analysis would be, in this case, shorter. Experiments were conducted in the temperature region 25-75 OC. Higher temperatures could be tried but temperature fluctuations need to be brought into account. Lower detection limits for hydrogen with annealed P d films were determined by the uncertainty associated with measuring AR.
ACKNOWLEDGMENT
d FILMS FILMS
25. 75.
The authors express their thanks to Michael Cox, for the vacuum deposition of the f i b s and the chemical analysis. We also thank Om Chawla of General Atomics, San Diego, Calif., for valuable suggestions.
LITERATURE CITED 200
400
eo
0
BOO
PPY I W I W ) H 2 I N He
Flgure 4. Change in the resistance vs. concentration of hydrogen in helium at 1 atm. Annealed films of titanium were used
various concentrations of hydrogen in helium at two temperatures are shown in Figure 4. As with palladium there is a saturation limit, but lower limits of hydrogen with titanium were detected than with palladium.
CONCLUSION This investigation shows that palladium and titanium films respond to hydrogen on helium. Either a freshly deposited palladium film or the conditioned palladium or titanium film can be used for this purpose. The effects of the other gases were not studied. In the case of annealed films, the response depended on the hydrogen concentration. Elovich equation (IO) may partially satisfy the conditions found, but Elovich equation itself (it describes the adsorption of hydrogen on chromia catalysts) is empirical. In any case, the initial rise of the curve could serve to analytically determine the hydrogen content using kinetic methods of analysis (11). Time for
R. E. Norburg, Phys. Rev., 86, 745 (1952). R. A. Oriani. E. McCliment, and J. F. Youngblood, J . Chem. Phys., 27, 330 (1957). W. H. Sachtler, J . Chem. Phys., 25, 751 (1956). P. Zwelterlng, H. L. K. Kdts, and C. Van Hearden, J. phys. Chem. Solkfs, 11, 18 (1959). E. B. Sandell, “Colorimetric Determinatlon of Traces of Metals”, 3rd ed., Interscience Publishers, New York, N.Y., 1950, p 488. R. B. Belser, J . Appl. Phys., 28, 109 (1957). K. S.Kim, A. F. Gossman, and Nlcholas Wlnograd, Anal. Chem., 46. 197 (1970). F. A. Lewis, “The Palladium Hydrogen Systems”, Academic Press, New York, N.Y., 1967, p 65. W. Von Benken and T. Kuwana, Anal. Chem.. 42, 1114 (1970). J. Sarmousakis and M. J. D. Low, J. Chem. Phys., 25, 178 (1956). S. R. Crouch, In “Spectroscopy and Klnetlcs”, J. S. Mattson, H. B. Mark, Jr., and H. C. MacDonaM, Jr., Ed., Marcel Dekker Inc., New York, N.Y., 1973.
RECEIVED for review September 10,1976. Accepted February 22,1977. We express our thanks to General Atomics Co., San Diego, Calif. for the financial support of the work. One of us (V.S.S.) thanks the Faculty Research Committee of BGSU for initial support. This paper was presented at the 169th National Meeting, American Chemical Society, Philadelphia, Pa., 1975.
Elimination of Chloride Interferences with Mercuric Ions in the Determination of Nitrates by the Phenoldisulfonic Acid Method Ahmed Mubarak, Reed A. Howald, and Ray Woodriff * Depafiment of Chemistry, Montana State University, Bozeman, Montana 59715
A new technique to eliminate chloride interferences in the determination of nitrates by the phenoidisulfonic acid method Is presented. I t simply involves addltion of an excess of mercuric sulfate to the nltrate-containing sample before evaporatlon. The dissociated mercuric ions tend to complex the chiorides present as mercury chloride complexes and greatly reduce their ability to react wlth the nitrates when the phenoidlsuifonic acld reagent is added, thus preventing nitrate loss. The technique appears to be simple and effectlve up to 500 ppm chlorides and presumably more. Also an explanation for the various irregularltles reported by several workers on nitrate loss In presence of chloride ions Is incorporated.
In America, the phenoldisulfonic acid method is probably the most frequently used method for nitrate determination.
The popularity of this technique comes from its sensitivity and its wide range of applicabilityto various nitrate-containing samples (I). Other colorimetric reagents such as diphenylamine and brucine have been reported but have found less acceptance (2). Ion selective electrodes and UV absorption techniques have found use for screening purposes (3). Polarographic and several other reduction techniques are thoroughly discussed elsewhere (3, 4). Although the phenoldisulfonic acid method is the most sensitive of all under ideal conditions, its sensitivity suffers from several interferences. A detailed study of all interfering substances is best illustrated in a series of papers by Chamot et al. (5). The most commonly and seriously interfering substance, however, is the chloride ion. Chloride ion interferences have long been attributed to the interaction of hydrochloric and nitric acids when the phenoldisulfonic acid reagent is added (5, 6). This interaction results in the formation of nitrosyl chloride gas (among other ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
857
gases), causing an intolerable loss of nitrates. The loss is believed to be governed by the following reaction (7): 3C1- t NO; t 4 H + Z NOCl + CI, + 2H,O (1) It is seen from reaction 1that three chloride ions take up one nitrate ion to form one nitrosyl chloride gas molecule. In reality, however, the loss is not regular and of much less magnitude. For accurate determinations, the chlorides (and other halogens, if present) must be removed. Silver compounds and notably silver sulfate has long been employed for the removal of chlorides. An equivalent amount of silver sulfate is added to the sample precipitating the chlorides as silver chlorides, then the precipitate is filtered off (3). Although this method has been in general use, it requires a prior chloride analysis in order to add an equivalent amount of silver sulfate because of the undesirable presence of any excess of silver ions (4-6). In the presence of silver ions, potassium or sodium hydroxides tend to impart off-colors, thus inducing uncertainties is the absorbing color (6). In trying to avoid such extraneous colors, workers often use calcium or magnesium carbonates before evaporation and ammonium hydroxide for final color development (8). Data herein and elsewhere (2, 5) show considerable loss of nitrates when ammonium hydroxide is used as a base for final color development. Mechanical losses of nitrates are almost inevitable in the presence of carbonates (5). We find that the use of mercuric sulfate effectively eliminates chlorides interferences in the phenoldisulfonicacid method, and that it has fewer undesirable side effects than silver sulfate. Mercuric sulfate can also serve as a preservative to inhibit microbial activity which can affect the amount of nitrate present between time of collection and analysis.
EXPERIMENTAL Reagents and Apparatus. Phenoldisulfonic Acid. Dissolve 25 g of CP colorless phenol in 150 mL concentratedsulfuric acid. Add 75 mL fuming sulfuric acid. Heat for 2 h on a steam bath. Although this reagent is stable for several months, it should be kept in a glass stoppered bottle to prevent absorption of water from the atmosphere. Stock Nitrate Solution. Dry several grams of CP potassium nitrate in a weighing bottle at 100-110 "C. Weigh as accurately as possible a 1.0-1.5 g quantity into 1-L volumetric flask. Dissolve, add a drop or two of 12 N potassium hydroxide solution to make basic and fill to the mark with distilled water and mix thoroughly. Standard Nitrate Solution. Pipet 50.0 mL of the stock nitrate solution,transfer to an evaporating dish, and evaporate to dryness over a hot water or sand bath. Add 10 mL of the phenoldisulfonic acid reagent to the residue and mix well, making certain that the reagent comes in contact with all parts of the evaporating dish that the original water sample covered. Add 50-100 mL of distilled water from a wash bottle down the side of the dish, slowly to avoid spattering, and wash the solution quantitatively into a 500-mL volumetric flask. Add enough concentrated potassium hydroxide to develop maximum color (about 75 mL) and dilute the solution to the mark. The concentration of this master solution is calculated in parts per million of nitrate nitrogen and used to prepare more dilute standard solutions of various concentrations for a calibration curve. Potassium chloride and mercuric sulfate used were Baker Analyzed Reagents. Absorbance measurements were taken on a Spectronic 20 at a wavelength of 420 nm. Other spectrophotometers can be used. However, the Spectronic 20 was chosen to test the procedures as they would be performed in a typical, small analytical laboratory. Recommended Procedure. To 100 mL of the nitrate containing sample, add approximately 0.1 gm of CP mercuric Sulfate. Mix well by shaking for 10 min. Adjust sample pH by adding a drop or more of 12 N potassium hydroxide to produce a pH slightly above 7. Then filter, discarding the filtrate. Pipet a desired volume of the filtrate, transfer to a casserole,and evaporate to dryness over a hot water or sand bath. Add 2.0 mL of phenoldisulfonic acid in the same manner described above and wash 858
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
Table I. Effect of HgSO, on Recovery of Nitrate-Containing Samples Free of Chlorides Evaporating Amount dish of NO,-N Sample diam- %SO, present, volume, eter, added, cm P P ~ mL €! 0.50 50.0 12.5 0.00 0.50 50.0 12.5 0.01' 0.50 20.0 12.5 0.20 0.50 20.0 12.5 0.05 1.00 20.0 7.5 0.10 1.00 20.0 0.30 7.5 5.0 10.0 0.10 7.5 5.0 10.0 0.03 7.5 10.0 5.0 0.10 7.5 10.0 5.0 7.5 0.01
NO,-N founda PPm With With KOH ",OH 0.50 0.52 0.50 0.51 1.0 1.0 5.0 5.0 10.1 10.0
0.41 0.48 0.40 0.46 0.92 0.90 4.7 4.8 9.6 9,6
a Reported value was the mean of three determinations. Average standard deviation was i 3% except for the lower values.
the solution quantitatively into a 100-mL volumetric flask with distilled water. Add 6-8 mL of 12 N potassium hydroxide and dilute to the mark. If a precipitate develops (when working with a natural water sample),filter the sample,then take absorbance readings at the absorbance maximum (410-425 nm) on the clarified sample against a blank prepared from identical volumes of phenoldisulfonicacid reagent and potassium hydroxide as used in the determination. A number of variations in this procedure, including omission of the mercuric sulfate addition, were tested to determine their effect, but are not recommended for analytical practice. The nature of these variations is explained at the appropriate point in the text.
RESULTS AND DISCUSSION Upon addition of mercuric sulfate to the sample, it hydrolyzes instantly to give a yellow or brown precipitate of basic mercuric salb depending on the acidity of the sample. When the sample is made basic by addition of potassium hydroxide prior to evaporation to dryness, mercuric chlorides are substantially precipitated as oxychlorides such as HgC12.2Hg0. The sample is then filtered, removing most of the mercury, and processed as recommended. It was necessary for us to know if mercuric ions would upset the nitrate recovery when chlorides are not present. The data of Table I indicate that nitrates were not affected by the presence of excess mercuric ions. We would like to note that after the sample is evaporated down to dryness, the residue appeared to have a light yellowish color. We believe this is due to the presence of HgO, but this didn't have any adverse effects on the determination as indicated by the data. We also noticed that loss of nitrate was encountered when ammonium hydroxide was used to develop the absorbing color. This may be due either to the weak ionization constant of NH4OH in contrast to KOH or to a side reaction of NHB with the strongly acidic species initially present. Similar losses have been reported by other workers (5). The data of Table I1 show the magnitudes of nitrate losses in the presence of various concentrations of chloride ions before and after treatment with HgS04. Taras and Chamot et al. have reported higher and irregular losses of nitrates at similar chloride concentrations (5,6).,In our opinion this can be explained on the basis of reaction rate theories. The principal proposal in the literature for the mechanism of NOCl formation (9) shows NOz+as an intermediate. NO2+is known to be an intermediate in the nitration of aromatic molecules (IO),and in fact the rate is determined by the rate of NOz+ formation. In this case it is likely that NO2+is formed and
Table 11. Effect of HgSO, o n Nitrate Recovery in Presence of Chlorides After treatment with HgSO, %
NO,-N present, PPm 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95
Sample volume, mL
Dish diam., cm
20.0 20.0 20.0 20.0 20.0
12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5
20.0
20.0 20.0 20.0 20.0' 20.0 20.0
Chloride concn, PPm 0 10
25 25 50 50 100
100 150 150 200 200
NO3-N found, PPm 0.95 0.90 0.85 0.83 0.80 0.79 0.72 0.73 0.63 0.65 0.60 0.60
%
Loss of NO3-N
HgSO4
0 5
0.10 0.10
11
0.10 0.01 0.05
13 16 17 24 23 34 32 37 37
added, g
0.10
0.20 0.10
0.15 0.01 0.10 0.01
NO,-N found, PPm 0.95 0.96 0.95 0.95 0.95 0.94 0.93 0.93 0.94 0.97 0.93 0.93
Loss of NO,-N 0
-
0 0 0 1
2 2 1
-
2 2
Table 111. Effect of Sample and/or Evaporating Dish Volume on Nitrate Recovery in Presence of Chlorides before and after HgSO, Treatment NO,-N present, PPm
Sample volume, mL
0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95
50.0 50.0 50.0 25.0 25.0 25.0 15.0 15.0 15.0
Chloride concn, PPm
NO,-N found, PPm
%
HgSO4
diam., cm
Loss NO,-N
added, g
NO,-N found, PPm
7.5 12.5 22.5 7.5 12.5 22.5 7.5 12.5 22.5 7.5 12.5 22.5 7.5 12.5 22.5
500 500 500 500 500 500 500 500 500 500 500 500 500 500 500
0.12 0.34 0.59 0.30 0.43 0.70 0.42 0.51 0.79 0.51 0.60 0.80 0.63 0.71 0.81
87 64 38 62 55 26 56 46 17 46 37 16 37 25 15
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
0.86 0.91 0.96 0.90 0.93 0.95 0.92 0.95 0.94 0.93 0.98 0.94 0.92 0.94 0.97
Dish
10.0 10.0 10.0
5.0 5.0 5.0
then can react with either HCl or phenoldisulfonic acid. NO;'+ C,H,S,O, -+ H + + C,II,S,O,NO, NO;' + HCl- NO' + HOCl NO++ HCI -, H' + NOCl (fast) HOCl + HCl- H,O + C1, (fast) The amount of nitrate lost will then depend upon the precise local concentration of HC1 at the time the reactive intermediate NOz+is formed. This will certainly increase as the total amount of chloride present to dissolve in 2 mL of phenoldisulfonic acid is increased. Also there will be some tendency to lose HC1 by the reaction KCl
+ C,H,S,O,
+
K+C,H,S,O;
+
HCI
and the local concentration of HCl may very well depend on the rate at which the sample is dissolved in the phenoldisulfonic acid and hence on the size of the dish used for the evaporation and solution. We are aware of the irregularities possibly induced by the presence of mono and tri acids in the phenoldisulfonic acid reagent (5). In this work, the reagent used was prepared in a procedure (11) that ensured the absence of mono- or trisulfonic acids and equal amounts of the reagent were used for all determinations. The data of Table I11 show the effect of sample volume on nitrate loss. For example, when 50 mL of sample was evaporated in a 7.5-cm dish, 87% loss of nitrate was found, while only 37% loss was encountered when a 5-mL sample was evaporated in the same dish under the same conditions. This effect was also
%
Loss NO,-N 11
4
-
5 2 0
3 0 1 '
2
-
1 3 1
-
studied using identical sample volumes but different sizes of evaporating dishes. As is shown by Table 111, a decrease of nitrate loss from 87% to 38% was noted when two different-sized dishes were used to evaporate 50 mL. These losses, however, were not simply proportional to evaporating dish dimensions. It is also seen from data of Tables I1 and I11 that the determination is not affected by the amount of HgSO, added to the sample, indicating that addition of an excess of HgSO, is permissible but not necessary. The slight loss of nitrate found when a 50-mL sample was evaporated in a 7.5-cm dish (Table 111) is, to our belief, due to the slight dissociation of mercuric chloride. Percent loss was substantially reduced upon using a larger dish or evaporating a smaller volume. CONCLUSION The proposed technique appears to have several advantages, among which are: a. Addition of excess mercuric sulfate could be tolerated, so no chloride determination is necessary. b. Potassium hydroxide could be used throughout the determination, thus eliminating large losses of nitrates. Also contamination of analytical laboratories by ammonia fumes is avoided. c. Mercuric sulfate would serve as a sample preservative; hence, acidification or cooling of the sample is no longer necessary. As is shown by the data of Tables I1 and 111,the technique appears to be satisfactorily effective in correcting for chloride interferences. At high chloride concentrations,we recommend dilution of the sample (after it has been treated with HgSO,) a t a sample to water ratio of 1 : l O for highest accuracies. ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
859
LITERATURE CITED (1) F. H. Rainwater and L. L. Thatcher, Geol. Survey Wafer Supply Pap., 1454, 216-219 (1960). (2) F. B. Hora and P. J. Webber, Analyst (London), 85, 567-69 (1960). (3) “Standard Methods for the Examination of Water and Waste Water”. 14th ed.,American Public Health Association, American Water Association, and Water Pdlutkn Control Federation, New York, N.Y., 1975, pp 420-23. (4) “Standard Methods for the Examination of Water and Waste Water”, 12th ed.,American Public Health Association, American Water Association, and Water PollutionCOnW Federatbn, New Yolk, N.Y., 1965, pp 202-205. (5) E. M. Chamot, D. S. Pratt, and H. W. Redfield, J. Am. Chem. SOC.,33, 366-85 (1911). (6) M. J. Taras, Anal. Chem., 22, 1020-22 (1950). (7) A. Macejunas, J. Am. Water Works Assoc., 59, 1190-93 (1967).
.,
(8) C. A. Black, “Methods of Soil Analysis”, Part 2, American Society of Agronomy, Inc., publisher, Madison, Wis., 1965, pp 1217-19. (9) J. H. Wise and M. Volpe, cited In M. Volpe and H. S. Johnston, J. Am. Chem. Soc., 78, 3903-10 (1956). (IO) F. H. Westhelmer and M. S. Kharash, J. Am. Chem. Soc., 68, 1871 (1946). (1 1) F. Snell and C. Snell. “Colorimetric Methods of Analysis”, Vol. 11, 3rd ed., D. Van Nostrand Co., Inc., Prlnceton, N.J., 1949, pp 785-801.
RECEIVED for review October 29, 1976. Accepted February 22,1977. We wish to acknowledge support of the Environmental Protection Agency for one of the authors.
Separation of Organic Acids on Amberlite XAD Copolymers by Reversed Phase High Pressure Liquid Chromatography Donald J. Pietrzyk” and Chi-Hong Chu Chemistry Department, The Unlversity of I o w a , Iowa City, Iowa 52242
Eluting conditions for high pressure llquid chromatographic separation of organic acids on columns packed with 45- to 65-pm particles of rigidly porous Amberlite XAD-2, -4, and -7 copolymers, which act as reversed statlonary phases, are evaluated. XAD-2 and -4 are polystyrene divlnylbenzene copolymers and XAD-7 Is an acrylic ester copolymer. Adds studied Include alkyl and other substituted phenols, substituted benzolc acids, chlorinated phenoxyacetlc acids and their corresponding chlorinated phenols, benzene and naphthalene sulfonic acids, amino acids, dipeptides, and sulfas. The advantages of each type of XAD copolymer are established relative to the broad range of acids studied. Depending on the type of acid, optimum eluting conditlons are based on water+rganlc solvent ratio with or without pH control (the XAD copolymers are stable in acldlc and bask solution) and by the inclusion of electrolyte in the eluting mixture. Establishing the column behavior allows the prediction of optimum stripplng conditions for organic acids. I t is also possible to predict the retention volumes or level of retention from a mlnlmum set of data.
Using organic polymers as the stationary phase in high pressure liquid chromatography (HPLC) has been limited the major reasons being that they often lack the required physical strength, often suffer from slow mass transfer, readily swell and contract, or are not available in a uniform, microparticle size. A group of very rigid, macroporous copolymers, which do not suffer from all of these limitations, are stable to acid and base solutions, and are readily available, are the Amberlite XAD copolymers (1-3). The XAD copolymers should act as reversed stationary phases similar to nonpolar bonded phases, and are unlike silica and diatomaceous earths, which are polar porous stationary phases. Other polymers which have recently been shown to be useful in HPLC are polyvinylalcohol(4) and 2,6-diphenyl-p-phenylene oxide (Tenax GC) (5). XAD copolymers have been widely used in stripping applications for the removal of organics; this has been surveyed previously (6-8). Only a few separations on columns prepared from these copolymers using gravity flow (6) or HPLC (7)have been reported. 860
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
Experimental results are provided which illustrate the advantages and limitations of the XAD copolymers as reversed phase stationary supports. XAD-2 and -4,polystyrenedivinylbenzene copolymers, and XAD-7, an acrylic ester copolymer, were studied. Simultaneously, conditions for the analytical separation of organic acids such as phenols, carboxylic acids, sulfonic acids, amino acids, and sulfas by HPLC are described.
EXPERIMENTAL Reagents. All organic compounds were obtained commercially and used as received or distilled or recrystallized. Solvents and salta were analytical reagent grade and were used without further purification. Amberlite XAD copolymers were obtained from Rohm and Haas Chemical Company and Mallinckrodt Chemical Works as 20 to 60 mesh (500 Irm average) particles. These copolymers were cleaned by a Soxhlet extraction procedure described previously (8) and subsequently crushed in a blender (HzO-alcohol slurry) or in a ball mill. Column Preparation. A narrow, particle size range was obtained by dry sieving in U.S. Standard Screens or slurry sieving (HzO-alcohol) with a screen-filtering system from Cistron Corporation. The 45-65 Hm range was placed in methanol, stirred, and allowed to settle. Particles that did not settle after 15 min were discarded; this procedure was repeated severaltimes allowing the particles to air dry before addition of the MeOH. If the fines are not completely removed, an excessive pressure drop will occur in the column. After air drying, the columns were packed by a dry-packing technique. (Recently, slurry packing was shown to be feasible (9)J Since the XAD copolymers swell, particularly XAD-7 (8,9),the crushed particles should not be thoroughly dried or baked; activation is not necessary. Initial conditioning of the column is completed by passing alcohol through the column. Thereafter, different eluting mixtures can be used; however, the polarity change should be gradual and time should be allowed for the column to recondition according to the new eluting condition. For example,if a 10% EtOH-90% HzOeluting mixture was to be used, 10 to 20 mL of 80% EtOH would be passed through the column, followed by 60%, 40%, 20%, and finally 10% EtOH. A very slow gradient change could also be used. The pressure drop of the XAD-7 column was observed to be very dependent on polarity of the mobil phase, more so than for the XAD-2 or -4 column, and is probably due to the fact that XAD-7 swells more readily (8, 9). XAD-7 columns with low pressure drops were best prepared by dry packing with XAD-7
