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IA'D UXTRIAL A N D ENGIA'EERING CHEMISTRY
Vol. 16, No. 1
Magnesium Perchlorate Trihydrate',2 I t s Use as Drying Agent for Steel and Organic Combustion Analysis By G.Frederick Smith, Morris Brown, and John F. Ross UNIVERSITY OF ILLINOIS, U R B A N A ,
ILL., AND UNIVERSITY O F MICHIGAN, ANN
ARBOR,MICH.
T
the following description, HE preparation of anMagnesium perchlorate trihydrate is so eficient a dehydrating magnesium perchlorate trihydrous magnesium agent that it may be used in the place of phosphorus pentoxide, and hydrate was found best. Perchlorate and its oboiously of other less e c i e n t drying agents, in the combustion trihydrate was described method for determining carbon in steel and carbon and hydrogen PREPARATION by Willard and Smith,2and in organic compounds. I t will absorb 30 per cent of its weight of it was shown that as drying Ammonium perchloratewater. I t is easily and cheaply prepared and shows other adoantages of reasonable purity can be agents they were ComParooer the drying agenfs now commonly used. able in efficiency with obtained on the market a t phosphorus pentoxide. A the present time a t a moder-substitute for phosphorus pentoxide in the absorption train ate price. This material serves admiGably for the preparation used for the determination of carbon in steel by direct of perchloric acid following themethod of Willard.3 The percombustion is desirable. The difficulties in handling the chloric acid solution thus obtained, without being distilled, pentoxide, its cost, and rapid deterioration during use are is slightly contaminated with nonvolatile impurities, chiefly marked disadvantages. Because the rate of flow of oxygen potassium perchlorate. The acid is concentrated in porcein steel combustions is usually high, no satisfactory substi- lain dishes until the temperature rises to 155" to 160" C . , tute for it has been found. For carbon and hydrogen de- or until it fumes strongly and an acid concentration of 68 termination in organic compounds by direct combustion, to 70 per cent is obtained. It is then ready for conversion concentrated sulfuric acid or calcium chloride is often used, to magnesium perchlorate. but the former has the disadvantage of being a liquid. A thick paste is made of finely powdered magnesium oxide The object of the present paper is to describe an improved or carbonate, by mixing with water. This is added in several method of preparing magnesium perchlorate trihydrate portions to the partially cooled perchloric acid contained in and of its successful application to the determination of car- the same porcelain dish used for its concentration. By this bon in steel and of hydrogen and carbon in organic compounds procedure a too vigorous reaction is prevented. A slight by the combustion method. excess of magnesium oxide or carbonate is added, followed by enough perchloric acid to dissolve the excess. The magCHOICE OF MATERIAL nesium perchlorate solution thus obtained is concentrated Magnesium perchlorate, when crystallized from water by boiling until crystallization sets in, and then cooled to. solution a t ordinary temperature, is obtained as the hexa- room temperature, a semifluid crystal mass being obtained. hydrate containing 32.62 per cent of water of crystallization, The crystals are drained on a Buchner funnel and centrifuged. and has a melting point between 145" and 147" C. The The magnesium perchlorate hexahydrate thus obtained is trihydrate of magnesium perchlorate containing 19.50 per dried to 110" to 120" C. to remove excess of water, and is cent of water was obtained by drying the hexahydrate over finally dehydrated to magnesium perchlorate trihydrate phosphorus pentoxide in vacuo a t ordinary temperature for by heating in a good vacuum a t 138" to 140" C., just below long periods of time. At 0" C . this does not occur. An- its melting point, during 15 to 20 hours. The completion hydrous magnesium perchlorate is prepared by heating either of the dehydration is readily determined by the loss in weight. the hexahydrate or trihydrate to 250" C. a t ordinary pressure, The product used in connection with this paper was analyzed" the pure anhydrous material being perfectly stable under by conversion to magnesium sulfate, and found to contain these conditions. 19.75 per cent of water, which is within 0.25 per cent of the Magnesium perchlorate holds its first three molecules of theoretical value for the trihydrate. water of crystallization perhaps more firmly than any other The hexahydrate, when dehydrated as described, retains salt. Since the anhydrous material acquires water as water its original crystal form, and is therefore extremely porous, of crystallization and not of chemical combination, as with requires no crushing and sifting to reduce to a suitable phosphorus pentoxide, it is not to be expected that the value fineness, and the small amount of impurity it contains does of both materials as drying agents would be equal. In the not cause the slightest decomposition at the temperature usual form of drying tubes, anhydrous magnesium per- employed in its preparation. When fused and heated to 180" chlorate is as effective as phosphorus pentoxide in the re- C., even in a high vacuum, there is no further loss of crystal moval of water from air saturated with moisture a t 25" C., water or decomposition. If the temperature is further provided the rate of gas flow does not exceed Yj liters per hour.2 raised to 250" C. a t atmospheric pressure, dehydrakion to, Magnesium perchlorate trihydrate is equally effective a t anhydrous magnesium perchlorate is slowly accomplished, 0" C., and very nearly so a t 25O C . Both anhydrous mag- but considerable decomposition results, chlorine and oxide of nesium perchlorate and its trihydrate were thought to have chlorine being given off , the product obtained being alkaline distinct possibilities in providing a cheap reagent which in reaction and not completely soluble in water. This decould be easily handled, and which would be very high, hydration to the anhydrous form a t 250" C. can be much not only in dehydrating efficiency, but also in absorptive hastened by being conducted in a vacuum, when the fused power. In choosing between these two materials, various material gives up its water of hydration rapidly enough to factors must be considered. For reasons made clear in cause ebullition, a porous, solid cake of anhydrous magnesium perchlorate finally resulting. Because it is necessary to pre1 Received July 26, 1923. Presented before the Division of Industrial and Engineering Chemistry at the 66th Meeting of the American Chemical pare a very pure material in order to prevent decomposition Society, Milwaukee, Wis., September 10 to 14, 1923. 1 J . A m . Chem. SOG.,14, 2255 (1922).
a J . A m . Chem. SOC.,39, 1480 (1912).
January, 1924
INDUSTRIAL AND ENGINEERING CHEMISTRY
at 250' C., and because the trihydrate itself gives perfectly satisfactory results, anhydrous magnesium perchlorate was not used in the analyses described below. No violent reactions were encountered in the preparation of either the trihydrate or anhydrous magnesium perchlorate; kilogram lots of material were prepared in one operation and practically 100 per cent yields were obtained.
DETERMINATION OF CARBONIN STEEL Standard steels supplied by the U. S. Bureau of Standards were analyzed, using magnesium perchlorate trihydrate in place of the phosphorus pentoxide generally employed. For these determinations the directions supplied by this bureau were strictly applied. Standard equipment, reagents, and apparatus were used, with the single exception of the magnesium perchlorate drying agent. The results of the analyses are listed in Table I. DETERMINATIONS I N S T E E L B Y DIRECTCOMBUSTION USIYG MAGNESIUM PBRCHLOBATE TRIHYDRATE AS DRYING AGENT Carbon B. of S. Certificate Carbon Sample Value Sample cos Found No. Percent Grams Grams Per cent 9b 0.184 2.0013 0.0132 0.180 9b 0.184 0,0099 0.180 1.5020 9b 0.184 0.175 2.0002 0.0128 0.414 0.414 2.0020 0.0304 1oc 1oc 0.414 0.411 1.5005 0.0226 1.5005 0.414 0.416 0.0229 1oc 220 0.577 0.581 0.0213 1.0000 220 1,5005 0.577 0.573 0.0315 220 0.677 0.578 0.0318 1.5000 146 0.814 0.0448 1.5015 0.817 14b 1.0012 0.813 0.0298 0.817 14b 1.0005 0.818 0.0300 0.817 35 1.0023 1.031 0.0379 1.03 35 1.0020 1.034 0.0380 1.03 35 1.034 0.0380 1.03 1.0020
T A B L E Y-CARBON
The teniperature was 1075" to 1125" C. The silica combustion tube was 60 cm. long and of 22 mm. inside dismeter. Twenty-five centimeters of the length of combustion tube was filled with a glass rod in order to diminish the volume of gas to be displaced following each combustion. Kickel boats were used, with a roll of copper oxide toinsure climplete Combustion. A Sargeant absorption tube for absorbing the carbon dioxide was used, one of the simpler types of tubes being selected in order to test the efficiency of the magnesium perchlorate under the least favorable conditions. Ascarite4 was employed to absorb the carbon dioxide, since it allowed a high rate of gas flow and therefore a more severe test of the perchlorate drying agent. The oxygen used was passed through the following units in order, starting from the tank: carbon dioxide absorbent, 9-cm. column; magnesium perchlorate trihydrate, 7-cm. column; combustion tube; 5-cm. tube of granular zinc; magnesium perchlorate trihydrate, 8-cm. column; Sargeant carbon dioxide absorbing tube having 6-cm. column of carbon dioxide absorbent and 6-cm. drying tube charged with magnesium perchlorate trihydrate. A small weighed tube containing phosphorus pentoside was used a t the exit end of the Sargeant tube to check the completeness of removal of water from'the gases leaving the absorption train. The rate of oxygen flow for the various analyses, as determined by the use of a flowmeter in the train, was 170 to 200 cc. per minute. By use of this flowmeter a blank correction, which was accurately determined, could be always applied. By using a previously burned nickel boat, this blank was found to be 0.2 mg. per analysis. The time used in single carbon determinations reported in Table I, from the time the steel sample was placed in the combustion furnace until the absorbing apparatus was detached for weighing, was 10 to 12 minutes. With the apparatus used this time could have been diminished considerably 4 The trade name of an absorbent for carbon dioxide, which is a sods dime-asbestos mixture.
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without altering the results. By use of this time interval a more severe test of the drying agent was made. The carbon dioxide absorbent used is thought to have as high absorptive power and efficiency as any available carbon dioxide absorbent. But one charging of the Sargeant tube was necessary for the analysis of Table I. After these fifteen determinations the carbon dioxide absorbent was approximately half saturated, as indicated by color change in the spent portion. The perchlorate drying agent was apparently less dissipated, as indicated by its physical condition. Actual tests in sufficient number to establish the point were not made because of the large number required, but it is believed that the life of the drying agent described is practically equal to that of the carbon dioxide absorbent employed when used in the popular forms of absorption apparatus for the determination of carbon in steel. The results in Table I show that magnesium perchlorate trihydrate is fully equal to phosphorus pentoside as a drying agent in the determination of carbon in steel. This is also shown by the fact that there was no increase in weight of the phosphorus pentoxide tube at the end of the train. The use of magnesium perchlorate trihydrate as a substitute for phosphorus pentoxide as drying agent is accompanied by the following advantages: 1-Facility in charging, since the drying agent does not become sticky while momentarily in contact with atmospheric moisture. 2-Channels do not form in the salt and decrease its efficiency. 3-The deterioration of the reagent is accompanied by contraction in volume, thus reducing the tendency of drying tubes t o clog. 4-On account of its porous nature, it offers almost no resistance to the passage of gas. 5-Cotton can be used as plugs. 6-The spent reagent can be readily recovered, if desired. 7-The reagent has large absorptive capacity, as will be subsequently shown.
DETERMINATION OF HYDROGEN I N ORGANICCOMPOUNDS A drying agent employed in determining carbon in steel by direct combustion must be efficient a t high rates of gas flow. A drying agent for use in determining hydrogen by the combustion of organic compounds must also have a large capacity t o provide for continued use without renewal. The remainder of this work deals with the use of magnesium perchlorate trihydrate in meeting this added requirement. A number of pure organic compounds were analyzed for carbon and hydrogen by combustion, following the usual method. Phosphorus pentoxide was first used in absorbing the water evolved. Magnesium perchlorate trihydrate was then substituted for phosphorus pentoxide in the analysis of the same compounds. Potassium hydroxide solution was used in both cases to absorb carbon dioxide. Standard equipment and apparatus was employed throughout. The results of these analyses are reported in Table 11. TABLEII"-COMPARISONOF PHOSPHORUS PENTOXIDE AND MAGNESIUM PERCHLORATE TRIHYDRATE AS DEHYDRATING AGENTSIN THE COMBUSTION
-
ANALYSIS OF ORGANIC COMPOUNDS Using Using PzOs and KOH Mg(ClOa)a.3HzO -Theor SUBSTANCE %H %C %H %C %H Succinic acid 5 . 0 3 40.39 5.05 4 0 . 3 9 5 . 0 7 40.60 Resorcinol 5 . 4 1 65.32 6.43 65.34 5 . 4 5 65.43 Benzoin 5 . 6 4 79.19 5.65 79.21 5.70 79.22 Anthranilic acid 5 . 1 3 61.24 5 . 1 7 61.24 5 . 1 5 61.35 #-Nitromethylbenzoate 3 . 8 8 53.01 3 . 8 7 5.1.04 Salicylic acid 4 . 3 2 60.84 4.30 60;85 Palmitic acid 12.56 74.86 12.68 74.91 Tetracosane, m. p. 51' C. . 14.60 85.02 14.88 85.12 The analyses reported here are the mean of from four to six separate closely agreeing determinations.
dC
..... . . . ...... .... .... ..... . . . ... . .
It will be seen from Table I1 that the use of magnesium perchlorate trihydrate in organic combustion analysis leaves little to be desired as regards accuracy. A U-tube commonly employed with calcium chloride as drying agent, when charged
Vol. 16, KO. I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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with 5 to 6 grams of magnesium perchlorate trihydrate, absorbed 15 per cent of the weight of absorbent before the tube became clogged. A drying tube was designed to increase the capacity of the reagent for absorbing water. (Fig. 1) This absorption tube was charged with 13 grams of magnesium perchlorate trihydrate and was used in the analysis of the hydrocarbon tetracosane until
FIG.1
passage of gas was obstructed. Twelve analyses of this material were made, during which 4.364 grams of water were evolved, the capacity of the drying agent being thus over 30 per cent of its weight. The absorption of this much water would correspond to 40 to 50 ordinary combustions. Magnesium perchlorate trihydrate was used for all parts of the preliminary absorption train, as well as the final absorption train. It served admirably in the drying tubes of popular types of potash bulbs. The perchlorate drying agent in the purification train of an organic combustion outfit can be used where sulfuric acid is used as drying agent in the preliminary absorption train without sacrifice in accuracy. Until supplied in the usual trade channels, a limited supply of magnesium perchlorate trihydrate for use as drying agent may be obtained through the Division of Organic Manufactures of the University of Illinois, by communication with the authors.
Apparatus for Maintaining a Constant Pressure of Gas' By David F. Smith GATESCHEMICAL LABORATORY, CAI.IFORXIA INSTITUTE
T H E maintenance of a constant pressure of gas is an important requirement for many scientific purposesfor example, in the use of flowmeters and indirectly for the purpose of maintaining a constant temperature. Thus one of the best ways to maintain constant and uniform moderately high temperatures is by means of liquids boiling a t various constant pressures. It was mainly for the latter purpose that the apparatus herein described was devised. The arrangement and operation of the apparatus a t pressures below atmospheric may be seen by reference to the accompanying figure. To the ordinary mercurial manometer A B with the arm A evacuated above the mercury level and extending upward to any desired height are attached the sealed-in platinum leads a t B. These leads form a continuous circuit through the mercury with a cell and the electrical relay C, which in turn controls the solenoid D in series with several cells. The solenoid D, when energized, draws down the soft iron plunger E, fitting loosely in the glass tube EF, opening the common bicycle valve which has been sealed inside the same glass tube with wax. Connections to a vacuum pump and to the apparatus are as shown in the figure. G is a tube with a capillary opening allowing a very slow passage of gas between the atmosphere and the interior of the apparatus. To adjust the apparatus to maintain any desired pressure, the 3-way stopcock is connected a t H to a leveling bulb filled with mercury. Communication between the leveling bulb and the arm B is established by means of the cock, and the level of mercury in arm B is adjusted precisely a t the upper platinum contact in B. Communication is then established between H and the arm A , and the level of mercury in the arm A is adjusted to the desired height. After connecting to the apparatus and pump as indicated in the figure, the cock is turned so as to connect the mercury in arms B and A. When the pressure inside the apparatus becomes greater than that corresponding to the adjusted difference in height of the mercury levels in A and B , the electrical circuit through the relay C is opened by the movement of the mercury in the manometer, the bicycle valve is opened by the 1
Received October 8. 1923.
OF
TECHNOLOGY, PASADENA,
CALIF.
solenoid D, and the pump evacuates until the contact a t B closes. The slow leak a t G assures that the pressure within the apparatus never becomes lower than the predetermined value. The foregoing operation is for pressures below atmospheric. To maintain pressures above atmospheric, the positions of bicycle valve and plunger a t F E are inverted, the connection to the pump being replaced by a connection to the large bottle and a pressure (greater than the desired pressure) applied below the valve a t F. A relay which makes rather than breaks contact when energized is also substituted a t C.
A SOFT IRON PLUNGE
This apparatus, which has been in use in this laboratory for many months, will maintain a pressure which is constant to less than 1mm. of mercury over long periods of time. For very close regulation it is desirable to have as small volume as possible between E and the pump, and also to have the reduced or elevated pressure a t the pump as near as convenient to the pressure desired inside the apparatus in order to avoid a sudden change of presmre when the bicycle valve opens. It is evident that the apparatus is adaptable for use either with air or with some inert gas such as nitrogen, which would be desirable, for example, with boiling organic liquids which are oxidized by air.
