Analytical Reactions Involving Ignition with Manganous Nitrate SISTER M. JOAN PREISING, OTTO F. SLONEK, AND J. H. REEDY Noyes Chemical Laboratory, University of Illinois, Urbana, Ill.
I
N ORDINARY analytical procedures, wet reactions are
The following dry test is based on the decomposition of perchlorate into chloride when it is heated in contact with manganese dioxide. Halides and other halates interfere and must be removed previously.
preferred to dry reactions for both practical and theoretical reasons. When wet reactions are lacking or are unsatisfactory, the analyst may have to resort to dry methods. Two such cases are reported below.
MATERIALS.The materials used in this test must be specially prepared, since the perchlorates and chlorates furnished by manufacturing chemists are not sufficiently pure. Ammonium perchlorate, free of chlorate and chloride, was prepared by neutralizing 70 per cent perchloric acid with ammonium hydroxide and recrystallizing the product. Pure potassium chlorate, free from perchlorate, was made by recrystallizing barium chlorate until free from perchlorate, and then adding a slight excess of potassium carbonate. The precipitated barium carbonate was removed by filtration and the solution was evaporated to crystallization. Owing to the presence of traces of chloride in the manganous nitrate of commerce, a chloride-free product was made by warming an excess of powdered metallic manganese with nitric acid. PROCEDURE. The solution is acidified with dilute nitric acid, and a slight excess of silver nitrate is added to remove the chloride group of anions. The chlorate and perchlorate ions are left in the solution. A reducing agent is then added to reduce chlorate ions t o chloride. A number of reagents were tried for this purpose, including sulfur dioxide, ferrous sulfate, and such metals as zinc, aluminum, etc. Of these, reduction by means of granulated zinc and dilute sulfuric acid on a water bath for half an hour was found most satisfactory. This reduces chlorate ions to chloride without affecting the erchlorate ion. Silver nitrate is again added, t o remove the ''ch?orate chlorine". After removing zinc by sodium carbonate, a few drops of 50 per cent manganous nitrate solution are added, and the solution is evaporated and ignited to incipient redness. The chloride formed by the decomposition of the perchlorate may now be present largely, if not wholly, as silver chloride, and ammonium hydroxide is therefore used as the extractant. The solution is acidified with nitric acid and, if necessary, a little more silver nitrate is added to complete the precipitation. The third silver chloride precipitate represents the perchlorate ions present in the original mixture.
Ignition with Manganous Nitrate Table I shows the principal products obtained when representative anions are ignited with manganous nitrate. Solutions of alkali salts of these anions were carefully neutralized with sodium carbonate and evaporated to dryness. The residues were moistened with concentrated manganous nitrate solution and heated to incipient red heat. After cooling, the black residues were extracted with warm water and the extracts were analyzed for the anions present. I n most cases the effect of this ignition was oxidation. The halates, on the contrary, were catalytically reduced to halides by the manganese dioxide. I n several cases the decomposition was not complete-e. g., bromide, cyanide, acetate, etc. The oxidation reaction generally occurs between 260 " and 300" C., and is sometimes accompanied by a flash or puff, harmless for moderate amounts of the reactants. The active oxidizing agent in these reactions seems to be manganese dioxide, formed by the decomposition of manganous nitrate. This is indicated, not only by the fact that pure manganese dioxide will give all the reactions noted, but by the fact that manganous nitrate is broken down into manganese dioxide and nitrogen dioxide below 200°, considerably below the reaction temperature. The same consideration precludes the assumption that the oxidation is due to nitrogen dioxide. The oxidizing action can hardly be attributed to the sodium nitrate formed by a reaction between manganous nitrate and sodium carbonate, since sodium nitrate alone shows no oxidation effects below 340". Manganous nitrate seems more effective than manganese dioxide, probably because of the finer division of the oxidant and a more intimate mixture of reactants. The proper condition for the oxidation reaction is a faint acidity. This is realized by adding sodium carbonate solution drop by drop as long as effervescence occurs. The slight excess of alkali is removed by the free acid present in the manganous nitrate solution. Excess of sodium carbonate must be avoided, since the oxidizing action of manganous nitrate is strongly diminished by the presence of alkalies. Besides, a faintly acid mixture is favorable to the oxidation of bromide and iodide.
SENSITIVENESS.The decomposition of perchlorate in the presence of manganese dioxide is quantitative, so that the procedure admits of high accuracy (Table 11). As little as 0.001 millimole (0.0995 mg.) of perchlorate can be detected, even in the presence of one thousand times that amount of chloride or chlorate, or both. The procedure is simple and fairly rapid. Its most serious defect is the slowness of the
TABLEI.
IGNITION O F ANIONS IN THE
Anion 1-
Br -
c1-
CN SCNFe(CN)a----
Detection of Perchlorate The precipitation of potassium perchlorate is not a sensitive
S--
soJ--
test for the perchlorate ion, owing to the appreciable solubility of the salt. The sensitiveness can be increased by adding alcohol or other organic solvent, but the anions whose potassium salts are insoluble in alcohol are apt to interfere. I n 1909 Rothmund (4) proposed a test based on the reduction of perchlorate to chloride by reduced salts of titanium, vanadium, or molybdenum, or by zinc in the presence of salts of these metals. The reliability of this test has been investigated in this laboratory, and the reduction was found t o be incomplete in every case. There always remained a small but appreciable amount of unreduced perchlorate.
PRESENCE O F M.4XQANOUS NITRATE
Products I, Brz (incomplete)
Anion
szo1--
N01NOaO C N - (incom lete) CIO so4-- OC$-(+ C N - ) Cl0:Fe(CN)s--czo4-SOr-CzHaOzso,-AsOi---
c1-
+
Products
so,--
Koa- oxides of N NOa-: oxides of N
c1C1cos--
C o s - - (incomplete) .ksOi---
TABLE11. DETECTION OF PERCHLORATE IN THE PRESENCE OF CHLORIDE AND CHLORATE Composition of Material NH~CIOI KCl KClOr Millimoles
875
0.001
...
...
0.001 0.001 0.001
... ...
0.1 1.0
1.0
1.0
AgCl Formed Slight precipitate light precipitate Turbidity Turbidity
876
Vol. 14, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE111. SOLVENT ACTIONOF CONCENTRATED NITRICACID ON SILVERTHIOCYANATE AND SILVERCHLORIDE Materials Used KC1 NHISCN Millimole 0.0116 0.0116 0.0116 0.0116 0.0116 0.0116 0.0116 0.0116
.. ..
.... 0.01 0.02 0.03 0.04 0.05 0.07 0.10 0.01
(Volhard procedure) --Weight of Residue-Calcd. Experimental Difference Qram Gram Gram 0.0166 0.0166 0.0166 0.0166 0.0166 0.0166 0.0166 0.0166 0.0000
0.0154 0.0151 0.0159 0.0165 0.0162 0.0172 0.0169 0.0178 0.0002
-0,0012 0.0015 -0,0007 -~0.0001 0.0004 + O ,0006
-
-
4-0.0003 f0.0012 0.0002
+
reduction of chlorate to chloride, which may be incomplete in the hands of the impatient analyst.
Detection of Chloride in the Presence of Thiocyanate Numemus procedures have been proposed for the detection of chloride ions in the presence of thiocyanate ions. A complete bibliography is too extensive for inclusion in this paper. Conflicting opinions have indicated that certain of these procedures should be reinvestigated. MATERIALS.Since traces of chloride are generally present in the ammonium and potassium thiocyanates furnished by manufacturing chemists, pure materials were prepared as f o l l o ~:s Ammonium thiocyanate was prepared by the reaction between carbon disulfide and alcoholic ammonium hydroxide, using the method of Millon (3). Potassium thiocyanate of “analyzed reagent” grade was recrystallized twice, and the product was shown to be chloride-free by the test below. The trace of chloride in the cupric sulfate used to precipitate cuprous thiocyanate was removed by silver sulfate, and the silver ions thereby introduced were removed by warming the solution with copper turnings. I t was sur rising to find that the liquefied sulfur dioxide, which wa8 use! for reducing cupric compounds, contained considerable amounts of chloride. These were removed by distilling the saturated solution over silver sulfate.
PREVIOUS METHODS. One of the best known procedures for the detection of chloride in the presence of thiocyanate is that of Volhard (6), based upon the assumption that concentrated nitric acid will dissolve silver thiocyanate, but will not affect silver chloride appreciably. Measured amounts of potassium chloride and ammonium thiocyanate were mixed with slight excesses of silver nitrate solution, and the mixtures were heated with 100 ml. of concentrated nitric acid on a hot plate just below the boiling point for 40 minutes. Results for one series of simultaneous runs are shown in Table 111. These results indicate that there is always a loss when silver chloride is heated with concentrated nitric acid. If the mixtures are distilled, the distillates are found to contain considerable amounts of chloride. This loss by volatilization is reduced by heating below the boiling point, but still is appreciable. It would be reduced by using a reflux condenser. The presence of silver chloride in the digestion mixture seems to exert a protective action on the silver thiocyanate, as indicated by the gradual increase in the residue with increase in the thiocyanate content. The values are roughly reproducible, varying slightly with the length of digestion, temperature, exposure, etc. Effervescence, due to the escape of the oxides of nitrogen when thiocyanates are present, also favors the loss of chlorine. Another procedure is based upon the separation of the thiocyanate as cuprous thiocyanate. In the procedure proposed by Mann (I), the solution is mixed with cupric sulfate and the cupric thiocyanate is then reduced to cuprous thiocyanate by hydrogen sulfide. Hall (6) improved this procedure by using sulfur dioxide as the reducing agent. To
determine the limitations of the method, solutions of thiocyanate and chloride were treated with 10 ml. of a 20 per cent solution of cupric sulfate and 10 ml. of a saturated solution of sulfur dioxide, making a total volume of 30 ml. After boiling for one minute, the mixtures were cooled and the precipitates separated. Results (Table IV) show that the precipitation of the thiocyanate is not complete, and that enough of this ion is left in the filtrate to give a slight precipitate with silver nitrate solution. The limiting sensitiveness of the precipitation of cuprous thiocyanate is 0.001 millimole (0.058 mg.), in contrast with less than 0.0001 millimole (0.00355 mg.) when precipitated as silver thiocyanate. Most of the chloride is removed as cuprous chloride during the cuprous thiocyanate precipitation, leaving about 0.1 millimole in the filtrate. It follows that the precipitate obtained when t h e filtrate is treated with silver nitrate solution may consist of either silver thiocyanate or silver chloride, or both. Another procedure (Z), sometimes used by manufacturing chemists, is based on the direct oxidation of the thiocyanate by nitric acid. Mixtures of thiocyanate and chloride in a total volume of 100 ml. were mixed with 25 ml. of concentrated nitric acid and heated just below the boiling point for 3 hours. In all cases a loss of chloride occurred. I n a blank test containing chloride only, a loss of 0.006 millimole (0.2130 mg.) of chloride was found. The losses are apparently greater with increasing amounts of thiocyanate.
NEW PROCEDURE. The complete decomposition of the thiocyanate group upon ignition with manganous nitrate affords a rapid and reliable method for separating thiocyanate from chloride. If the medium is kept neutral by the addition of a drop of sodium carbonate solution, no chloride is lost during the ignition. The solution is made slightly alkaline with sodium carbonate, and any pretipitate is filtered out. The filtrate is evaporated almost to dryness in a small crucible, and the residue is moistened with a drop of 50 per cent manganous nitrate solution and heated to incipient red heat. If the residue after ignition is not black, more manganous nitrate is added and the ignition is re The residue is extracted with dilute nitric acid and boilegtz:pel any hydrogen cyanide. The solution is then tested for chloride with silver nitrate in the regular way.
TABLEIV. SEPARATION -OF THIOCYANATE FROM CHLORIDE AS CUPROUS THIOCY.4NATE
(Mann’s procedure) Materials Used KSCN KCI Millimoles 0.1 0.01 0,001 0.0005 0.0002 0.0001
... ...
... ...
... ... ... ...
1.o
0.5 0.3 0.2 0.1
Reaction with CUSO4 son
Reaction of AgN0a with Filtrate from Cuprow Ppt
Precipitate Precipitate Slight precipitate Slight precipitate Precipitate on standing No effect Precipitate Precipitate Precipitate Precipitate on standing No effect
Turbidity Turbidity Turbidity Turbidity Turbidity Turbidity Precipitate Precipitate Precipitate Precipitate Precipitate
+
Several interferences with this test should be noted. Bromides are not completely oxidized upon ignition with manganous nitrate, and some approved procedure must be used to detect chlorine in their presence. When large amounts of thiocyanate are present, or when insufficient manganous nitrate is used, some sulfide will be formed. This will be eliminated during the boiling with dilute nitric acid. Chlorates and perchlorates are changed to chlorides during the ignition; hence they must be separated prior to heating with manganous nitrate. This separation is effected by precipitating the halide group of anions as silver salts, and
ANALYTICAL EDITION
November 15, 1942
TABLEV. DETECTION OF CHLORIDE IN
THE
PRESENCE OF
THIOCYANATE Cornpoiition of Material NHdBCN KCI Millimole
0.1 0.1 0.01 0.01 0.1
.
AgCl Formed
...
No effect Precipitate Precipitate Slight precipitate Turbidity
0.1
0.01 0.001 0.001
reforming the halide ions by warming the precipitate with zinc and dilute sulfuric acid in the usual way. Some hydrogen sulfide and hydrocyanic acid may be formed owing to the reduction of thiocyanate by zinc, but this has no effect upon the test for chloride. SENSITIVENESS. This test is very sensitive (Table V). As little as 0.001 millimole (0.0355 mg. or 35.5 micrograms) of chloride can be detected in this way, even in the presence of one hundred times that amount of thiocyanates. This sensitivity is superior to that of the older tests, and the procedure is comparable in speed and accuracy.
877
Summary Many anions are oxidized when ignited with manganous nitrate in neutral medium. The actual oxidiaing agent is manganese dioxide. A very sensitive test for perchlorate is based upon the formation of chloride when heated in the presence of manganese dioxide. The test is better than the methods based upon the reduction of perchlorates by titanous solutions. Certain procedures in common use for the detection of chloride in the presence of thiocyanate are not sensitive. A new procedure, based on the removal of the thiocyanate by oxidation with manganese dioxide, has been extended to include chlorate and perchlorate.
Literature Cited (1) Mann, C., 2. ana2. Chem., 28, 668 (1889). (2) Merck, “Chemical Reagents”, 2nd ed., p. 146, New York. Merck & Go., 1914. (3) Millon, M. E., Jahresber., 1860, 237. (4) Rothmund, V., 2. anorg. Chem., 62, 108 (1909). (5) Treadwell-Hall, “Analytical Chemistry”, 9th ed., Vol. I, p. 343, New York, John Wiley & Sons, 1937. (6) Volhard, J., 2. a d . Chem., 18, 281 (1879).
Analysis of Plant Extracts for Chlorophylls a and b Using a Commercial Spectrophotometer C. L. COMAR, Michigan Agricultural Experiment Station, East Lansing, Jlich.
T
HERE has long been the need of a simple method for the determination of chlorophyll which could evaluate the ratios of the individual components and avoid,the difficulties and uncertainties incurred by the use of solid chlorophyll standards (6, 7 , 8 ) . A simple, rapid, and accurate determination for ohlorophylls a and b in green plant tissue has been reported by Comar and Zscheile (2, 3), based upon the fundamental absorption spectra of highly purified chlorophyll a and chlorophyll b preparations (8). The original measurements and analyses were made with a photoelectric epectrophotometer similar to thbt described by Hogness, Zscheile, and Sidwell (4) but employing a large Muller-Hilger Universal double monochromator in the optical system. Errors due t o the size of the spectral region isolated or to scattered radiation were negligible. The utility of the method was limited due t o the expensive optical apparatus required; for to the writer’s knowledge there are only five installations of this type in the United States. The purpose of the present work, therefore, was to determine whether the reported absorption coefficients could be applied to yield accurate values using a more generally available instrument. Instrumental A Cenco-Sheard spectrophotelometer was used in this work. The instrument as described by Sheard and States (6) has an absorption cell carriage made to carry two 1-cm. cells of the open top type. It was riot found practical t o use this type of cell, mainly because of the rapid loss of diethyl ether during the
measurements. However, it was possible to obtain from the manufacturer a 5-cm. cell carriage attachment which accommodated I-, 2-, and 5-cm. cells with ground stoppers. Both the 1- and 2-cm. cells were used in the reported analyses, although the latter have been found more convenient. The light source was an incandescent lamp drawing 70 to 80 amperes at 4 to 5 volts.
In all measurements the 25 b. exit slit was used and the entrance slit adjusted to 0.4 mm. According to Sheard and States ( 6 ) the total spectral region isolated would be 41 A. with the major portion of the energy in the 25 A. band. For all measureme@ above 6500 b. a red filter, which did not transmit below 6000 A., was inserted direc$ly before the entrance slit. For the measurements near 6600 A. it was necessary to use the lamp bulb at its maximum current rating in order to secure satisfactory galvanometer deflections. This should not decrease the life of the lamp unduly, since each measurement usually required less than a minute. The red absorption peak of the plant extract in diethyl ether solution, .prepared as herein described, has always been found at 6600 A. This is a convenient means of checking the wave length calibration of the instrument in this region; t$e same correction may usually be applied to the setting for 6425 A. The calibration in regions of shorter wave length (around 5460 It.) may be checked with a mercury arc either visually or photoelectrically. The concentration of the solution to be used for the measureI ment in the red region should be adjusted to give a log,, f value of 0.5 to 0.8 a t 6600 b. In the normal plant extract, the same I solution is also used for the 6425 reading and yields a log,, 2 I value which is in the accurate range. At other wave lengths I the log,, value should be restricted to the customary limits of 0.2 to 0.8 with the optimum value a t about 0.4.
-x.
Spectroscopic Analysis Figure 1 shows the absorption spectra of chlorophylls a and b in ether solution as determined by Zscheile and Comar (8). Beer’s law is used in the form: IO log10 7 a=-
cl
