Anal. Chem. 1987, 59, 1481-1483
Table 11. Comparison of Calculated and Measured pH Changes Induced by Additions of Acid and Base to Tris (Seawater) Buffer R 0.8209 0.7983 0.7677 0.7155 0.8191 0.8423 0.8833 0.9609
pH (eq 5)
8.199 8.186 8.168 8.136 8.197~ 8.2104 8.232 8.271
A p H (spec)’
0
0
-0.013 -0.018 -0.032 0
-0.013 -0.018 -0.032 0
+0.013 +0.022 +0.039
‘spec = spectrophotometric. tentiometric.
A p H (titr)*
+0.015 +0.023 +0.044
A p H (potn)‘
0
-0.015 -0.022 -0.034 0
+0.012 +0.020 +0.039
*titr = titrametric. c p o t n = po-
Equation 5 then indicates that this buffer had undergone considerable alteration (-0.027 pH unit) during storage. For indicator-buffer pairs such that R N 1, 1-cm path lengths and 5 7 X M sulfonephthalein concentrations produce absorbances on the order of 1.0 at each absorbance peak. In the case that R N 1, HL- and L2- are present in approximately a 5:2 concentration ratio. For absorbances at each peak on the order of 1.0, a systematic 0.001 absorbance overestimate at one peak coupled to a 0.001 underestimate at the other will produce a pH misestimate on the order of 0.001. A 0.01 overestimate at one absorbance peak and a 0.01 underestimate at the other will produce a pH misestimate on the order of 0.01. For the purpose of absorbance ratio measurements it is important that spectrophotometers exhibit a linear (Beer’s law) response to an absorber’s concentration. It is, however, interesting to note that in some respects the outlined spectrophotometric procedures are not especially demanding of absolute absorbance accuracy. If indicator-buffer pairs are chosen such that R N 1, even such potentially troublesome effects as base line shifts will not substantially degrade spectrophotometricbuffer assessments. In our experience with spectrophotometricmeasurements, both in the laboratory and at sea, base line shifts typically consist of “jumps” in absorbance that are, for the most part, independent of wavelength. In such a case that an extraordinarily large base line shift, +0.010 occurs at both 596 and 435 nm and, initially, ASQ6= 0.8261 and A435 = 1.000, the concomitant variation in R constitutes only a change between R = 0.8261 and R = 0.8361/1.010 = 0.8278. This creates a buffer pH missestimate equal to only 0.001 pH units. As an example of the use of eq 5 in assessments of Tris/ Tris.HC1 seawater buffer (6),we titrated our buffer with acid (1 M HCl) and subsequently with base (1 M NaOH). Aliquots of the titrated buffer was used for spectrophotometric pH assessments according to the outlined procedures, and also for potentiometric measurements of the pH change (ApH) induced by acid and base. Our potentiometric experiments were performed by using a Corning 130 pH meter and an Orion combination pH electrode (Orion No. 810200). Our potentiometric and spectrophotometric measurements were obtained a t 25.0 f 0.05 “C.
1481
Column two of Table I1 shows that with no titrant added
(R = 0.8209 and R = 0.8191) the pH of our buffer was 8.198, f O.OO0,. Our measurements thereby indicated that, over a period of 4 months, our buffer became more acidic than the original freshly prepared standard by 0.0026 pH units. Columns three, four, and five show assessments of the pH change induced by each titrant addition. Potentiometric ApH is calculated as the millivolt difference measured by the electrode, before and after each titrant addition, divided by 59.16 mV. Titrametric ApH is calculated from the known, titrant-induced modification in the buffer conjugate pair ratio. Agreement between ApH (spectrophotometric) and ApH (titrametric) is generally very good. Further, in every case, ApH (spectrophotometric) is either (a) indentical to ApH (titrametric) or ApH (potentiometric),or (b) intermediate to ApH (titrametric) and ApH (potentiometric). It is important to note that, although buffer comparisons can be obtained by potentiometric means, potentiometric comparisons require conjugate measurements in two or more buffers and provide only a difference between buffers being directly compared. In the event that two buffers are distinctly different, each buffer’s relationship to a true standard may involve the preparation of a fresh buffer. In contrast, the outlined spectrophotometric procedures directly reveal a buffer’s relationship to the standard in a single spectrophotometric determination. The outlined procedures are rapid, precise, and permit direct quantitative comparisons between independent buffer preparations at diverse locations. Through the results of many independent preparations, various investigators are free to judge for themselves the absorbance ratios, R, that best characterize each standard buffer. In addition to the indicator-buffer pairs examined in this work, a variety of indicators are available for use in standards at both higher and lower pH. The outlined spectrometric techniques should prove quite useful as an instructional device in conjunction with an intiate’s first buffer preparation and, in diverse laboratory settings, should prove useful in assessments of the care prerequisite to preservation of standards. ACKNOWLEDGMENT The author gratefully acknowledges R. M. Garrels, R. W. Ramette, and an anonomous reviewer for their constructive criticism of this work. LITERATURE CITED (1) Zirino, A. Limnol. Oceanogr. 1975, 2 0 , 654-657. (2) Cuiberson, C. H. I n Marhe Elecfrochemlsfry; Whitfield, M., Jagner, D., Eds.; Wiiey: New York, 1981; pp 187-261. (3) Chen, C. T. Department of Energy Technical Report, DOE/EV/106114; 1984; 118 pp. (4) Bates, R. 0.Determination of p H , Theory and Practice, 2nd ed.; Wiley: New York, 1973. (5) Hansson, I. Deep-sea Res. 1973, 2 0 , 479-491. (6) Ramette, R.; Cuiberson, C. H.; Bates, R. G. Anal. Chem. 1977, 49, 867-870. (7) Robert-Baldo, 0.; Morris, M. J.; Byrne, R. H. Anal. Chem. 1985, 5 7 , 2564-2567.
RECEIVEDfor review October 23,1986. Accepted January 30, 1987.
Apparatus for the Rapid Equilibration of Moisture-Sensitive Materials John R. Moody* and Ellyn S. Beary Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899 For the certification and use of any moisture-sensitive reference material, it is necessary to state a condition by which
all users may obtain the same moisture content. Ideally, a simple, universally applicable method is preferred td assure
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
that all users may reproduce the exact condition of the material specified in the certificate. For the certifying body, however, a considerable effort is required to find a set of conditions that give the best results for any prototype reference material (1). A variety of conditions must be explored to give the best method for drying. In addition, allowances must be made for variable environmental conditions at different locations around the world. To approximate these conditions in our laboratory, we have used desiccators containing salts or solutions to establish an atmosphere of defined relative humidity (RH). The reference materials are exposed to a number of different conditions over a period of time ranging from weeks to months. By following the sample weights, the period of time required for equilibration may be calculated. In addition, the samples may be dried and resubjected to the same moisture equilibration conditions to determine whether these changes can influence the dry basis of the material (2, 3). Because of the need to evaluate a large number of potential reference materials, a more rapid means of sample equilibration and evaluation is desirable. In this communication, we describe the construction of a simple apparatus from readily available components that permits rapid sample equilibration and simultaneous measurement of sample weights over a wide range of temperature and humidity changes. The measurements made with this apparatus identify materials that require special packaging as well as help to develop and evaluate methods for sample drying. The apparatus should be easily adaptable to computer control. EXPERIMENTAL SECTION A Blue M Model VP-100 AT-1* controlled temperature and humidity chamber was used to provide the desired temperature and humidity. This device utilizes forced air convection within a glass covered chamber. The humidity is controlled by a webbulb and dry-bulb sensor, which are not calibrated directly in "C. For this reason a time wet-bulb/dry-bulb thermometer is placed in the chamber to provide a means of reading the environmental conditions in the chamber and calibrating the front panel controls of the instrument. Cooling water from the tap can be circulated through the chamber to achieve lower wet-bulb temperature. At higher humidities, the uninsulated glass chamber will fog up due to water condensing on the cooler glass surface. A tent-shaped shield above the sample area is used to deflect any condensate that might drip from the glass dome under high-humidity conditions. The apparatus may be used to obtain relative humidities (RH)between -20% and 95%. Most of our experiments were carried out between 20 and 55 "C and 30% and 80% RH. To evaluate the rate of equilibration and other factors, we measured changes in the sample weight over the course of the experiment. The apparatus was modified by drilling two holes in the controlled temperature/humidity chamber to permit the suspension of the sample from a digital electronic balance by means of a stainless steel wire. The complete apparatus is depicted in Figure 1. In use the actual measurement of wet-bulb and dry-bulb temperatures was made from the wet- and dry-bulb thermometers held within the environmental chamber. The relative humidity was determined by interpolating from tables for hygrometric application. To measure the sample weight, the air circulation fan is turned off for -30 s to eliminate oscillationsof the readings due to sample suspension. The long term (several days) stability of the balance used in this manner was found to be ztO.001 g (the reproducibility of the balance). The chamber itself was stable to within k0.5 "C and k2-4% RH from low to high relative humidities, respectively. RESULTS AND DISCUSSION To test the apparatus, the behavior of several NBS Standard Reference Materials (SRM's) was investigated. The first example, a proposed NaCl SRM was taken as a typical hygroscopic salt. NaC1, though hygroscopic, will maintain a
f
gl
I
l Ig
Flgure 1. Functional representation of controlled environment apparatus consisting of the following parts: (a) dlgltal balance, (b) holes drilled for balance suspension, (c)deflector shiaM, (d) 5-cm glass petri dish and suspension wire, (e) wetldry-bulb thermometers, (f) glass dome, (9) controller-Blue M Model VP-100 AT-1. Dimensions are approximate, not critical.
constant moisture content as long as the relative humidity is below a critical level. Samples of exactly 1O.OOO g of NaCl were weighed into a 6-cm-diameter Petri dish before loading into the apparatus. The NaCl was exposed to humidities of 35%, 40%, 4570, 50%,55%, 60%, 65%, 70%, and 75% RH for periods of 1 h at 37 "C. During the entire course of the experiment from 35% to 75% RH only a 0.02% weight change was observed. At 80% RH, very rapid absorption of H20was observed with >3% water adsorption reached in only 2 h. Manual adjustment of the RH control until a significant weight change occurred, would enable the operator to find the same 75-80% RH threshold for NaCl in only 1 h. Another material, SRM 332, copper concentrate, was selected to test its stability to high relative humidity levels. One sample of the material was dried at 105 "C for 2 h and lost 0.20% of its original weight. Another sample of 10 g was loaded into the environmental chamber. At low RH (28% RH, 50 "C) conditions, the sample lost about 0.05%. Below 60% RH, there was no evidence for weight gain by the material. However, at 37 "C and 60% RH, a change of +0.24% was found within 2 h. After 12 h, no further changes were observed. Most of the observed gain occurred in less than 1h. By changing the relative humidity to 80% at 37 "C, a further increase to a total of +0.47% in weight was observed. When the RH was lowered to 30-35% RH, approximately 80% of the previously observed gain was lost within 1 h. Another sample of SRM 332 was taken from 40% to 80% to 40% RH at 37 "C a total of 3 times. After the third equilibration at 40% RH, a final weight gain of +0.38% was found. The discrepancy between the expected loss based upon drying the untreated sample for 2 h a t 105 "C and the loss actually found for the treated sample was only 0.01%, equivalent to the uncertainty in the measurements. On the basis of these experiments, we conclude that SRM 332 suffers no short-term change in assay due to high relative humidity or even cyclic changes in the relative humidity. Finally, a sample of SRM 329, zinc concentrate, was obtained, which had been suspected of environmentally induced changes in the Zn assay. As received, the material lost 0.39% when dried a t 105 "C for 1 h. When placed in the environ-
Anal. Chem. 1987, 59, 1483-1485
mental chamber, the material gained weight up to 1.8% or lost up to 0.15% depending upon the conditions employed. No significant weight gain was observed at 55% RH or lower. When a single exposure to 60% RH or more occurred, approximately 80% of the observed gain could be lost by reequilibration at 35% relative humidity. Repeated or cyclic exposures, however, led to a permanent weight gain. A previously untreated sample of SRM 329 was then subjected to three cycles of 40% to 80% to 40% RH at 37 OC with a 1-h exposure at each condition. The sample was then dried for 1 h a t 105 "C as recommended by the certificate and a weight loss of 1.72% was recorded. Of this, 1.66% may be accounted for by the weight gain during the humidification/dehumidification experiment. Thus, the net loss on drying is only 0.06% compared to 0.39% for the untreated SRM. This discrepancy of 0.33% is quite significant and would invalidate the certificate values by the same relative amount. Thus, conditions have been found that for some users could lead to erroneous results. SRM 329 is a zinc sulfide which slowly oxidizes to zinc sulfate. The mechanism of this reaction is not clear but high-humidity conditions are at least one of the probably causes. SRM 329 had been withdrawn from sale as a result of this study and customer feedback. In addition, we have used the apparatus to assist in developing sample processing methods for new SRM's where there was insufficient prior experienceto predict the behavior of the material under dry to humid conditions. A sample of 10 g will yield precision of *0.01% with a balance of 1 mg precision. A more sensitive balance would permit either better precision with the same sample size or the use of smaller
1483
samples. Smaller sample sizes should achieve a more rapid equilibration. This apparatus differs from other approaches primarily because of the dynamic nature of the sample equilibration caused by forced air movement. A smaller chamber should permit better humidity control and perhaps faster sample equilibration. The present apparatus was only meant to demonstrate the feasibility of the experiment with available apparatus. For reference materials, ideal behavior with respect to repeatable moisture content can be demonstrated by the cyclic humidificationfdehumidification process described here. Other processes, such as bacterial degradation or oxidation, occur over a longer time scale and may be influenced by the presence of water. This procedure does not address those problems. LITERATURE CITED (1) Michaelis, R. E. The SRM Story at NBS; NBS Special Pubiicatlon 408, National Bureau of Standards: Washington, DC, 1975; pp 246-257. (2) Ward, R., Chairman Non Stoichiometric Compounds; 141st Meeting of the Amerlcan Chemical Society; Advances in Chemistry Series No. 39; American Chemical Society: Washington, DC, 1963. (3) Barnes, I. L.; Murphy, T. J., personal communication, National Bureau of Standards, Gaithersburg, MD, M a y 1986.
RECEIVED for review November 18,1986. Accepted February 3, 1987. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedures. 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.
Indirect Cationic Chromatography with Fluorometric Detection Jeffrey H. Sherman and Neil D. Dadelson*
Department of Chemistry, Miami University, Oxford, Ohio 45056 Indirect photometric chromatography (IPC) is a technique by which analyte separation is effected through an ion exchange process, following which, analyte detection is achieved through a photometric process. IPC uses light-absorbingions in the mobile phase, with which photometrically inactive, injected sample ions substitute in the column effluent as sample elution occurs causing a decreased absorbance at the detector and negative peaks to be recorded. A number of different eluents such as Cu(I1) (1, 2 ) , benzyltrimethylammonium ion (3-5), picolinic acid (6, 7), and benzylamine (7) have been used for the separation of either inorganic or organic cations by IPC. The advantage of IPC is that it enables chromatographs equipped with UV detectors to perform ion chromatography. There are two major ways peaks in chromatograms generated by IPC can be obscured. Fmt, some analyte ions might absorb light a t the wavelength of detection. Second, the sample matrix may contain an interferent which could also absorb at this wavelength. However, few alternative detection modes for IPC have been reported (8). Indirect fluorescence chromatography (IFC) using sodium salicylate as the eluent has been reported for the separation and detection of anions (9, 10). Detection in IFC is achieved by simply monitoring the column effluent at the emission wavelength of the counterion. As an analyte ion elutes, the background fluorescence of the mobile phase decreasesand a negative peak is recorded. However, none of the eluents cited previously for IPC would be expected to be good candidates for IFC of cations. 0003-2700/87/0359-1483$01.50/0
The fluorescence of aqueous solutions of cerium(II1) has been known for some time and its use in analytical chemistry has been reported (11-15). Prevously in our laboratory, cerium(II1) was characterized as an eluent for indirect photometric chromatography of cations (16). We now wish to extend the utility of Ce(II1) to include serving as a counterion for indirect fluorescence chromatography. The advantage of IFC over IPC for the determination of Na+, NH4+,and K+ in urine is demonstrated. EXPERIMENTAL SECTION Reagents. All solutions were prepared with doubly distilled, deionized water and were kept in Pyrex glassware. Cerium mobile phase solutions were prepared from the sulfate salt unless otherwise indicated, while all sample solutions were prepared from the respective chloride salt. These salts were obtained from a variety of sources and were reagent grade or better in quality. Instrumentation. The chromatographic system consisted of an IBM LC/9533 ternary gradient liquid chromatograph and operator station (IBM Instruments, Danbury, CT). An IBM Model 9522 (254 nm) UV detector was employed for IPC and a Kratos Spectroflow 980 programmable fluorescence detector was used for IFC (Kratos Analytical Instruments,Ramsey, NJ). The chromatograms were recorded on a Fisher Recordall Series 5000 recorder (Houston Instruments, Austin, TX). The separations were effected with a 10 cm X 3.2 mm i.d. ION-210 Transition Metals column (Interaction Chemicals, Mountain View, CA) at room temperature. The column was packed with 5 pm sulfonic acid derivatized poly(styrene4ivinylbenzene) resin particles. The capacity of this strong cation exchanger was about 500 pequivjg. 0 1987 American Chemical Society
