A. Determination of Carbon and Hydrogen
With few exceptions, organic compounds always contain both carbon and hydrogen. When an organic compound is decomposed by heating at high temperatures (combustion) in the presence of oxygen, carbon dioxide and water are produced:
The resultant water vapor and carbon dioxide can be collected sequentially in separate receivers (absorption tubes). From the weights of carbon dioxide and water obtained, the percentages of carbon and hydrogen are calculated by the following formulas:
wt of CO2 at wt of C %C = -:-— x -;-T-TT- X 100 (2)
wt of sample mol wt of CO2
and wt of H2O at wt of H
wt of sample mol wt of H2O
This is the absolute method for the determination of carbon and hydrogen. The apparatus and technique were developed in the early part of the nineteenth century. Then at the turn of the twentieth century, Pregl (Nobel Prize laureate, 1923) improved the method and demonstrated that carbon, hydrogen, and some other elements in organic compounds can be determined accurately by using a few milligrams of the sample.
B. Dumas Method for Nitrogen Determination
If a nitrogen-containing organic compound is decomposed by oxidation, nitrogen oxides are formed:
Organic nitrogen compound-> Nitrogen oxides.
All nitrogen oxides can be converted to elementary nitrogen by passing them through metallic copper at high temperature:
The nitrogen gas is then purified, collected, and measured.
C. Simultaneous Determination of Carbon, Hydrogen, and Nitrogen
Currently the common practice in analyzing organic compounds for carbon, hydrogen, and nitrogen is to use the
CHN analyzer, an apparatus that determines all three elements simultaneously on the basis of the chemical reactions discussed above. Several models of this apparatus are available commercially. An example is shown in Fig. 1, in which the CHN analyzer is connected to a microcomputer. With this equipment, after the accurately weighed sample has been placed in the combustion tube and the sample weight entered on the keyboard, the analysis is started and all operations of combustion: measurement of CO2, and H2O and N2; calculations; and printout of results are carried out automatically. Figure 2 shows a schematic diagram of the combustion train. The boat that contains the organic sample is put in the ladle and introduced into the combustion tube by opening the sample fitting plug. By means of the magnet, the boat is pushed to the high heat coil area. The automated process now commences. The organic compound is combusted in the pyrolysis tube in an oxygen atmosphere under static conditions. The carbon dioxide, water vapor, and nitrogen oxides produced are then swept by a stream of helium into the reduction tube, where nitrogen oxides are converted to nitrogen gas and the excess oxygen is removed. The gaseous mixture is conducted into the analyzer, where the measurement of
CO2, H2O, and N2 is performed by three pairs of thermal conductivity cells connected in series. A trap between the first pair of cells absorbs H2O from the gas mixture before it enters the second cell so that the signal is proportional to the amount of H2O removed; another trap between the second pair of cells removes CO2 so that the signal is proportional to the CO2 removed; and the last pair determines N2 by comparing the remaining sample gas plus helium with pure helium. The instrument is calibrated with a pure known organic nitrogen compound such as acetanilide before samples are run.
Adaptation of the CHN analyzer for total automatic operation when a large number of samples have to be analyzed within a short period involves changing the combustion train from horizontal to the vertical arrangement. The head of the combustion tube is connected to a turntable device known as the autosampler. Figure 3 depicts the schematic flow diagram of the PE 2400 CHN Analyzer equipped with an autosampler which holds 60 samples in the carousel sample tray. Figure 4 shows a complete picture of this instrument connected to the microprocessor and microbalance. Instead of being placed in an open mi-croboat, each sample is weighed in a tin or an aluminum
capsule. The capsule is then sealed and placed in its corresponding location in the 60-sample tray. Sample weight and identification are stored in memory. As the automatic analysis mechanism is activated, the capsules drop sequentially from the turntable into the combustion tube. The reaction products N2, CO2, and H2O are separated by frontal chromatography and measured by thermal conductivity, respectively. Oxygen is used during the combustion, while helium serves as the carrier gas for chromatography. At the completion of each sample analysis, results are printed out to record the identification number, sample weight, and C, H, and N percentages.
In the Leco CHN Analyzer, infrared spectroscopy is utilized to determine carbon and nitrogen. After combustion, the gaseous mixture containing nitrogen, carbon dioxide, and water vapor is conducted through two infrared absorption cells where H2O and CO2 are measured, respectively. Helium serves as the carrier gas.
Kjeldahl discovered in 1883 that when agricultural materials were heated with concentrated sulfuric acid, the nitrogen originally present in the organic matter was transformed into ammonium bisulfate. He accomplished the determination of nitrogen by liberation of ammonia through the action of a strong alkali and titration of the ammonia. The chemistry involved can be depicted as follows:
FIGURE 3 Schematic flow diagram of the PE 2400 CHN Analyzer. [Courtesy of Perkin-Elmer Corporation.]
Organic nitrogen compound
NH4OH + HCl
The Kjeldahl method is probably the most frequently used method in organic elemental analysis. It is performed generally on complex mixtures, and it is routinely carried out in agricultural stations, food processing plants, and clinical and biochemical laboratories. The equipment and experimental procedures vary widely, depending on the nature of the organic material. Thus, in some determinations milligram quantities of the sample are used, while in other cases the sample size may be as large as 5 g. The reaction vessels employed for the decomposition of the sample range from 10 to 800 mL in capacity. The modes of finish can be titrimetric, colorimetric, or based on the ammonium ion-specific electrode.
A simple procedure for analyzing milligram amounts of organic nitrogen compounds can be carried out as follows. The sample is weighed into a micro-Kjeldahl digestion flask, which is commercially available or can be homemade from a 150-mm test tube by blowing out its bottom to form a bulb of about 10-mL capacity. Ten milligrams of selenium powder and 40 mg of copper sulfate-potassium sulfate mixture are added, followed by 1 mL of concentrated sulfuric acid. The reaction mixture is boiled gently for about 10 min until it becomes colorless. On cooling, the solution is diluted with water and the ammonia is liberated by using the micro-Kjeldahl distillation apparatus shown in Fig. 5. In operation, the ammonium bisulfate solution in the micro-Kjeldahl digestion flask is transferred through funnel B to the bottom of distilling flask D, followed by 8 mL of 30% sodium hydroxide solution. Funnel B is then closed by putting the Teflon plug in place. Steam is then conducted from generator A into flask D, whereupon ammonia is driven from flask D into condenser C. The distillate (ammonium hydroxide solution) is collected in a 50-mL conical flask containing 5 mL of 2% boric acid solution. The ammonium hydroxide is titrated with 0.01 N hydrochloric acid [see Eq. (9)], with methyl red-bromcresol green as the indicator.
When the Kjeldahl method is used for nitrogen determination, it should be remembered that the nitrogen present in the organic compound must be the amino type. Other types such as nitro and nitroso compounds can be reduced to amino compounds by suitable treatment prior to concentrated sulfuric acid digestion. Complex organic materials such as coal and blood samples require the addition of catalysts and prolonged heating to achieve complete recovery of nitrogen as ammonium bisulfate.
In the mid-1990s, Collins, Chalk, and Kingston developed a microwave digestion method which eliminates the need for a catalyst and reduces the amount of sulfuric acid
considerably. For biological materials, 10 mL of concentrated sulfuric acid per gram sample is recommeneded, followed by 6-12 mL of 30% hydrogen peroxide. The digestion time is about 20 min.
be achieved by passing the gas through copper oxide at 600° C,
or through anhydroiodic acid at 130°C,
Several finishing modes are applicable. The carbon dioxide can be retained in an absorption tube packed with Ascarite (sodium hydroxide mixed with asbestos) and weighed, or absorbed in a solution and titrated. The iodine produced in Eq. (14) can also be determined by titration. Alternatively, the iodine vapor can be led by a stream of nitrogen into an electrolysis cell, where iodine is reduced at controlled potential and the amount of electricity is recorded. Figure6 shows, from right to left, the complete equipment for oxygen determination, which comprises the combustion train, the furnace for the oxidation of carbon monoxide, and the assembly for electrometric finish.
The PE 2400 CHN Analyzer (Fig. 4) can be modified to perform automatic determination of oxygen. The combustion tube is filled with platinized carbon reagent. The samples are pyrolyzed in an inert atmosphere of argon or helium. The reaction product CO is separated by frontal chromatography and measured by thermal conductivity. In the Leco CHN Analyzer, carbon monoxide is converted to carbon dioxide to be measured by infrared absorption. Neither CHN method is suitable for the analysis of organic substances which contain fluorine, phosphorus, silicon, or most metallic elements.
IV. DETERMINATION OF SULFUR
When an oxygen-containing compound is pyrolyzed at 950°C in the presence of platinized carbon in a nitrogen or helium atmosphere, the organic molecule is broken up and all oxygen is converted to carbon monoxide according to the following reactions:
Compound containing C, H, O
Hence measurement of the amount of carbon monoxide produced will indicate the oxygen content of the organic substance. In practice, the carbon monoxide is determined indirectly by transforming it to carbon dioxide. This can
Sulfur in organic compounds is usually determined in the form of sulfate. When organic material is heated in a large excess of oxygen, as in the burning of petroleum, its sulfur content is converted to sulfur trioxide, which combines with water to produce sulfuric acid:
Organic sulfur compound —2 SO3 (15)
A simple technique for decomposing organic substances in a flask filled with pure oxygen is closed-flask combustion (or oxygen-filled flask combustion) with platinum as the catalyst. This method is suitable for sulfur determination. The appartus is illustrated in Fig. 7. When the sample size is between 3 and 25 mg, the flask (Schoniger flask) has a capacity of 500 mL. For the decomposition of larger quantities of organic materials, 1000-mL flasks are used. The stopper of the flask is sealed to platinum gauze. In operation, the sample to be analyzed is placed on a piece of paper cut in the shape shown at the left in Fig. 7. The wide part of the paper is then folded and inserted into the platinum gauze. Meanwhile, 10 mL of distilled water and 0.3 mL of 30% hydrogen peroxide are added to the flask to serve as absorption liquid, and the air inside the flask is displaced with pure oxygen. Next the tip of the paper is ignited and the stopper is immediately attached to the flask. To prevent escape of sulfur trioxide, the analyst must tilt the flask as illustrated at the right in Fig. 7.
After cooling to room temperature, the contents of the conical flask are quantitatively transferred to a 200-mL graduated beaker. The pH of the solution is adjusted to 3.0± 0.2 by addition of 0.5 N ammonium hydroxide. Then 50 mL of acetone and 0.3 mL of dimethylsulfonazo-III indicator are added. While the mixture is stirred vigorously, the sulfate ions are titrated with 0.01 M barium chloride:
The end point is a sky-blue color that persists for at least 30 sec.
When the organic compound to be analyzed contains only carbon, hydrogen, oxygen, and sulfur after closed-flask combustion as described above, the resultant solution will be dilute sulfuric acid. In this case, it is more convenient to use acidimetry as the mode of finish. For this purpose, the solution in the Schoniger flask is transferred into a 100-mL conical flask, boiled for 2 min to remove residual hydrogen peroxide, and then titrated with 0.01 N sodium hydroxide, with methyl red as the indicator:
In the CHNS/O analyzer, sulfur is determined in the form of SO2. The flow diagram of the Leco Analyzer for CHNS/O (Fig. 8) is shown in Fig. 9. The organic sample is weighed in a tin capsule and dropped into
FIGURE 8 Leco Analyzer for CHNS/O. [Courtesy of Leco Corporation.]
FIGURE 8 Leco Analyzer for CHNS/O. [Courtesy of Leco Corporation.]
the oxidation furnace containing copper oxide and silver tungstate. Heating at 1000°C in an oxygen atmosphere produces CO2, H2O, N2, nitrogen oxides, and sulfur oxides. The gaseous mixture then passes through the reduction furnace containing copper metal at 600° C in a helium atmosphere, whereupon all nitrogen oxides are converted to N2 and sulfure oxides to SO2. With helium as the carrier gas, the mixture is conducted through three infrared (IR) cells to measure H2O, SO2, and CO2, sequentially. Finally, these three components are removed by Anhydrone (magnesium perchlorate) and Lecosorb (sodium hydroxide), which leaves N2 to be measured by thermal conductivity. The Perkin-Elmer Analyzer uses gas chromatography to separate all four components. Neither CHNS/O method is applicable to the analysis of materials containing metallic elements which form thermally stable sulfates.
A special sulfur test apparatus is used for the determination of sulfur in petroleum products by lamp combustion. The apparatus consists of the burner flask, chimney, spray, and absorption tube. The sulfate that is collected can be determined by a turbidimetric method.
V. DETERMINATION OF CHLORINE, BROMINE, AND IODINE
The nature of the organic substance containing chlorine, bromine, or iodine determines what is the best method for decomposition. On the one hand, organic compounds that have ionizable halogens, such as the alkaloid halides, are soluble in aqueous solution, which liberates the halide ions, which can be titrated directly. Similarly, some compounds can be dissolved in a nonaqueous solvent like ethyl alcohol or liquid ammonia; then addition of metallic sodium dislodges the halogen from the organic molecule and produces sodium halide. On the other hand, certain compounds, especially the polyhalogenated ones, require drastic reactions at high temperatures to destroy the organic molecule in order to convert the halogens into ionic forms. The closed-flask combustion technique (see Fig. 7) is commonly used for this purpose. The absorption liquid for chlorine or bromine contains sodium hydroxide and hydrogen peroxide and that for iodine contains hydrazine sulfate, so that chloride, bromide, and iodide, respectively, are obtained as the final products.
Another technique for decomposing organic halogen compounds utilizes a metal bomb constructed of nickel. Commercial metal bombs are available in two sizes: a 2.5-mL bomb for decomposing up to 50 mg of organic material, and a 22-mL bomb that can handle as much as 0.5 g of sample mixed with 15 g of solid reagents. The reagents are sodium peroxide and sucrose or potassium nitrate. After being locked tightly, the bomb is heated. Vigorous oxidation reactions take place, resulting in the transformation of the chlorine, bromine, and iodine originally present in the organic substance to chloride, bromide, and iodate, respectively.
For the determination of iodine, one mode of finish consists of titration with standardized mercuric nitrate solution, with diphenylcarbazone as the indicator:
Another method suitable for the determination of small amounts of iodine in organic materials involves the amplification technique. The iodide ions obtained after decomposition of the sample are oxidized to iodate by the addition of bromine in an acetate buffer. Excess bromine is removed with formic acid. Then the iodate is determined by liberation of iodine on addition of iodide in sulfuric acid solution, followed by titration of the liberated iodine with standardized sodium thiosulfate solution with starch as the indicator. The sequence of reactions can be depicted as follows:
B. Modes of Finish
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