disturb the indicator circuit and, furthermore, no proper mixing would be achieved. The measurements were evaluated on the basis of value Q determined from the recorded two symmetrical titration curves.
RESULTS AND DISCUSSION The programed coulometric titration technique worked out has been applied for the determination of chloride content of streaming water samples. Two curves recorded after a single shot reagent generation are shown in Figures 2 and 3. As expected each recording can be divided into two well defined parts; two potentiometric titration curves. The first one is a result of the chloride titration, while the second is that of the linearly decreasing silver ion generation rate. According to the theoretical considerations, the two parts should be the mirror picture of each other. The distortion is due to the tailing effect taking place in the tube. The titration curves were almost symmetrical to each other in the case of high chloride concentrations (C 1 10-*M) and small n values. However, the tailing seemed to be more noticeable a t low chloride concentration and a t higher n values. Furthermore, the distortion from symmetry appeared to be more significant in the lower range of sample flow rate. Owing to the huge change in the electrode potential recorded a t the inflection points, the time interval Q existing
between them can easily be determined. Accordingly, for analytical evaluation, calibration curves can be made by plotting the Q values vs. the actual chloride ion concentration of standards. As an example, Figure 4 shows a calibration curve which is almost linear and provides an excellent way for the accurate chloride determination in streaming sample solutions. I t must not be overlooked that Equation 9 is completely valid only under ideal conditions, i.e., if no tailing appeared in the flowing system. Additionally, it is worth mentioning that, in our laboratory, promising experiments are in progress to investigate further the properties and the possible applications of this programed coulometric titration technique carried out in streaming solutions. Amperometric, biamperometric, and different potentiometric detector cells have already been constructed and applied for this work. LITERATURE CITED (1) T. S. Light, "Ion Selective Electrodes Industrial Application", Industrial Water Engineering, 1969, 5 pp. (2) L. A. Eifers and C. E. Decker, Anal. Chem., 40, 1658 (1968). (3) M. S. Frant and J. W. Ross, Jr., Anal. Chem., 40, 1169 (1968). (4) R. Di Martini, Anal. Chem., 42, 1102 (1970). (5) W. J. Blaedel and R. H. Laessig, Adv. Anal. Chem. Instrum., 5 , 69 (1966). (6) D. L. Eichler, "Technicon Symposium 1969", Vol. I. Mediad Inc., New York. NY, 1970, p 51 (7) B. Fleet and A. Y. W. Ho, Anal. Chem., 46, 9 (1974).
RECEIVEDfor review October 1, 1974. Accepted March 24, 1975.
Determination of Total Sulfur and Total Phosphorus in Soils Using Fusion with Alkali Metal Nitrates Neil R. McQuaker and Tony Fung Chemistry Laboratory, Wafer Resources Service, 3650 Wesbrook Crescent, Vancouver, B.C. V6S 212, Canada
Methods used to bring total phosphorus in a soil into solution have included both digestion with strong acids such as HC104 and fusion with alkalis such as Na2C03 ( I ) . Corresponding procedures, however, have been found unsatisfactory for the determination of total sulfur in soils. Hesse indicates that volatile sulfur compounds are lost during acid digestion procedures (2). Volatile sulfur compounds may also be lost during alkali fusion methods since melt conditions are obtained only a t relatively high temperatures (e.g., 895 "C in the case of NaZC03). This compares with fusion using alkali metal nitrate salts where melt conditions are reached a t relatively low temperatures (3). In addition, these salts form powerful oxidizing systems and, indeed, Bowen has proposed using a NaN03/KN03 (50/50 mol %) melt for the destruction of organic matter in biological materials ( 4 ) .Bowen further suggests that the NaN03/ K N 0 3 fusion process should be considered for determining certain volatile elements such as sulfur. Since an equimolar solid solution of NaN03/KN03 achieves melt conditions a t 218 "C sulfur compounds which are volatile above this temperature are not expected to be lost. Reference to the literature indicates t h a t both Mg(N03)2 and K N 0 3 fusion procedures have been previously used for the determination of total sulfur in both soils and vegetation (5, 6). However, crystalline Mg(N03)2 decomposes a t 380-450 "C ( 7 ) and so cannot provide an effective oxidizing 1462
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8 , JULY 1975
system. This fact has evidently been overlooked by previous workers (5, 6). Crystalline K N 0 3 has a melting point of 334.5 "C (3) and, above this temperature, will provide an efficient oxidizing system if the nitrate salthample ratio is sufficiently high. However, work done in this laboratory has shown that the KNOahample ratio used by Chaudry and Cornfield (6) is too low to provide melt conditions and thus an efficient oxidizing system (8).In addition, the nitric acid pre-digestion step in Chaudry and Cornfield's procedure is suspect, since it too may result in the loss of volatile sulfur compounds. Evidently, a suitable fusion procedure for total sulfur in soils has not been reported and, in the present work, we have used a variation of Bowen's procedure ( 4 ) to bring both the total phosphorus and total sulfur of soil samples into solution. Once a suitable extract was obtained, the orthophosphate was determined using the Murphy-Riley colorimetric method (9) and the sulfate was determined using a variation of the turbidimetric procedure described by Massoumi and Cornfield ( 1 0 ) . EXPERIMENTAL Apparatus. Absorbance measurements for the total phosphorus test were made a t 885 nm using a Beckman Acta I11 Spectrophotometer and 5.0-cm cells. Calibration was achieved by using KHzP04 standards (see below) over the range 0-0.5 ppm phosphorus.
In the case of the total sulfur test, the absorbance measurements were made at 490 nm using a Beckman Acta I11 Spectrophotometer and 5.0-cm cells. Calibration was achieved using K2S04 standards (see below) over the range 0-12.0 ppm sulfur. Reagents. The following reagents and solutions were prepared according to the following instructions. NaN03IKN03 (50/50 mol %) Solid Solution. Add 85.0 g reagent grade NaN03 and 101.1 g reagent grade KN03 to a Pyrex beaker. Heat to 350-400 "C and then allow the resulting melt to cool and solidify. Crush to a powder. Stock Phosphorus Solution (10 ppm P). Dissolve 0.4393 g predried reagent grade KHzP04 in deionized water; dilute to one liter and then dilute 1:lO. Stock Sulfur Solution (100 ppm S). Dissolve 0.5434 g pre-dried reagent grade K2S04in deionized water and dilute to 1liter. Nitric Acid Reagent. Add 250 ml concd nitric acid t o 100.0 ml sulfur stock solution and dilute to 1 liter. Nitric AcidlNitrate Salt Reagent. Dissolve 150 g powdered NaN03/KN03 in 600 ml deionized water. Add 50.0 ml sulfur stock solution together with 125 ml concd nitric acid and dilute t o 1 liter. Phosphorus Standard Solution. Add suitable aliquots of the 10 ppm P solution to 100-ml volumetric flasks and then dilute to volume. Sulfur Standard Solution. Add suitable aliquots of the 100 ppm S solution together with 10.0 ml nitric acidhitrate salt reagent to 50 ml volumetric flasks and then dilute to volume. Procedure. The samples as received by the laboratory were dried a t 105 "C and then ground so as to pass a 0.15-mm sieve. Approximately 0.5 g of the prepared sample was then weighed accurately and transferred to a 50-ml Pyrex beaker. Next, 1.50 g NaN03/KN03 powder was added-about 1.0 g was mixed intimately with the sample by gently swirling the beaker, and the remaining 0.5 g was added evenly to the surface of the sample. The sample was then transferred to a muffle furnace set to 200 "C and the temperature was subsequently raised to 450 "C. The sample was then allowed to digest at 450 "C for 1 hour. After the sample has been removed from the furnace and allowed to cool, 5.0 ml of 0.5N HC1 was added to the sample. The sample was then brought to a boil. (This step destroys any native carbonates and/or carbonates resulting from the oxidation of organic carbon.) Next 5.0 ml of nitric acid reagent was added (this will result in the addition of 1 ppm sulfur to the final extract to enhance detection limit), and the sample was allowed to boil gently at a volume of approximately 10 ml (deionized water was added as required) for 15 min. Once the sample had cooled, it was transferred quantitatively to a 50-ml volumetric flask, diluted to volume, and filtered through dry Whatman No. 42 filter paper. In the case of the total phosphorus test, an aliquot of the filtrate, which had been diluted 1:100, was used for the colorimetric determination of orthophosphate. For the total sulfur test, a 20.0-ml aliquot of the filtrate was used for the turbidimetric determination of sulfate. The procedures followed for both these tests appear elsewhere (11).
RESULTS AND DISCUSSION T h e three parameters influencing the effectiveness of the fusion procedure are fusion temperature, fusion time, and nitrate salt/sample ratio. Bowen has shown that fusion parameters of 410 "C and approximately 0.25 hr were sufficient t o completely destroy the organic material in powdered kale; a nitrate salt/sample ratio of 20/1 by weight is suggested ( 4 ) . Since t h e organic carbon content of vegetation samples is typically an order of magnitude greater t h a n that found in most soils (12),a correspondingly smaller nitrate salthample ratio should be sufficient t o allow for the destruction of the organic matter in soils. Accordingly, we have used a ratio of 3/1. This compares with a ratio of 1/1 used by Chaudry and Cornfield (6). Bowen found that, after the destruction of the organic material in the kale samples, a clear melt was obtained. This contrasts with soil samples where a considerably greater residue of inorganic material will remain in the melt. In the present work, when the samples were first removed from t h e furnace, a dark colored inorganic residue could be seen dispersed in the molten nitrate salt. The presence of t h e molten nitrate salt indicates that melt conditions existed during the fusion
"-1
I
I
I
Figure 1. Absorbance curves for total sulfur (as sulfate) in the presence of 3.0% NaNOSlKN03 (50/50 mol % ) and in the absence of the N a N 0 3 / K N 0 3
process. It is t o be noted that if the nitrate salt/soil ratio is too low, melt conditions cannot exist and the sample will appear as a dry porous cake when removed from the furnace. During t h e course of this work, it was found t h a t if the organic carbon content of the sample exceeded a p proximately 5-6% the situation just noted was likely t o occur. For such samples it is necessary to increase the nitrate salt/soil ratio so that the increased depletion of the nitrate ion, due t o the oxidation of organic carbon to C032and CO2 together with other accompanying oxidation processes, will be provided for. T h e oxidation processes may result in the liberation of NOz-, N2, 0 2 , and oxides of nitrogen (3). For t h e purposes of this work, the remaining two fusion parameters were set at 450 "C and 1 hour. These parameters allow for somewhat more drastic digestion conditions than those suggested by Bowen for vegetation samples. However, Bowen considered only the complete oxidation of the organic material, and there is evidence that the oxidation of some inorganic compounds may require more drastic digestion conditions than those required for organic compounds ( 3 ) . T h e extraction of the sample with acid after the fusion process brings the sulfate into solution and hydrolyzes the phosphates to orthophosphate. As has already been indicated the orthophosphate in the extract is determined colorimetrically. T h e colorimetric procedure used in this work (11) has a detection limit of 0.003 pprn P. Thus, if the 50 ml extract is diluted 1/100 and if a 0.5-g sample is used, the minimum detectable concentration (MDC) for the total phosphorus test is 0.03 mg/g P. I t is t o be noted that, at the concentrations present in the diluted extract, the nitrate salts did not interfere. T h e sulfate in the extract is determined turbidimetrically as a B a s 0 4 suspension. Reference to Figure 1 shows the absorbance curves obtained using standards containing 3.0% NaN03/KN03 and standards from which the NaN03/ K N 0 3 has been withheld. T h e discrepancy between t h e two curves is a reflection of the interferences due t o both the NaN03/KN03 and sulfate impurity in the nitrate salts. Thus, it is imperative t h a t the standards be prepared with t h e appropriate nitrate salt background. It is t o be noted t h a t the curves are nonlinear at low (sulfur) concentrations-this is not unusual in the case of turbidimetric measurements (13). T h e turbidimetric procedure used in this work (11) has a detection limit of 0.3 p p m S. Thus, if t h e 50-ml extract is not diluted and if a 0.5-g sample is used, ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1463
Table I. Comparison of Methods for the Determination of Both Total Sulfur a n d Total Phosphorus in Soils Total sulfur, mg/ga Sample No.
NaN03/KN03 method
LECO method
1 2 3 4 5 6 7 8 9 10 11 12
0.18 0.19 0.84 1.70 0.11 2.27 0.08 0.35 0.06 0.46 0.82 0.27
0.18 0.18 o .a7 1.95 0.11 2.20 0.09 0.37 0.05 0.50 0.81 0.27
Total sulfur, mglg'
Total phosphorus, mglgb NaNO3/KN03~O3/HC10~ method method
0.53 0.52 1.94 4.17 0.93 4.81 0.29 0 -54 0.28 1.78 1.67 0.55
0.50 0.50 1.91 4.25 0.92 4.90 0.29 0.53 0.28 1.77 1.74 0.55
M e a n of three results. M e a n of three results-the orthophosphate in the extracts was determined colorimetrically (If).
the MDC for the total sulfur test is 0.03 mg/g S. In this work, the equivalent of 1 ppm sulfur was added to samples and standards in order to enhance the MDC. When referenced against deionized water, the reagent blank (containing the 1 ppm sulfur) had an absorbance of about 0.030. Table I presents total phosphorus and total sulfur results for 12 soil samples which came from diverse sources. The total phosphorus results as obtained using the NaN03/ K N 0 3 fusion procedure may be compared with the results obteined using an HN03/HC104 digestion (14). When referenced against the HN03/HC104 method, the NaN03/ K N 0 3 results, in the range 0.1-5.0 mg/g P, can be shown to yield an average percent recovery of 100.7% (and a standard deviation of 2.7%). The total sulfur results as obtained using the NaN03/KN03 fusion procedure may be compared with results obtained using a LECO (Laboratory Equipment Corporation) induction furnace (Model 523300) coupled with a titrimetric analyzer (Model 518-000). When referenced against the LECO method (15, 1 6 ) , the l\!aNOS/KN03 results, in the range 0.1-2.5 mg/g S,can be shown to yield an average percent recovery of 98.1% (and a standard deviation of 5.5%). The above recoveries of 100.7 and 98.1% are excellent and compare with recoveries of 99.8 and 99.0%, which were obtained when 0.100 mg/g spikes of P-Po43- and s-So4'(added to sample No. 12) were carried through the fusion process. Replicate analyses of sample No. 12 yielded relative standard deviations (95% confidence interval) of 1.8% a t a level of 0.554 mg/g P and 3.0% a t a level of 0.267 mg/g
S. Finally, Table I1 compares the results for total sulfur obtained using three digestion and/or fusion procedures. These are NaN03/KN03 fusion, HN03/HC104 digestion ( 1 4 ) ,and HN03/KN03 digestion plus fusion (6). When referenced against the NaN03/KN03 method, the results from the latter two methods, in the range 0.1-2.5 mg/g S, can be shown to yield average percent recoveries of 85.6 and 82.096, respectively, These low recoveries are indicative of the expected losses of volatile sulfur compounds. I t is to be noted that in the case of the HN03/KN03 digestion plus fusion procedure, the losses may be due to the acid pre-digestion and/or a KNO3/soil ratio which is too low to provide melt conditions (8). The standard deviations associated with the recoveries are 8.0 and 11.1%and indicate sulfur losses of approximately 15 f 10%. Thus, of the three methods using digestion and/or fusion procedures, the 1464
Table 11. Recoveries of Total Sulfur by Various Methods
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Sample KO,
1 2 3 4 5 6 7 8 9 10 11
IiaNO3/~03 method
0.18 0.19 0.84 1.70 0.11 2.27 0.08 0.35 0.06 0.46 0 .a2
M e a n of three results-the m i n e d t u r b i d i m e t r i c a l l y (If).
HN03/HC104 method
0.17 0.18 0.70 1.40 0.10 1.79 0.09 0.31 0.05 0.40 0.57
"03/m03 method
0.17 0.17 0.63 1.17 0.10 1.63
o .oa 0.30 0.06 0.31 0.77
sulfate in the extracts was deter-
NaN03/KN03 method gives the best recoveries and is to be preferred. In addition, the NaN03/KN03 method, when compared with the HN03/KN03 method, has the added advantage that it is much quicker. About 4-5 hr are required to bring the total sulfur into solution using the HN03/KN03 procedure (6). This compares with less than 2 hr for the NaN03/KN03 procedure. Since an equivalent time is required for an HN03/HC104 digestion, it is recommended that, in those cases (14, 1 7 ) where an acid digestion has been used in the determination of total phosphorus and/or total sulfur, it be replaced by the procedure of this work. This will allow for equivalent recoveries in the case of total phosphorus and improved recoveries in the case of total sulfur. In addition, the hazards associated with the use of perchloric acid are avoided. Addendum. The hazards of explosions when fusing soils of high organic content with various oxidants have been reported in the literature (18). These hazards are increased when fusions are carried out in enclosed furnaces as compared to open beakers. The use of a NaN03/KN03 (50/50 mol %) melt as the oxidant, which requires a lower fusion temperature, probably reduces such hazards. LITERATURE CITED (1)P. R. Hesse, "A Textbook of Soil Chemical Analysis", John Murrary (Publishers) Ltd., London, 1971,p 255. (2)kid, p 305. (3)D. H. Kerridge. "The Chemistry of Molten Nitrates and Nitrites", in "lnorganic Chemistry, series one", H. J. Emeieus, Ed., MTP Int. Rev. Sci., Voi. 2, 1972,p 29. (4)H. J. M. Bowen, Anal. Chern., 40, 969 (1968). (5) B. Butters and E. M. Chenery. Analyst(London), 84, 239 (1959). (6) I. A. Chaudry and A. H. Cornfield, Ana/yst(London), 91, 528 (1965). (7)J. Heubei, Bull, SOC.Chirn. Fr., 19, 162 (1952). (8)N. R. McQuaker, Report No. 7403, The Chemistry Laboratory, Water RESOURCES Service, Province of British Columbia, 1974. (9)J. Murphy and J. P. Riley, Anal. Chim. Acta, 27,31 (1962). (10)A. Massoumi and A. H. Cornfield, Analyst(London), 88, 321 (1963). (11)N. R. McQuaker, "The Chemical Analysis of Waters, Wastewaters, Sediments and Biological Materials", Chemistry Laboratory, Water Resources Service, Province of British Columbia. 1974,pp 316 and 376. (12)J. M. Bremner and D. S. Jenkinson, J. SoilSci., 11, 394 (1960). (13)G. W. Ewing, "Instrumental Methods of Chemical Analysis", McGrawHili, inc., New York, 1960,p 60. (14)P. R. Hesse, Reference (l),p 377. (15)A. R. Tiedemann and T. D. Anderson, Plant Soil, 35, 197 (1971). (16)L. M. Lavkulich. "Methods of Soil Analysis", Pedology Laboratory, Department of Soil Science, University of British Columbia, 1974,p 47. (17)H. D. Chapman and P. F. Pratt, "Methods of Analysis for Soil, Plants and Waters", Division of Agricultural Sciences, University of California, 1961,pp 161 and 184. (18)T. Greweling, Anal. Chern., 41, 540 (1969).
RECEIVEDfor review December 17, 1974. Accepted February 19,1975.
