should always be kept constant. By using this special standard curve, Equations 1, 3, and 5 will be changed as follows:
+ L3)
Equation 1becomes (Lac (Lz Equation 3 becomes (Lac Equation 5 becomes (.Lac
=
+ L,) - LI + L 3 ) = Lt
(6) (7)
+ L3) = LS - LI
(8)
+
where (Lea L3) and its corresponding urea concentration are directly shown on the special standard curve. Interference. Since this method is based upon the additivity of the conductivities of the components of a solution, it is not applicable when the conductivities vary irregularly, because of the influence of interfering substances. The activity of urease may become depressed and its conductivity irregular when heavy metal ions, such as Ag, Hg, etc., and protein-destroying substances are present. Electrolytes such as NaC1, KC1, etc., have no significant influence on urease activity; however, high concentration of these Flectrolytes may introduce a larger error, especially in a dilute urea solution (Table 11). To keep the electrolytes in the urease stock solution a t low concentration, no buffer solutions were used; this results in a constant but moderate increase in pH with the hydrolysis of urea in the solution. The p H changes from 7 to 9 when the urea concentration is increased from 1 to 2000 p.p.m. The increase in p H from 7 to 9 cuuscs a slight decrease in the
conductivity of urease, La. For this reason, the value of Lac is not exactly equal to the conductivity of a pure (NH&C08 solution of the same concentration. However, this disparity is theoretically eliminated because L,,, but not the conductivity of pure (NH4)p CO,, is used for constructing the standard curve. The p H value of the urease stock solution is about 7, which is the optimum p H for urease activity. If the original p H of the sample solution is less than 6 or higher than 8, it may be adjusted to neutral by the addition of NaOH or HC1 solution.
tivity is influenced by substance(s) in the solution, may be tested easily by examining the recovery of a known amount of urea added to the sample. If recovery is not satisfactory, the procedure may not be applicable. ACKNOWLEDGMENT
The authors thank K. W. King, L. K. Brice, and G. W. Thomas for their review and for valuable suggestions. LITERATURE CITED
(1) Brown, H. H., ANAL.CHEM.31, 1844 (1959). - - ,.
CONCLUSION
Data presented indicate high recovery and precision of this new method. Because of its simplicity and rapidity (100 urea determinations may be made within 4 to 5 hours) and its wide testing range of urea concentrations (0.1 to 2000 p.p.m.), it is well suited for routine analysis, such as determination of urea in blood, serum, and urine (3) and in fertilizers ( 2 ) . Two other advantages are: The values of the standard curve a t a specified temperature are constant, and interferences due to the presence of colored impurities are eliminated. This method, however, is not applicable to samples containing substances that interfere with urease activity, such as Ag and Hg ions and protein-destroying substances. Its adaptability to a specified sample solution, to determine whether urease ac-
\--
(2) Chin, W. T., Kroontje, Wybe, unpublished data. (3) Chin, W. T., Kroontje, Wgbe, King, K. W., unpublished data. (4) Daniels, F., Alberty, R.A., “Physical Chemistry,” 6th ed., pp. 383,400, Viiiley, New York, 1959. (5) Frederick, C. H., Martin, E. H., “Practical Methods of Biochemistry,” p. 314, Williams B Wilkins Co., Baltimore, Md., 1953. (6) Hubener, H. J., Bode, F., Mollat, H. J., Hoppe-Seyler’s Z. Physiol. Chem. 290,136 (1952). ( 7 ) “International Critical Tables,” p. 230, Vol. VI, McGraw-Hill, New York, 1929. (8) Natelson, S., “Microtechniques of Clinical Chemistry for the Routine Lahoratorv,” D. 386. Charles C Thomas, Srrinefield.’Ill.. 1957. (9)’Van Slyke, b. D., Cullen, G. E., J . Biol. Chem. 24, 117.(1916). (10) Watt, G. IF‘., Chrisp, J. D., ANAL. CHEM.26,452 (1954).
RECEIVED for review February 10, 1961. -4ccepted August 14, 1961.
Determination of Traces of Sulfur, Fluorine, and Boron in Organic Materials by Oxygen Bomb Combustion J. J. BAILEY’ and D. G. GEHRING Repauno Developmenf laboratory,
E. 1.
du Ponf de Nemours &
b A platinum-lined Parr oxygen calorimeter ,bomb has been employed successfully for quantitative decomposition of organic samples. Water is introduced into the bomb prior to combustion and the resulting combustion products are absorbed in the water. This aqueous liquid may then b e analyzed for trace quantities of the desired cation or anion by standard microanalyiical methods. A comparatively large (1 gram) sample may b e completely decomposed in the oxygen combustion bomb. Trace quantities of fluorine, boron, and sulfur in organic compounds were determined successfully. 1760
ANALYTICAL CHEMISTRY
M
Co., Inc., Gibbsfown, N. J.
of sample decomposition have been proposed and accepted, among the most common perhaps are the oxygen flask micro methods of Schoniger (IO,11), the procedures of Agazzi, Fredericks, and Brooks ( I ) , Arthur, Annino, and Donahoo ( 2 ) , Granatelli (5),Hinsvark and O’Hara (6), Hudy and Mair ( 7 ) ,and the well known Parr peroxide bomb techniques (9). Although these and other combustion procedures are satisfactory for a multitude of analytical problems, they usually have limitations for trace ion analysis. These include complex equipment (scrubber solutions etc.), extensive neutralization and evaporation steps, ANY MEANS
high salt concentrations, impracticality with solid samples, and milligram-sized samples. A technique was desired that would retain the simplicity, cleanliness, and rapidity of the oxygen flask method, but, in addition, would permit the combustion of a much larger sample. Combustion in a platinum-lined Parr oxygen calorimeter bomb fulfilled these requirements, The bomb is platinum lined to eliminate corrosion of the bomb interior and possible loss of desired ions 1 Present address, E. I. du Pont de Nemours & Co., Inc., Beaumont, Tex.
the 125-ml. flask allowing the cell to drain completely. Do not rinse out the cell. Pipet 2 ml. of barium chloride solution into the Erlenmeyer flask and shake the solution vigorously for 30 seconds. Wait 5 minutes, and again shake the solution for about 10 seconds. Immediately transfer the solution into the 10-cm. cell and measure the absorbance as rapidly as possible. Correct the net absorbance (before and after barium chloride addition) for the reagent blank and determine the sulfur concentration from a calibration curve.
from the effects of surface corrosion. In fact, the platinum-lined bomb (together with platinum ignition wire) may be employed for determination of various metal ions. The techniques of sample preparation and bomb combustion are relatively simple and the procedure may be utilized easily for control analysis. A 1-gram sample may be burned and prepared for analyses within 10 to 15 minutes. Methods for the trace determination of fluorine, boron, and sulfur are presented. pq. B o r o n
EXPERIMENTAL
Apparatus. Oxygen combustion bomb, platinum lined, No. 1106 C including all accessories (Parr Instrument Co., Moline, Ill.). Reagents. Fluorine Determination (3, 8 ) . Alizarin complexone (Hopkins & Williams, Ltd., Chadwell Heath, Essex, England) ; fluoride solution, 2 pg. per ml. Boron Determination (4). Standard boron solution prepared from reagent grade boric acid; 1 pg. per ml. Sulfur Determination (12). Standard solution prepared by dissolving sodium sulfate to contain 2 pg. of sulfur per ml. Salt-acid solution (No. 8043, Hellige Go.), ethyl alcohol-glycerol solution, 55 to 45. General Combustion Procedure. Press a maximum 1-gram sample into a pellet, orl if liquid, pipet directly into the sample holder. If the liquid is volatile, use a gelatin capsule. Pipet 10 ml. of water into the bomb. The water may contain 1 meq. of base if desired t o neutralize the nitric acid formed during combustion. Assemble the bomb and fire in the inverted (inlet and exit valves up) bomb position. After firing, thoroughly shake the bomb for 1 minute to absorb all of the combustion products in the water. Slowly and carefully release the bomb pressure and transfer the aqueous liquid quantitatively into a beaker or flask for chemical analysis. FLUORINE DETERMINATIOX. Add 10 ml. of water containing 1 meq. of sodium hydroxide t o the bomb. After firing, transfer the liquid to a small beaker using a minimum amount (10 ml.) of water for transfer. Quantitatively transfer the liquid to a 50ml. volumetric flask, add the reagents and buffer solution as described by Belcher, Leonard, and West (5, 8))and carry out measurements on a Beckman DU spectrophotometer using either a 5- or 10-cm. path. Read micrograms or parts-per-million of fluorine directly from a previously prepared calibration curve. BOROX DETERMINATIOX. The method of Burkhalter and Peacock (4) was adapted to quantitative measurement of trace amounts of boron. After combustion, quantitatively transfer the bomb solution into a beaker. Add a drop of phenolphthalein indicator and make basic with 0.3N sodium hydroxide. Evaporate the sample solution and known standards until about 5 ml. remain. Then pipet exactly 2 drops of sodium chloride solution into
Figure 1.
Boron calibration curve
each beaker and evaporate all to complete dryness. Pipet exactly 1 ml. of water and exactly 4 drops of bromothymol blue indicator into each beaker. Add dropwise 0.01N sodium hydroxide or 0.01N hydrochloric acid until the color visually approximates that of the prepared blank (refer to blank preparation below). Quantitatively transfer the beaker contents to a 5-ml. volumetric flask, dilute to the mark, and mix well. The blank solution must always be a slightly deeper blue than the sample. Withdraw and discard exactly 1 ml. of solution from each of the volumetric flasks. Transfer the entire remaining solution into the 1cm. sample cell. Place the sample cell in the reference path and place the blank solution in the sample path of the spectrophotometer. Measure the absorbance a t 615 mp. Carefully remove the sample cell and add approximately 0.2 gram of mannitol directly into the cell. Replace the glass cell cover, dissolve the mannitol by inverting the cell, and again measure as before a t 615 mp. Plot the concentration against the net absorbance, i.e., the difference in the absorbances measured before and after the addition of mannitol (Figure 1). Prepare the blank solution by addition of the same ratio of reagents used in the analysis to water. Adjust the solution t o a deep blue color with base and mix thoroughly. The blank solution is stable for about 2 days. SULFUR DETERMINATION. The method of Toennies and Bakay (12) was adapted to the trace detection of sulfur as the sulfate ion. For this determination, add 5 ml. of 3% hydrogen peroxide and 5 ml. of water to the bomb prior to combustion. After combustion, quantitatively transfer the aqueous portion into a 25-ml. flask containing 5 ml. of salt-acid solution and adjust to volume. Transfer the solution into a 125-ml. glass-stoppered Erlenmeyer flask. Do not wash or rinse the 25-ml. volumetric flask, but allow the sample solution to completely drain into the Erlenmeyer flask. Add 20 ml. of glycerol-ethyl alcohol reagent and thoroughly mix the solution. Carefully fill a 10-cm. cell with a portion of the sample solution. Do not spill any of the solution. Measure the absorbance a t 400 mp against a distilled water blank. Transfer the sample solution back into
RESULTS AND DISCUSSION
Combustion Procedure. A large metal shield was erected as a safety precaution. The 1-gram sample size and a pressure of 25 atmospheres of oxygen were never exceeded. During the actual firing, the bomb was immersed in a cold water bath. Disadvantages encountered were relatively few. Bomb maintenance and the replacement of parts (usually gaskets) are periodically necessary. The inlet and exit Kel-F valve seats frequently become stopped up because of too much wrench pressure exerted by the analyst in closing the valves. It is a simple matter to free the valve apertures by use of a small drill mounted on a screw drivertype handle. Chemical disadvantages are twofold. A slight discoloration of the liquid inside the bomb takes place during combustion because of oxide formation from the decomposed Nichrome ignition wire. This slight discoloration may interfere with very sensitive spectrophotometric methods where slight color differences are to be measured. The fluorine and boron methods presented are unaffected by this oxide formation; however, a measurement must be made in the sulfur procedure to correct for the discoloration due to both the Nichrome wire and peroxide. The use of platinum ignition wire will virtually eliminate these effects of oxide formation. Secondly, the nitrogen from the air and the oxygen combine during combustion to form about 1 meq. of nitric acid. For this reason, the bomb cannot be used for nitrate ion microanalysis. Nitrate ion formation may be greatly reduced by initially purging the bomb with oxygen. However, it was virtually impossible to remove all traces of air, and some nitrate ion was always formed. Bomb misfiring was usually due to the ignition wire not being in direct contact with the sample. Some solid pellet samples did not burn properly when the pellet had been compressed too tightly. This problem was eliminated completely by wetting each pellet with pure methanol before ignition. Fluorine Determination. This determination and reaction were dis\IOL. 33, NO. 12, NOVEMBER 1961
1761
Table 1.
Fluorine Synthetic Sample Results
(1-gram sample) Fluorine, P.P M. Calcd. FOUL o-Fluorotoluene 63 65 33 30 15 13 5 6.5 Trifluoroacetic acid 25 22 10 8 Sodium fluoride 5 4 2 4
‘
Table II.
Boron Synthetic Sample Results
(1-gram sample) P.P.M. Boron Added (as boron triacetate) 0.50 0.50 0.50 1.00 1 .oo 2.0 2.0
Table 111.
P.P.M. Boron Found 0.35 0.40 0.60 0.85 0.90 1.7 1.9
Sulfur Synthetic Sample Results
(1-gram sample)
P.P.M. Sulfur Added (aa sulfanilamide) 25.0 25.0 25.0 12.5 12.5 12.5
P.P.M. Sulfur Found 23.0
2i.5
24.0 10.0 10.0 11 .o
cussed previously (9, 8). Table I illustrates results obtained by the analysis of samples of known fluorine concentration. The known samples were prepared by combustion of the required quantity of compound in p-xylene. Boron Determination. Burkhalter and Peacock (4) have presented a detailed discussion of this boron-polyhydric alcohol cornplev formation. However, this reaction can be made quantitative by the procedure indicated here. The calibration curve (Figure 1) is not linear and must be checked with each series of samples to be analyzed. The known calibration samples do not have to be burned or prepared in the bomb. Table I1 indicates results obtained from the complete analysis of samples of known boron concentration, prepared by adding boron triacetate to acetic acid. The boron procedure outlined here is designed for the 0.2- to 5-p.p.m. concentration range. The scope of the method may be increased by adjusting the blank and sample solutions to a deeper blue color prior to addition of the mannitol or by using large volumes. The color change is nearly instantaneous and the resulting yellow solution is stable for a t least 30 minutes. To ensure a positive absorbance reading, the color of the blank solution must be as intense as, or deeper than, the color of the sample. Sulfur Determination. Turbidimetric measurement are generally conceded to be lacking in reproducibility for the reasons cited by Toennies and Bakay (12). However, in trace ion analysis, a certain degree of inaccuracy is generally tolerable. As illustrated in Table 111, the results ob-
tained were felt to be exceptionally good. Synthetic samples were prepared by addition of sulfanilamide to cyclohexane and completely analyzed. The calibration curve obtained was essentially a straight line. I t was necessary to measure the sample absorbances before and after addition of barium chloride as discoloration of the combustion absorbant (water) was not constant and contributed slightly to the absorbance a t 400 mp. The success of this method largely depends on the care and technique of the analyst. The analyst must be thoroughly trained in the techniques involved in this method and suitable practice must be obtained by analysis of known samples. LITERATURE CITED
(1) Agazai, E. J., Fredericks, E. M., Brooks, F. R., ANAL. CHEM. 30, 1566 (1958). (2) Arthur, P., Annino, R., Donahoo, W. P., Ibid., 20,1852(1957). (3) Belcher, R.,Leonard, M. A., West, T. S., J. Chem. SOC.1959, 3577. (4) Burkhalter, T. S., Peacock, D. W., ANAL.CHEM.28,1186 (1956). (5) Granatelli, L., Ibid., 27, 266 (1955). (6) Hinsvark, D. N.,O’Hara, F. J., Ibid., 29, 1318 (1957). (7)Hudy, J. A., Mair, R. D., Ibid., 27, 802 (1955). (8)Leonard, N. A,, West, T. S., J. Chem. Soc. 1960,4477. (9) Parr Instrument Co., Moline, I11 Parr Peroxide Bomb Apparatus ana Methods No. 121,1950. (10) Schoniger, W., Mikrochim. Acta Heft 1,123 (1955). (11) Schoniger, W., Ibid., Heft 1-6, 869 (1956). (12) Toennies, G., Bakay, B., ANAL. CHEM.25, 160 (1953). RECEIVEDfor review April 24, 1961. Accepted August 28, 1961. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1961.
Determination of Oxygen in Organometallic and Inorganic Compounds by a Modified Unterzaucher Method MITCHELL KAPRON and MANUEL BRANDT Research laborafories, Ethyl Corp., Detroit, Mich.
b Direct determination of oxygen in organometallic and inorganic compounds may be accomplished by a modified Unterzaucher method in which the sample is reacted with cuprous chloride in the presence of excess carbon. The method gives quantitative recovery of oxygen in cases where the conventional Unterzaucher method fails. It is applicable to in1762
ANALYTICAL CHEMISTRY
organic oxygenated compounds and organometallic compounds in which the metal is bonded to oxygen.
0
can be determined directly by the Unterzaucher method (6) in oxygenated organic compounds and in organometallic compounds in which the metal is bonded to carbon and the oxygen is in the organic moiety (Table XYQEN
I), However, the method is unsatisfactory for analyzing compounds whose metallic oxides are not readily reducedi.e., aluminum, calcium, and magnesium compounds. Huber (3) determined oxygen in alkali and alkaline earth salts of organic acids by treating the sample with silver chloride and heating in B hydrogen atmosphere. Sheft and Katz
