maximum) at the moment of total desiccation. The residue is then weighed as an indication of dissolved residue. It is widely known that for ideal solutions the colligative properties, such as boiling point elevation, freezing point depression, and osmotic pressure, are an indication of total solute content. Unlike the drying procedures commonly used for residue measurements, the colligative properties are related to total molality rather than to dissolved mass. Nevertheless, total molality provides the wastewater engineer with a gross indication of treatment progress and, therefore, can serve much the same purpose as the traditional "TDS" measurement. Similarly, total molality provides a quality control check for the analyst, since the individual components should sum to the total molal strength, providing that the major components have been measured correctly. Thus,if the colligative properties of retort wastewaters could be measured and related to total solute strength, they would fill much the same role as the total residue measurement. This paper describes efforts to investigate both lyophilization and the measurement of colligative properties as an indication of total solute content. The objective of the work described below is to develop a method for measuring total dissolved material in retort wastewaters which is simple and rugged enough to be performed in a field laboratory in support of pollution control tests. The analysis should also be rapid enough to provide timely and pertinent data to the pollution control plant operator. To be of most value, the technique developed also should be applicable to other synfuel wastewaters, most of which contain similar major components as oil shale retort waters.
EXPERIMENTAL SECTION Standards of known osmolaity (osmotic strength)were prepared from "Baker Analyzed" reagent-grade NaCl dissolved in deionized, distilled water. Solutionsof NH4HC03/NH3were prepared from reagent grade ammonium hydroxide (Fisher Scientific) and analytical reagent NH4HCOB(Mallinckrodt). The ammonium hydroxide was standardized by titration with 0.1 N H2S04prepared from Acculute concentrated standards by using a pH meter to monitor the progress of the titration. For this titration the pH at the equivalence point varies with the concentration, a factor which was taken into account. Retort waters of various types were obtained from the the Laramie Energy Technology Center (DOE),mainly in conjunction with operations at their North Site Facility. Selected retort water samples were analyzed on a Digimatic Model 3DII (Advanced Instrumenta, Inc.) to establish repeatability with retort waters. This instrument operates by automatically determining the freezing point depression of a sample. Additional 0003-2700/84/0356-0582$01.50/0
Table I. Total Solute by Freezing Point Depression
sample 1.0 M NH,HCO, t 0.1 M ",OH 0.1 M NH,HCO, t 0.01 M ",OH 0.01 M NH,HCO, t 0.001 M ",OH 1.0 M NH,HCO, + 0.1 M ",OH t 0.921 M acetone Omega-9 retort water
ideal osmolality, mol/L
measd osmolality, mol/L
equiv, mol/L
2.1
1.90
2.0
0.21
0.216
0.21
0.021
0.021
0.021
3.02
2.64
2.8
NaCl
0.58
freezing point depression measurements,specifically those listed in Table I, were obtained on a Knauer manually operated cryoscope operated according to the manufacturer's instructions. Samples were stored at 4 "C from preparation until analysis.
RESULTS AND CONCLUSIONS Various retort water samples were analyzed for total dissolved residue using the standard drying technique at 103 "C. As expected, residue measured by this procedure was only a small fraction of that expected based on analysis of the individual components. Lyophilization of retort waters gave similar results, implying that the ammonium carbonates also evaporate under cooled, low-temperature conditions. To confirm this implication, solutions of ammonium bicarbonate and ammonium carbonate were evaporated by using both lyophilization and thermal drying. With either procedure over 98% of the dissolved material was lost. It is thus clear that neither lyophilization nor thermal drying, as carried out in this experiment, are appropriate for retort wastewaters. It is natural to ask at this point whether lyophilization carried out at a different temperature would more selectively evaporate water in comparison to the dissolved material. The answer to this question can be found by examining the vapor pressures of water and the major constituents in retort water as a function of temperature. Application of simple thermodynamic formulas indicates that the vapor pressure of ammonium bicarbonate, one of the major components of retort waters, exceeds the vapor pressure of water at all temperatures above approximately -10 "C, corresponding to 0.003 atm. In theory, then, lyophilizationcarried out well below -10 "C could selectively evaporate water from retort water samples. However, because the vapor pressure of most organic compounds is less affected by temperature than is water, as the temperature is lowered the organic compounds in the retort water would be more selectively evaporated than they are at 103 "C. It thus appears that evaporation techniques, regardless of the temperature under which they are carried out, are inappropriate for retort wastewater. The most common colligative properties for measuring total solute content include osmotic pressure, boiling point elevation, and freezing point depression. The fiist of these requires a semipermeable membrane capable of passing water but not the dissolved materials. In the case of retort wastewaters, the membrane must be impermeable to both ammonia and carbon dioxide, as well as additional organic compounds. Most membranes are permeable to both of these gases, and for this reason osmotic pressure was not examined. The depression of the dew point, which is equilvalent to the elevation of the boiling point, can be measured by com@ 1984 American Chemical Society
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mericially available instruments, one of which was evaluated by using NaCl standards, NH3/NH4HC03standards, and retort waters. While the NaCl standards yielded reasonable results, no meaningful numbers could be obtained with the other samples. In retrospect, this result is reasonable since the proportionality between dew point depression and solute content can only be expected for nonvolatile solutes (3). Hence, dew point depression (and boiling point elevation) are inappropriate methods for measuring solute content of retort wastewaters. Freezing point depression also has been evaluated by using both artificial standards and actual retort waters and has proven to be the best method of those tested. Table I illustrates the application of this technique to real and simulated retort waters. In this table ideal osmolality refers to the number of moles of solute added, counting each ion separately. Measured osmolality is the freezing point depression in OC divided by 1.86 “C, the freezing point depression per mole at infiiite dilution. The measured and ideal osmolality will agree to the extent that the sample behaves like an ideal (i.e., infinitely dilute) solution. The measured osmolality is sufficiently close to the actual molal strength for most field applications. The final column in the table shows the concentration of NaCl which gives the same freezing point depression as the solution under test. Comparing the data in columns 2 and 4 in this table suggests that the “NaC1 equivalent molarity” is a slighlty better indication of total solute content than is the measured osmolality. The data for the artificial samples in Table I suggest that the total solute content could be measured to an accuracy of 7% (relative) at concentrations up to 3 mol/L. At higher concentrations, solution nonideality would likely become increasingly important, and the freezing point method is therefore not recommended for such samples. The accuracy of most devices for measuring freezing point depression is typically *0.001 mol/L, which constitutes the lower limit of this technique. Of course, experience with retort waters which have been completely analyzed by reliable techniques for all major and minor components is necessary in order to best assess the accuracy of this method. Unfortunately, such samples were not available at the time of this investigation. However, the one retort water sample shown in the bottom of Table I has subsequently (2 years later) been analyzed for most inorganic species using methods developed in the author’s laboratory ( 4 ) with a resulting total solute content of 0.57 mol/L. For the purpose of checking the total of the individual analyses, the data in Table I suggest that the “NaC1equivalent
molarity” should be compared to the sum of the individual species. Since most species are determined in units of mg/L, they can be easily converted to mol/L and compared directly to the “NaC1 equivalent”. This approach may prove to be difficult with waters containing high levels of total organic carbon (TOC) which are not analyzed for individual organic species. In this case the TOC measurement could not be related directly to molarity. In any case, a high molar organic content suggests that at least the major organic species should be determined. Repeatability of the freezing point method was determined by analyzing a variety of retort waters with the commericially available instrument listed in the experimental section of this report. Most results could be repeated to within *0.001 osmols, indicating that adequate precision is available from commerically available instruments. With such instruments, the freezing point measurement can be made in a matter of minutes with a minimum of equipment. The freezing point depression thus appears to be a method which could easily be performed in field laboratories in support of water pollution control tests. However, this author has not yet applied this test under field conditions nor to a large number of samples and therefore cannot yet attest to its ruggedness and reliability under such conditions. ACKNOWLEDGMENT Freezing point depression measurements from the Knaure cryoscope were obtained by Huffman Laboratories (Wheat Ridge, CO). Registry No. Water, 7732-18-5. LITERATURE CITED (1) EPA, Methods for Chemical Analysis of Water and Wastes, US EPA600/4-79-020, 1979. (2) APHA Standard Methods, American Public Health Association, WashIngton, DC, 1976. (3) Lewis, G. N.; Randall, M. “Thermodynamics”; Revised by K. S.Pitzer and Leo Brewer; McGraw-Hili: New York, 1961. (4) Wallace, John R.; Alden, Linda; Bonomo, Francis S.;Nichols, John; Sexton, Elizabeth Methods of Chemical Analysis for Oil Shale Wastes; Report Prepared under USEAP Contract No. 68-03-2791 by Denver Research Institute, 1982.
John R. Wallace* Francis S. Bonomo Denver Research Institute University of Denver Denver, Colorado 80208
RECEIVEDfor review February 28, 1983. Resubmitted November 14,1983. Accepted November 14,1983. This work was supported by U.S. EPA Contract No. 68-03-2791.
AIDS FOR ANALYTICAL CHEMISTS Use and Abuse of Digital Signal Processors Mark R. Thompson and Raymond E. Dessy* Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
The use of digital signal conditioning is increasing in popularity as the cost of the required hardware decreases. Many signal processing tab that have traditionally been performed by analog circuits can now be performed by digital circuits which have many advantages over their analog counterparts. 0003-2700/84/0356-0583$01.50/0
The digital devices are not as susceptible to drift and can implement time constants not available with analog filters. They also allow filter functions which are symmetrical. Recently read-only-memory-baseddigital signal processors have been introduced which are designed so that the average 0 1984 American Chemical Society
