HYDROGEN SULFIDE * e * * e PRECIPITATION @
L. J. CURTMAN AND S. M. EDMONDS The City College of the College of the City of New York
Pressure and non-pressure methods of hydrogen sulg'i.de precipitation have been subjected to experimental study in order to determine their relative merits. The conditions have been found under which the pressure method may be used (in conjunction with a central gas supfly system) by students in quulitatiwe analysis. The shortcomings of the non-pressure method under the same conditions are pointed out.
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HERE are two general methods for precipitating sulfides with hydrogen sulfide. These are: (1) the pressure method, usually carried out in an Erlenmeyer flask and (2) the non-pressure method, generally conducted in a beaker. The purpose of this investigation was to ascertain the relative merits of the two methods, particularly when applied to student work. PRESSURE METHOD
In this method the gas is not bubbled into the solution but absorbed over it under the full pressure of the generator. The method offers so many advantages over that of direct bubbling into the solution, that it has been generally adopted for all quantitative sulfide precipitations. When used by the experienced analyst the method unquestionably possesses the following favorable features: (1) rapidity of precipitation, (2) certainty of saturation, (3) economy of gas, (4) prevention of the escape of the gas into the laboratory. The apparatus suitable for this method is shown in flask Figure 1. I t consists of a 250-cc.
FIGURE1
bottle by means of a piece of black rubber tubing about six inches in length. The precipitation throughout this work is carried out as follows. The solution, the volume of which does not exceed 100 cc., is introduced into the flask and the stopper is tightly inserted. The screw clamp is opened and a rapid stream of the gas is allowed to pass through for 30 seconds to insure the complete displacement of air over the solution by the gas. The screw clamp is then closed and, with the wash-bottle stopcock open, the flask is gently shaken by means of a swirling motion, care being exercised not to shake so vigorously that the solution wets the upper wall of the flask or the stopper. The points to be considered in the study of this method are: (1) its adaptability to a central gassupply system, (2) its advantages and disadvantages in oreci~itatine . " Grouos 2 and 3, resoectively. Earlv in the work it becameapparent that k would-be necessary to consider separately the application of the method to the precipitation in acid and alkaline solutions. Adaptation to a Central Gas Sufifly System. In all the experiments to he described, a large Breithut generator1 was employed. It was provided with a main supply tube from which branched six separate outlets, one in each of six hoods. When the gas is absorhed over the solution a wash bottle is needed at each outlet. The ordinarv wash bottle filled with water to a depth of about an inch was permanently connected a t each outlet. ~ h design , of a suitable device for controlling the flow of gas a t each outlet is of paramount importance. Experience over a period of years has shown that the only safe and reliable method for gas regulation when the gas is to be bubbled into a solution is the simple valve consisting of a glass bead in the rubber outlet tuhe. Stopcocks and pinch clamps have the very serious disadvantage that elementary students are prone to be forgetful in turning them off. As a consequence, any steady large drain of the gas from an H2S generator, aside from being wasteful and injurious to health, may lead to an explosion in the generator due to the development of excessive pressure. It was thought that bead valves attached to the wash bottle a t each outlet would solve this difficulty. However, with this arrangement students were frequently misled into assuming saturation and complete precipitation long before this condition was actually attained. The method proved so troublesome and TmTMAN, I,. J., "Qualitative & e m i d analysis," The Mac-
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provided with a 2-hole stopper. Through one hole a right-angle delivery tube extends just below the stopper while through the other there is inserted a straight glass tuhe about one inch long and fitted with a short piece rubber a screw The delivery tuhe is connected to the hydrogen sulfide wash millan Co.. New ~ o r City, k 1931, p. 504. 305
undependable that the beads were finally discarded and as nickel nitrate, 2 g. of ammonium chloride, and 2 cc. glass stopcocks substituted. To permit their use, it of concentrated ammonium hydroxide was heated to became necessary to make the generator system safe boiling and treated with HzS by the pressure method. against any large drain occasioned by a steady leak Gas absorption was rapid for five minutes; thereafter, through a stopcock carelessly left open. The insertion the rate of flow of the gas diminished very slowly until of one-inch capillaries (0.5 mm. bore) in the rubber at the end of 15 minutes it was one to two bubbles every tubing connecting each wash bottle to the main pipe thirty seconds. In student work the time for saturation (one bubble in 30 sec.) usually extended to 20 to 25 line effected this admirably (see Figure 1). Under these conditions a steady stream could then be minutes and was never less than 15 minutes. It was had at each outlet and, with a full generator supplied therefore of considerable interest to determine whether with fresh acid, the stream was slow enough to permit or not there is any need for saturation under these one or two outlets to remain open continuously with conditions. The rate of gas absorption falls off so perfect safety. The use of capillary tubing serves slowly and so gradually with time that it is not possible also as an efficientpressure regulator, for a sudden drain to perceive a decided lessening of flow that might at any one of the outlets has no effect on the rate of gas yield some indication of the completeness of precipitadelivery a t any of the others. This becomes a very tion. (2) Determination of Time Required for Cmfilete important consideration when the generator is being used by many students at one time since changes in the Pren'pitatwn. One hundred cc. of solution containing rate of gas-bubbling through the wash bottles causes 500 mg. of the metal ion (Ni, Co, Fe, Mn, or Zn), 2 g. of ammonium Chloride, and 2 cc. of concentrated amconsiderable difficulty in testing for saturation. monium hydroxide was heated to boiling and treated CROUP 2 PRECIPITATION with hydrogen sulfide. In all cases, three minutes In order to study the precipitation with hydrogen were found sufficient to allow for complete precipitasulfide in acid solution, 500 mg. of copper as copper tion. At the end of this time the gas absorption was nitrate in 100 cc. of 0.3 M HCl were introduced into still quite rapid. In numerous student experiments on a 250-cc. Erlenmeyer flask and the method of pre- known solutions of Group 3 containing 100 mg. each of cipitation outlined above applied. Many trials showed Al, Cr, and Zn, and 50 mg. of Fe, Ni, Co, and Mn, comthat precipitation was complete within three minutes. plete precipitation was attained in three minutes. Since the rapidity and completeness of precipitation At the end of this time absorption of the gas into the solution through the wash bottle was still quite rapid, might be expected to depend, to some extent, on the indicating that the solution was still unsaturated. gas pressure in the generator system, it was important However, once the rate of absorption had noticeably to determine the r61e of this factor. Tests were made fallen from its high initial rate it soon decreased to after the generator was filled with 1:l HC1 and also about one bubble every 30 seconds. From then on, toward the close of the laboratory period when the acid absorption continued with one or two bubbles per was nearly spent. The rate of gas absorption and time minute for a very considerable time before true satura- for complete precipitation were the same under both tion was reached. With this arbitrary test for satura- conditions. It is thus evident that it is only necessary to control tion (one bubble every 30 seconds), 100 cc. of a 0.3 M HC1 solution containing 500 mg. of copper was satu- the time for precipitation. The two other possibilities rated in from 5 to 5l/* minutes. In all cases, as was to were nevertheless tried out with students. When a student was directed to continue precipitation until the be expected, precipitation was complete. The striking advantage of this method,. when em- bubbling became very slow, the time consumed was ployed by the experienced analyst, is the certainty generally from 10 to 15 minntes. When the student with which complete saturation and precipitation are was merely directed to continue the precipitation until indicated by the cessation of the gas flow. I t was of the bubbling through the wash bottle ceased, the test considerable importance to determine whether the being a rate of a bubble every 30 seconds, then, as method still retained this favorable feature when noted above, the process became time-consuming becarried out by the average student in elementary quali- yond all reason. Transfer of the Precipitate. In connection with the tative analysis. Fifteen students, chosen a t random, pressure method it is of interest to consider the question analyzing typical Group 2 unknown solutions required from 5 to 7 minutes to saturate their solutions (test: of transferring the sulfide precipitate and the conseone bubble in 30 seconds). In all cases complete quent cleaning of the precipitation flask. Students precipitation was attained. In numerous other trials were directed to transfer the precipitate to the filter the time for saturation was from 5 to 6 minutes and in with the aid of water. One or two treatments with some few cases as much as ten minutes. Complete about 10 cc. of wash liquid is sufficient to transfer all of a large sulfide precipitate except a thin film that precipitation, however, was always obtained. frequently adheres to the walls of the flask. Although AMMONIUM SULFIDE CROUP PRECIPITATION it is needless for qualitative purposes to collect the very ( 1 ) Determination of Time for Saturation. One small amount of sulfide constituting the film, neverhundred cc. of solution containing 500 mg. of nickel theless it is necessary to provide a convenient method
for cleaning the flask. In many cases i t is difficult or impossible to clean the flask mechanically; in such cases the use of hot aqua regia or HCl is recommended. The thin sulfur film that sometimes remains can then be easily removed with a properly bent test-tube brush.
When the beaker method is employed the time required for complete precipitation depends upon the rate of flow of hydrogen sulfide into the solution. In class use this rate depends, to a very considerable extent, on the freshness of the acid in the generator as well as on the number of gas outlets open a t the time. CONCLUSIONS The above experiments were made under ideal condiPrecipitation in 0.3 M HC1 Solution. The pressure tions, using fresh acid and with only one outlet open. method proves highly successful with students in eleAnother method sometimes recommended for prementary qualitative analysis, using a central supply cipitation with hydrogen sulfide is a combination of the system for hydrogen sulfide, provided the following two methods considered above. The solution to be conditions are observed: treated is contained in an Erlenmeyer flask; the latter (1) Each outlet must be provided with a wash bottle is stoppered with a two-hole rubber stopper, through of the design suggested and supplied with stopcock and one hole of which passes the gas delivery tube extending capillary as indicated in the diagram. to the bottom of the flask. The gas is bubbled through (2) The student should be directed to continue pre- the solution for some time; it is then tested for saturacipitation until the gas absorption, as indicated by the tion by closing the second hole in the stopper with the bubbles passing through the wash bottle, has fallen to thumb and shaking the flask. The solution is conone bubble in 30 seconds. sidered saturated when upon closing the hole and Precipitation in NfiOH Solution. Under the above shaking, the gas no longer continues to bubble into conditions the pressure method becomes unnecessarily the solution. The actual time required to precipitate confusing and wasteful of time when applied to a Group 500 mg. of copper in 100 cc. of 0.3 M HCI was three 3 precipitation. However, by setting an arbitrary time minutes. However, only after 14 to 16 minutes' treatlimit of 3 minutes for precipitation, perfectly satis- ment with a fairly rapid stream of the gas was the factory results may be obtained. The conditions solution saturated as indicated by the test. When chosen are those generally in use for Group 3. If the employed by students, the test recommended for conditions are widely different it is then necessary to saturation is very likely to lead to confusion and to determine the minimum time required for complete extend unnecessarily the time of treatment. It offers precipitation. one advantage over the beaker method by providing NON-PRESSURE METHOD some indication of saturation. However, this test so In order to compare the relative merits of the pres- prolongs the precipitation time with consequent waste sure and non-pressure methods the experiments already of gas that the method can hardly be recommended. described with the former were repeated with the SUMMARY latter. The test solution, contained in a 250-cc. beaker, was treated with a rapid stream of the gas for various 1. A comparative study has been made of the two periods of time, the mixture filtered, and the filtrate general methods for precipitation with hydrogen tested for complete precipitation. The gas was introsulfide using a central gas supply system: the duced into the solution through a small piece of 3pressure method in which the gas is absorbed over mm. glass tubing connected by means of rubber tubing the solution and the non-pressure method in which to the hood outlet of the central gas supply system. the gas is bubbled directly into the solution. The rate of flow was controlled by means of a bead 2. The pressure method can be successfullyadapted to valve in the tubing. The results showed that in class use when certain necessary modifications are Group 2 precipitation is complete in from 3 to 4 introduced. Only under these conditions is the minutes while in Group 3 complete precipitation is atmethod to be recommended as preferable to the tained in from 4 to G minutes. non-pressure method.
