Determination of PerchI orates by Fusion with Nitrite ELISABETH KURZ, GERSHON KOBER, and MAX BERL Central laboratory, Military Industries, and Scientific Department, Ministry of Defence, Tel-Aviv, Israel
b Determination of perchlorates by fusion with sodium nitrite gives reproducible and exact results, if the conditions detailed in this paper are observed. The perchlorate-nitrite ratio should b e within the limits of 1 to 7 and 1 to 10 and the temperature of 20" C. The fusion should be 500' resulting chloride is titrated either potentiometrically in the presence of nitrite or visually by the Volhard method, two variants of which are given, both requiring destruction of excess nitrite.
*
(1). REAGENTS
Sodium nitrite, analytical reagent grade, chloride-free. Potassium permanganate, analytical reagent grade, chlorate- and chloridefree (cold saturated solution, about 4%): hitric acid, &V. Silver nitrate, 0.1S solution. Potassium thiocyanate, 0.1N solution. Ferric ammonium sulfate, cold saturated solution, acidified with nitric acid. Kitrobenzene.
T
are few satisfactory methods for the quantitative determination of perchlorates. Some are based on the reduction of perchlorates in solution (wet methods) and others on the reduction by fusion or ignition with a suitable reducing agent (dry methods). \Then the perchlorate is embedded in a matrix of insoluble matter-as for example, in pyrotechnic mixtures-the wet methods are inadequate, beca ise complete extraction of the perchlorate cannot be assured. The dry methods in general use are repeated ignition with ammonium chloride ( 2 , 6) and ignition with sodium perolide in a Parr bomb (4, 6). The former requires the constant attention of a n experienced operator and involves considerable loss of platinum. The determination of perchlorate in the Parr bomb requires elaborate equipment and special precautions. Another method (5) mentioned in earlier literature is the reduction of perchlorate by fudion n.ith nitrites. This shares the advantage with the Parr bomb method that any organic matter present is destroyed simultaneously n-ith the reduction of the perchlorate, unlike the amnioniuni chloride method. Inaccurate results that were alnays on the loiv side were obtained, honever, n hen this procrdure was tested. This paper presents two alternative procedures under which reproducible and accuiale results ma). be obtained by the nitrite fusion method. If grcatt)r precision is desired, the chloride obtained by the reduction of the perchlorate is titrated potentiometrically in the presence of nitrite. For routine analysis, the chloride may be determined by the T'olhard method
potassium chloride solution and connected by a liquid bridge, fitted with porous glass disks at the ends, and dipping into the solution in the titration vessel. The bridge was filled lvith 10% potassium nitrate solution.
after destruction of the excess nitrite
PROCEDURE
Fusion. For perchlorate alone, 0.10 t o 0.15 gram is taken for analysis. If t h e perchlorate is present in mixtures t h a t contain other substances that also consume nitrite-e.g., organic substances and metal powderssamples containing correspondingly less perchlorate should be taken. Some examples are given in Tables I and 11. One gram of finely powdered, dried sodium nitrite is n-eighed. The bottom of a nickel crucible of suitable size (height 42 mm.. diameter 42 mm.) is covered with part of the nitrite. Thrn the exactly rreighed, finely pondered sample is introduced and covered with the remainder of the nitrite, taking care that the sample does not touch the bottom or sides of the crucible. The covered crucible is transferred to a
HERE
APPARATUS
A Fisher Titrimeter was used with a silver-silver chloride indicator electrode and a saturated calomel reference electrode. If the saturated calomel electrode was immersed in the titration vessel, leakage of chloride from the saturated calomel electrode occurred. The electrode mas therefore immersed in a separate beaker containing saturated
Table
I.
Detn. so.
41 42 43 44 45 46
Table
Determination of Ammonium Perchlorate in Mixtures with Acrylate Polymer
(Potentiometric method) Amounts Taken SH4C104Found A4crylate polymer, KH4C104, Total, NH,C104, mg. mg. mg. yo Mg. '5 22 22 21 __ 16 20 25
II.
Dev., mg.
-0 76 -0 32 -0 19 n 111 u i8.i c) 5 110 5 127 0 00 0 4 116 3 136 7 29 1 0 6 103 8 129 4 68 +O Mean error of XH4C104determination on total, 0 22% 2 2
109 4 120 7
131 6 142 9
83 84 82 87 85 80
13 46 33 00 07 21
108 9 120 5 111 7 110 5 116 6 104 4
82 84 82 87 85 80
5 2 2 3 6
Error on
total, yo -0.38 -0.14 -0 14 0 +0 22 $0.47
Determination of Potassium Perchlorate in Pyrotechnic Mixture
(Indicator method A ) Basic mixture, Ba(?;03)2:LIg:phenolic resin = 1 : 1 06:O 375 parts by weight Amounts Taken Basic - KC104 Found Detn mixture, KC104, Total, KC104, Dev., Error on mg. nig. % Mg. 5 mg. total, 7 0 SO. mg. 47 48 49 50 51 52 53
80 58 102 100 74 76 46
-0 37 I 31 60 14 4 +0 19 91 -1 17 84 22 4 -1 19 90 25 2 20 1 21 36 +O -0 17 E 18 64 0 13 1 22 64 Mean error of KCIOl determinatlon on total, 0 5 0 5
6 2 0 0 5 4 8
37 14 23 26
9 1 6 6 19 8 18 0 13 7
5 3 5 6 1 94 4 60 5
118 i2 126 126 94
31 19 18 21 21 19 22
98 50 72 01 04 06 64
5 3 1 4 3 4
VOL. 30, NO. 12, DECEMBER 1958
-0 +O -0 -1 +0 -0 0
39 41
88 11 31 42
1983
6 " r -----
400
500
600
Temperature
700 OC.
Figure 1. Negative error of determination of potassium perchlorate as a function of heating temperature
muffle furnace and removed after ll/z hours a t 500" f 20" C. After cooling, the contents of the crucible and particles adhering to the lid are dissolved and transferred quantitatively, using 40 to 50 ml. of hot distilled water, into a beaker of 100- to 150-ml. capacity, if the potentiometric method is to be used. If the indicator method is to be used, the transfer is made to a 300-ml. Erlenmeyer flask. Before titration, the solution is cooled. TITRATION
Potentiometric Method. The stirred solution is titrated with 0.1N silver nitrate added from a microburet, using the electrode setup described above. Towards the end of the titration, the silver nitrate solution is added in amounts of 0.04 ml., so that the maximum potential change obtained a t the end point of the titration (about 20 mv.) differs from the preceding and following increment by a few millivolts. The end point of the titration can be determined to 0.01 to 0.02 ml., if the potential changes preceding and following the maximum potential change are taken into account and linear interpolation is applied. Indicator Method A. The solution is acidified with 10 ml. of 6hT nitric acid. Saturated potassium permanganate solution is added, until a very faint pink coloration persists for a few minutes. Twenty-five milliliters
Table I l l .
O
425-450 450-475 475-500 500-530 530-650 550-575 575-600 600-650 650-700
1984
DISCUSSION
Any chloride, chlorate, or other chlorine-containing compounds which mag give chloride upon fusion with nitrite, must be determined separately and the proper amount of chloride deducted from the result of the argentometric titration. A blank on the reagents is also desirable, because the nitrite might contain some chloride, and the permanganate some chlorate. Certain critical points of the determination should be noted. The fusion must ensure destruction of all organic matter present and complete reduction of the perchlorate within a reasonable time. Foi this purpose a large excess of nitrite and high temperature would be favorable. Heating too strongly, however, causes a loss of chloride by volatilizaton, and a large excess of nitrite interferes both in the potentiometric titration and in Method A. Therefore, the optimal heating conditions and quantity of nitrite mere determined. TEMPERATURE AND DURATION OF FUSION
The original method (6) mentions slow heating, until complete fusion is effected, and keeping the substance molten for half an hour at the IoTvest possible temperature.
Error of Determination of Potassium Perchlorate as Function of Fusion Temperature (Heating continued for 1 hr., 30 min.)
Temp.
Interval,
(or for smaller amounts of perchlorate, 10 ml.) of 0.1N silver nitrate solution are added, followed by 3 ml. of nitrobenzene, and 1 ml. of ferric alum solution. The excess silver nitrate is backtitrated with 0.1N potassiuni thiocyanate solution, and the flask is shaken vigorously, until a faint reddish brown color persists. Indicator Method B. ( 3 ) . TKO milliliters of ferric alum indicator and an excess of 0.1N silver nitrate (20 ml.) are added to the solution. The solution is acidified with 10 ml. of 6N nitric acid and boiled, until all nitrous acid is destroyed. After cooling, the solution is filtered through a fine or medium sintered-glass crucible and the precipitate is washed with 2% nitric acid. The excess silver nitrate is determined in the filtrate by titration with 0.1N thiocyanate.
RIean
Error, %
Error of Detn., %
C.
-3,i -1.3, -0.5, $ 0 . 1 -0.4, -0.4, -0.3, -0.1, -0.3, - 0 . 3 0, + O . l , +0.2, -0.5, -0.3, -0.1, -0.4, -0.4, - 0 . 3 -0.6, -0.4, - 0 . 6 -1.4 - 1_ .4-
-5.2, -2.5, -2.5 -8.2, -9.5, - 7 . 6 ANALYTICAL CHEMISTRY
-0.3,
-3.7 -0.6 -0.3 -0.26 -0.5 -1.4 -1.4 -3.4 -8 4
Table I11 and Figure 1 summarize the relative errors of several determinations made by potentiometric titration after fusion for 11/2 hours a t different temperatures. A minimum of the error exists a t about 500' C. which is caused by two factors. On the left branch of the curve, a t low temperatures, the reduction of the perchlorate proceeds slowly and is not completed in 1'12 hours. Therefore too little chloride is found. At high temperatures loss of chloride occurs because of volatilization. A few experiments made with heating for 1 hour showed that the results are essentially unchanged when heating to temperatures from 480' even up to 550" C., while at lower temperatures, from 425' to 450' C., the error was about 20% (as against 3% when heating mas continued for ll/zhours). This explains why too low results were found when the directions given in the original method (6) mere followed. INFLUENCE OF NITRITE ON DECOMPOSITION OF PERCHLORATE
Quantitative measurements of the influence of nitrite on the rate of decomposition of potassium perchlorate were made by Tian and Svilarich ( 7 ) . They found that while the decomposition of pure perchlorate proceeds only to 24.9% after heating t o 520" C. for 65 minutes, in the presence of five times its weight of sodium nitrite the decomposition of the perchlorate was nearly complete at the same temperature. At loner temperatures the acceleration of the decomposition by nitrite was still greater, while the effect vanished at about 600" C. Potassium Perchlorate is reduced by fused sodium nitrite according to the equation KC104
+ 4 NaNOp = KC1 + 4 Kay03
+
(formula weight: 138.55 276.04) corresponding to a weight ratio of approximately 1 to 2. For efficient reduction, however, there must be an excess of nitrite. One gram of sodium nitrite was sufficient for the complete reduction of up to 0.15 gram of pure potassium chlorate under the conditions given in the procedure. If organic matter (or other oxidizable substances, such as powdered metals) is present, part of the nitrite is used for their oxidation and the amount of sample must be decreased accordingly for 1 gram of sodium nitrite to be sufficient. INFLUENCE OF NITRITE ON ARGENTOMETRIC TITRATION
Potentiometric Method. No stable potential can be obtained in the pree-
g. N,NO, 5
4
3
2
added
Table IV.
c
1
0
Determination of Potassium Perchlorate KClOi Detn. Taken, Found, Dev., NO. mg. mg. blg. Error, yo
Method Potentiometric
1 L n
s
4 5 6 7 85 9 106 11 12 13 14 15 16 17 18 19 20
Indicator A
I
1
0-20
I
I
I
I
40
60
60
700
I
Volume of solution in ml. Figure 2. Potential change a t end point of titration of potassium chloride 1 mmole KCI in 40 m M solution with various quantities of NaN02 added (upper abscissa scale) 2. 0 1 mmole KCI 3. A 1 mmole KCI 0.5 gram N a N 0 2 4. H 1 mmole KCI 1 gram NaNOz in various volumes (lower abscissa scale)
a
113.8 113.3 130.2 119.2 134.9 125.6 109.2 121.1 140.0 151.8 103.6 121.2 97.6 122.7 137.3 ii5.2 130.4 119.8 124.7 122.2
113.6 112.8 130.2 .. 118.6 134.7 125.0 108.9 121.1 139.9 151.8 102.8 121.2 96.7 122.3 135.9 ~
ii3.9 128.8 118.6 124.7 120.8
-0.2 -0.5
-0.17 -0.44
0
0
-0 6 -0.2 -0.6 -0.3 0 -0.1 0 -0.8 0 -0.9 -0.4 -1.4
-0.5 -0.15 -0.48 -0.27 0 -0.07 0 -0.8 0 -0.9 -0.3 -1.0
-1.3 -1.6 -1.2
0
-1.4
Mean Error, yo
0.21
-i.i
-1.2 -1.0 0 -1.1
0.8
Heated 1 hour. Heated 2 hours.
1.
++
ence of nitrite solutions acidified with nitric acid. I n neutral solutions, however, stable potentials can be attained if the solutions are stirred. Nitrite reduces the height of the potential change a t the end point of the titration, the interference being greater a t higher nitrite concentrations (Figure 2, curve 1). It might be assumed, therefore, that lowering the nitrite concentration by dilution of the solution containing chloride and nitrite would give a higher potential change than with the undiluted solution. This is, however, counteracted by a second effect. The potential change in solutions containing only potassium chloride is reduced by dilution (Figure 2, curve 2). The result of diluting the solutions containing both potassium chloride and sodium nitrite-a slight decrease in the maximum potential change-can be seen in Figure 2, curves 3 and 4. Consequently, dilution of a solution containing given quantities of potassium chloride and of nitrite not only does not increase, but even lowers somen-hat the maximum potential change. To obtain the greatest possible potential change a t the end point of the titration, the major requirement is to use the minimum quantity of nitrite for the fusion that still ensures complete reduction of the perchlorate. The solution of themelt should not be diluted more than necessary for its quantitative transfer to the beaker. Indicator Method. A. When known amounts of potassium chloride were titrated in solutions t o which a
Table V. Method Potentiometric
Indicator A
Determination of Ammonium Perchlorate IiH,C10, Detn. Taken, Found, Dev., NO. Error, % Mg. mg. mg. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
100.7 122.0 108.2 95.8 134.8 113.9 129.1 117.5 121.4 128.0 121.4 131.8 103.6 133.0 99.0 135 2 114 5 95 9 146 3 132 6
large excess of nitrite was added, too low results for chloride were obtained. This may be explained as the result of adding to the solution large quantities of permanganate to oxidize the excess of nitrite (which oxidizes thiocyanate, as does also permanganate). To avoid precipitation of insoluble manganous acid, the solution must be strongly acidified with nitric acid before the addition of the permanganate and this may cause losses of chloride by oxidation in the presence of nitrite, when nitrous oxides are evolved. B. When silver nitrate is added to the solutioh before the addition of nitric acid, no loss of chlorine occurs and a large excess of nitrite is not critical. APPLICATIONS AND RESULTS
Results of the analysis of C.P. potassium perchlorate and ammonium
100.6 121.8 108.3 95.4 135.1 113.7 128.8 117.6 121.7 127.9 120.4 131.2 102.0 132.0 98.8 133 2 114 5 95 2 144 0 130 6
-0. I -0.2 +0.1 -0.4 +0.3 -0.2 -0.3 +0.1 +0.3 -0.1 -1.0 -0.6 -1.6 -1.0 -0.2 -2.0 0 -0.7 -2.3 -2.0
Mean Error, %
-0 09 -0 16 +o 09 -0 42 +O 15 -0 17 -0 25 +o 09 +o 25 -0 08 -0 8 -0 5 -1 6 -0 8 -0 2 -1 5 0 -0 7 -1 6 -1 5
0.17
0 0
perchlorate by potentiometric titration and indicator method are summarized in Table IV and V. The relative mean error of the potentiometric determination is 0.2% and of Indicator Method A about 1%. I n general, the deviation is negative. To indicate possible applications of this method, results of the analysis of two kinds of synthetic mixture are given which include mixtures of ammonium perchlorate and acrylate polymers, and a green signal composition containing potassium perchlorate. I n both cases the perchlorate and the remainder of the mixture were weighed in separately, to avoid possible deviations from the nominal composition resulting from sampling. Results are summarized in Tables I and 11. The decomposition of the perchlorate - polymer mixtures is smooth and the results obtained by the potentioVOL. 30, NO. 12, DECEMBER 1958
1985
metric method are in the same range of accuracy as for perchlorate alone. K i t h pyrotechnic mixtures, the determination of perchlorate is piacticable and metallic powders and organic binders do not interfere appreciably. Organic compounds, such as tartrates, that are easily oxidizable and evolve large quantities of gas, cause deflagration and loss of material when present in considerable quantities. Some general precautions must be observed in the fusion of pyrotechnic mixtures. The smallest possible quantity of sample should be taken and the crucible inserted into a cold furnace and gradually heated t o 500” f 20” C. With some pyrotechnic mixtures the melt does not dissolve in water completely, as magnesium powder may
remain partly unchanged, while part appears as insoluble hydroxide. With such mixtures the indicator method should be used, because the insoluble material dissolves upon the addition of a sufficient quantity of acid. Table 11shows that the absolute error of the determination in pyrotechnic mixtures by the indicator method is of the same order as in the determination of pure perchlorate. As smaller quantities of perchlorate are present in the sample (because part of the sodium nitrite is used for the oxidation of the other components of the mixture and larger quantities of the mixture may cause deflagration), the relative error of the determination of the perchlorate is much larger than in the case of perchlorate alone.
LITERATURE CITED
(1) Berl-Lunge,
“Chemisch-technicche Untersuchungsmethoden,” Vol. 11, Part I, 8th ed.. p. 568, Julius Springer, Berlin,
1932. (2) Blangey, L., in Treadwell-Hall, “ilnalytical Chemistry,” Vol. 11, 9th English ed., p. 392, Wiley, Xew k’ork, 1942. ( 3 ) Crump, N. L., letter of Sept. 23, 1957. (4) Crump, S . L., Johnson, Ir;. C., 4 x . 4 ~ . CHEW27. 1007 f 1965). ( 5 l Dittrich, M.,Bollenbach, H., Ber. 38, (51 (1905). (6) Joint Army-Navy Specification, JanP-217, May 29, 1954, F-4L, F-4L(l). ( 7 ) Tian. .4..Svilarich. Bull. soc. chim. 47.
RECEIVEDfor review August 17, 1957. Accepted July 28, 1958. Published with the permission of the General Directors of the Rlilitary Industries and Scientific Department, Ministry of Defence.
Effect of Trichloroacetic Acid on Determination of Silicate and Phosphate with Molybdate Reagent EDWARD S. DELLAMONICA, ELIZABETH W. BINGHAM, and CHARLES A. ZITTLE Eastern Regional Research laboratory, Eastern Utilization Research and Development Division, U. S. Department o f Agriculfure, Philadelphia 18, fa.
strong increase (about fourfold) in the Trichloroacetic acid enhances the color obtained with silicate and the color of reduced silicate-molybdate complex. The quantitative results molybdate reagent for phosphate leading to this conclusion are reported (high concentration of sulfuric acid) because of the importance of the about fourfold. Because of this, small trichloroacetic acid effect for the deamounts of silicate can cause serious termination of both phosphate and error in the determination of inorganic silicate with the molybdate reagent. phosphate when trichloroacetic acid is This effect may be of analytical usepresent. T i ichloroacetic acid also enfulness in silicate determination. hances the color obtained with the molybdate reagent for silicate (low concentration of sulfuric acid), but to a REAGENTS considerably smaller degree (26y0). In addition, when trichloroacetic acid i s A11 chemicals used were reagent grade present the silicate color i s independent unless stated otherwise. of the sulfuric acid concentration and i s Sodium caseinate solution, 4%. Prenot influenced by the volume to which pare by dissolving purified isoelectric the molybdate is added. casein in water with 0.1Y sodium
T
effect of trichloroacetic acid on’the molybdate reagent for phosphate ( 2 ) or silicate ( 2 ) was observed in determining the inorganic phosphate in casein solutions before and after heating. The casein was precipitated 1% ith 12.5% trichloroacetic acid and inorganic phosphate was determined in the filtrate m-ithout digestion. The apparent quantity of inorganic phosphate in the unheated casein filtrate was much too high, and was found to be due to the presence of a small amount of silicate, which alone was not very reactive with the molybdate reagent for phosphate. The trichloroacetic acid, however, produced a HF
1986
ANALYTICAL CHEMISTRY
hydroxide (5.5 ml. per 1 gram of casein). The solution had an approximate pH of 6.8. Trichloroacetic acid (TCA) solution, 25%. Dissolve 100 grams of TCA (sulfate-free) in 400 ml. of water. The reagents used for the phosphate determination (1. 4 ) and silicate determination ( 2 ) with ammonium molybdate are described in the respective references. Sodium silicate. Prepare the silicate solution from sodium metasilicate (I\’azSiO3.9H20) which has been dried a t 130’ C. to a constant weight. Prepare a stock solution containing 2.4 mg. of the anhydrous sodium metasilicate per nil. and dilute 1 to 240 with water. 24 hours or more before use, to give a standard solution containing 10 y of
silicate. equivalent to 2.3 y of silicon per ml. APPARATUS
The Fisher Electrophotometer, Model AC, was used for colorimetric analysis with the red filter ( S o . 660) and with matched 23-ml. absorption cells (inside diameter approximately 2 em.). The Beckman Model B spectrophotometer was also used for colorimetric analysis a t a ware length of 650 mp with 18-mm. outside diametw matched absorption tubes. PROCEDURE
Analytical Methods. Phosphate (and silicate) was measured by the Fiske and Subbarow method ( 1 ) mith ferrous sulfate as the reducing agent (4). The amounts of each reagent used for digested samples differed slightly from the described procedure and are. as follow: 2.5 ml. of 2.5% ammonium molybdate, 1.25 ml. of 7 . 5 5 sulfuric acid, and 2.5 ml. of 10% ferrous sulfate, in a final volume of 25 ml. For nondigested samples. 2.5 ml. of 7 . 5 N sulfuric acid should be added. In some experiments. 1 nil. of aminonaphtholsulfonic acid (ASSA) was used as the reducing agent ( 2 ) , replacing the ferrous sulfate. Silicate mas determined by the method described by Kenyon and Bewick ( 2 ) . The amounts of reagents used in the present study were the same as in the phosphate method above. except that only 0.1 ml. of sulfuric acid
