October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
A cleaning method was finally discovered which permits the platinum electrode to indicate accurately the correct potential within 2 to 5 minutes. (In any electrometric pH measurement a variable amount of time is always necessary for the establishment of equilibria conditions.) The method of cleaning the platinum electrode consists in boiling the electrode in a 5 to 10 per cent sodium bisulfite solution for at least 3 minutes. The electrode is removed, washed in water, and inserted into the flow channel. Care must be taken not to touch the electrode or contaminate it in any way. Obviously such a cleaning will not remove many things that deposit on the electrode from industrial solutions. Therefore, in cleaning an electrode that has been contaminated, it is better to boil the electrode until it becomes bright in 50 per cent nitric acid, followed by a water wash. But it is absolutely essential to follow this acid treatment by
407
boiling the electrode in the 5 to 10 per cent sodium bisulfite solution. The thoroughness of terminating the cleaning of a platinum electrode with the sodium bisulfite treatment was further demonstrated with a battery of five flow channels operating in parallel on the same solution. The five platinum electrodes were cleaned by this new method and each inserted into a flow channel. Within a few minutes every electrode was indicating the correct pH within 2 millivolts. This experiment was repeated several times to show that the results were not accidental. Literature Cited (1) Biilman and Jensen, Bull. SOC. chzm., 41, 151 (1927). (2) Clarke, "The Determination of Hydrogen Ions," 3rd ed., p. 205, Williams & Wilkins, 1928. (3) Luther and Leuhner, J . $yak#. Chem., 96, 314 (1912): 2. onorg. Chem., 74, 389 (1912).
Ignition Losses in Potash Analyses of Triple Superphosphate Mixtures' L. B. Lockhart TENNESSEE COPPER & CHEMICAL CORP.,ATLANTA, GA.
OR several years it has been known that fertilizer mix-
F
tures containing potash salts and superphosphate may give erratic results, usually low, upon analysis for potash content by the Official Method. A number of papers have appeared on this subject from 1925 to date, the difficulties being usually ascribed to the presence of phosphoric acid or its salts which are present in the solution used for evaporation and ignition. Various methods for removal of phosphates have been proposed (1). Kerr (6) found that where a small quantity of phosphoric acid was present in the potash solution, the potash results were from 0.2 to 0.3 per cent too low. Bible (2) found low results with silica dishes, especially with "treble" superphosphate mixtures. Some of his results in porcelain were 30 per cent too low, with low results also in platinum. Haigh (4)' using the Official Method without removing phosphates, obtained losses as high as 36 per cent of the true potash content with fertilizer mixtures containing up to 16 per cent of phosphoric acid and 4 to 5 per cent of potash. Satisfactory checks were not obtained. I n a special study of the effect of silica dishes, Haigh (3, 6) found low results as compared with ignition in platinum dishes. The present paper deals primarily with the temperature of ignition. None of the papers referred to states the time, temperature, or method of ignition. The Official Method (1) states: "Maintain a full red heat until the residue is perfectly white.'! This temperature may be between 600" and 900" C. With no precautions indicated as to the effect of excessive temperatures, the higher temperatures are often used. The samples used were commercial mixtures of superphosphates, potash salts, and in most cases mineral and organic ammoniates. The procedure was to prepare the solution by the Official Method, using 2.425 grams of the fertilizer, making to 250 cc., and taking a 40-cc. aliquot for evaporation. A smaller aliquot was used for samples of high potash content. Thus the size of the aliquot represented a somewhat smaller sample than specified in the Official Method. Silica 'Received September 26, 1930. Presented before the Division of Fertilizer Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12,1930.
dishes 4 inches (10.16 cm.) in diameter and with flat bottoms were used, except where platinum is designated. These dishes were in regular use and in good condition. For each comparison, aliquots were taken from the same flask so that variations in making up the solution would not affect the reTable I-Comparative
Potash Results under Different Ignition Conditions POTASSIUM
KINDOF PaOa IN CHLOROPLATINATE FERTILIZER ALIQUOT 4-24-12
Gram
Gram
0.0438
0.2750 0.2764 0.2752a 0.2724
4-24-12
0.0232
4-24-12
....
0-24-24
0.0242
0.1200 0.1178 0.1180 0.1170 0.1092 0.2344 0.2350 0.2282
0. 2326a 0.2326 0.2338 0.1175 0-24-24 n iim 0.1167 4-24-4 0.0511 0.0922 0.0904 0 . 089Za 2-14-4 0.0050 0.0810 0.0772 2-12-6 0.0064 0.1222 O.ll60 0.0852 2-10-4 0.0802 Laboratory 0.2640 0.1246 sample: b.0 KC1 85% HsPO4 0.1306 0.1084 0.0203 a 0.1287
....
IGNITION PERIOD Min. Veryfaintred, Cone022 (590OC.) 3 Dark cherry red, Cone 014 (830' 5 C.) Dark cherry red 5 Very bright cherry, Cone 012 (890O C.) 6 Low red 3 Bright cherry red 3 Lowest red 2 HEATOF IGNITION
Full red Lowest red Dark cherry red Bright cherry red, Cone 011 (920' C.) Bright cherry red Full cherry red no HnSO4 added Lowest red, no 'H&Ok added Lowest red
10 3
4
a
4 4
~
4
2
Bright red Lowest red Bright cherry red Bright cherry red Lowest red Bright cherry red Lowest red Bright cherry red Lowest red Full red Just below white (above 900' C.)
lo
Just below white (above 900' C.)
8
0.1114 Just below white (above 900' C.)
13
. . ..
3 4 4 4 10 4 10 2 10 2
+
O>b
0.1316 Ignited in platinum. b Additional potash found in silica dishes by boiling with hydrochloric acid. Dishes used were badly damaged, and from 25 to 29 mg. of silicon dioxide were present with the potassium chloroplatinate precipitate. a
ANALYTICAL EDITION
408
sult. I n transferring the ignited residue from silica to porcelain, the solution was not filtered, but the potassium chloroplatinate was dissolved in hot water after weighing, so that the net weight was obtained. The aliquots were evaporated on the water bath, fumed off over Gilmer heaters a t a low temperature, and then placed in the electric furnace a t the temperature and for the period stated. It was found that 2 minutes in the electric furnace removed practically all of the ammonia, when various types of dishes were used. Even where the lowest visible red heat was used for 2 minutes only, the maximum amount of ammonia found was 0.2 mg. It may be noted that the Alternative Method (1) for potash specifies: “Ignite over a free flame beIow a red heat until all volatile matter is driven off .” Early in these determinations it was noted that after ignition small amounts of potash might sometimes be left undissolved in the silica dishes. Unusual care was used in dissolving the ignited residue, particularly in regard to the time. All of the dishes were tested for potash by boiling out with hydrochloric acid, and amounts of potash varying from none to 0.4 mg. of potassium chloroplatinate were found. These amounts were included with the amounts reported. In one case where a high temperature was used, 1.0 mg. of potassium chloroplatinate was noted. The ignited residue, although somewhat slow in dissolving, can be readily dissolved in a reasonable time, especially if the ignition has been made at a low temperature. Five minutes were found to be ample for solution before transferring to porcelain. I n ignition where phosphates are present, the phosphoric
Vol. 3, No. 4
acid may replace in part the sulfuric acid which is more volatile at high temperatures. Low-temperature ignition prolongs the life of silica dishes and reduces the amount of silica loosened from the dishes. I n this series of analyses, the maximum amount of silica noted in the precipitated potash was 3.2 mg. where a high temperature was used with triple superphosphate mixtures. The usual amount of silica varied, with low-temperature ignition, from none to 0.4 mg. Dishes deteriorate more rapidly with triple superphosphate mixtures. All mixtures containing acidulated phosphates normally contain phosphates in the potash solution. Ignition residues containing triple superphosphate melt at a very low temperature, forming a glassy instead of a crystalline or granular residue. Although few duplicate analyses are shown, no exception to low results with high-temperature ignition was obtained. As the only object of ignition is to remove ammonia and organic matter, the results with low-temperature ignition are probably nearest the true potash content. It is suggested that low-temperature ignition be stressed in any future study of potash methods for mixed fertilizers, and that adequate time be given for complete solution of the ignited residue. Literature Cited (1) Assocn. Official Agr. Chem., Methods, p. 15 (1925);also references cited below. (2) Bible, C. M., J . Assocn. Oficial Agr. Chem., 8,420-3 (1925). (3) Fraps, 0.S.,Ibid., 9, 192 (19261. (4) Haigh, L. D.,Ibid., 10, 220-3 (1927). (6) Haigh, L. D.,I b i d . , 11, 219-20 (1928). (6) Kerr, A. P.,Ibzd., 8, 419-20 (1925).
Practical Methods of Detecting and Estimating Methyl Chloride in Air and Foods With a Description of a Modified Combustion Method’ Mathew J. Martinek and Wm. C. Marti BUREAUOP LABORATORIES AND RESEARCH, CHICAGO DEPARTMENT OX HEALTH, CHICAGO,
Comparative studies of available field tests for methyl chloride in air showed the copper gauze-flame test to be the most efficient and rapid. A study of laboratory methods for the qualitative detection of methyl chloride in miscellaneous samples resulted in developing a practical combustion method which proved satisfactory. It is sensitive to about 50 parts of methyl chloride per million parts of air. For quantitative work, a modified combustion method
ILL.
was devised in which the methyl chloride gas, diluted with air, is oxidized, and the halogen products of combustion3. e., the hydrochloric acid gas and the free chlorine and its oxides-are absorbed in a solution of sodium carbonate and arseniousacid. The resultant chloridesare precipitated with silver nitrate and titrated by a very sensitive modified Volhard method, the accuracy of the method being about 50 parts per million. It is applicable to various types of samples, including air, food materials, urine, etc.
. .. .. HE recent occurrence of poisonings and fatalities from methyl chloride gas used in mechanical refrigerating systems in Chicago (2) made it necessary to seek practical tests for the rapid detection of this gas and to develop and perfect accurate quantitative methods for its estimation in air and miscellaneous substances. I n this report the subject will be treated under two headings, field methods and laboratory methods, with special attention to the development of a modified combustion method.
T
Field Methods
A method commonly used by service men for detection of gas leaking from refrigerating systems is to coat the pipes f
Received May 26, 1931.
with a film of soap, using a shaving brush and shaving stick to produce a lather, Wherever a leak occurs there will be a formation of bubbles. This test is useful but is not suitable for detecting leaks in inaccessible places. In refrigerating systems containing a mixture of methyl chloride and sulfur dioxide, the test used is to pass an open bottle of ammonium hydroxide along the pipes, the formation of a white cloud indicating a leak. I n small concentrations where the gas cannot be detected by odor, the copper flame test for organic halogens was found to be the most sensitive and convenient. The following apparatus has been adopted as an official field test by the Inspection Bureau of the Chicago Department of Health: A small alcohol lamp (Figure l), fitted with a cone of copper gauze, is constructed as follows: A wide-
