US1944738A - Method for controlling oxidation of halide solutions to liberate free halogen - Google Patents

Description

Jan.- 23, 1934. J. J GREBE ET AL 1,944,738

METHOD FOR CONTROLLING OXIDATION OF HALIDE SOLUTIONS TO LIBERATE FREE HALOGEN Filed July 31, 1930 HI UN Hlc 301 I l i i W -H /HgQ1 g Pas a. e i

4 "0 50 5F loo IO 20 .30 0 .5? G0 ereznfaae. Bra/"rune Liberated F 9- INVENTORS BY M ATTORNEY Patented Jan. 23,1934

METHOD FOR CONTROLLING OXIDATION OF HALIDE SOLUTIONS FREE HALOGEN TO LIBERATE John J. Grebe and Ray H. Boundy, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation o1'Micheigan Application July 31, 1930. Serial No. 472,027 4 Claims. (01. 23-21'0' This invention relates to a method for controlling the oxidation of halide-containing solutions for the liberation of the halogen in the elemental state, and has particular regard to a method permitting an immediate and continuous indication .to be made of the progress of the oxidation which shows directly the degree of liberation of bromine at any time.

Heretofore the control of bromine liberation, for example, has been based upon chemical analytical methods for determining bromide in the presence of chloride ions, such methods involving tedious manipulation and requiring from hour to 1 hour or more for carrying out. The very considerable time lag between the treatment of the bromide-containing solution and the determination of the result has made virtually impossible any close or immediate control of the oxidation. The usual practice, consequently.

' blowing or steaming out, is analyzed to determine by the residual bromine content thereof whether any material loss has been incurred in the ex,-'

traction. Obviously, such analytical method does not prevent a loss but merely indicates the fact ing solutions which is based upon the oxidation potential of free bromine in contact with a solution of its ions. Our improved method enables a continuous indication and record of the progress of the oxidation to be made contemporaneously therewith and substantially eliminates the time lag which has seriously limited and handicapped all chemical control methods. By providing suitable means responsive to the aforesaid indication an immediate and positive control of the flow of materials to the reaction may be maintained continuously during the process. To the accomplishment of the foregoing and related ends, the invention, then, constats in the method hereinafter fully described and particularly pointed out in the claims, the annexed drawing and following description setting forth in detail but a few of the various ways in which the principle of the invention may be used.

In said annexed drawing:-

Fig. 1 is a diagrammatic representation-of an apparatus adapted for determining the oxidation potential of a solution in which bromine is being liberated. Fig. 2 is a chart showing the relationship between the observed potential and the degree of water.

The oxidation', ,or solution, potential of a nonmetal in contact-with a solution of its ions at 25 C. is expressed by the formula;

liberation of free bromine in sea where E=E. M. F. in volts; eo=E. M. r. in volts of a solution of normal ion concentration; 1L=

valence of the ion, and c=concentration of ions in gram ions per liter, The potential E increases as the ionic concentration of the solution decreases. When free bromine is present in a solution of .bromide ions there exists a certain definite potential between the molecular bromine and Br. ions which is capable of being measured. This potential is represented by avalue which is characteristic for bromine and differs from the potential of other negative ions, e. g. 01- ions,

being lower than the value for the latter ions under equivalent conditions and higher than the value for other non-metallic ions. The actual value thereof depends primarily upon the concentration of bromide ions present, but also is influenced by the simultaneous presence of other ions and by the property of free bromine in aqueous bromide solutions to form complex ions. Whatever the actual value as measured under any given conditions, thisvalue will regularly increase as the concentration of free bromine increases and that of the bromide ions remaining in solution decreases, until a maximum point is reached whereat all of the bromine has been liberated and no bromide ions remain. When the liberation of bromine is eifectedby'the action of chlorine, any further increase in the observed potential above the aforesaid maximum point will be due to the presence of free chlorine having ,a higher oxidation potential value than that of bromine By determining the limits 'ior any bromide solution between which-the oxidation potential varies as bromine is liberated therefrom and installing a suitable instrument to register or' record such potential, it is possible to follow the progress of the bromine liberation directly to the point whereat such action is comconnected with a suitable instrument for indicatthe potential difference of the electrodes. One such electrode is a standard of known potential and the second is one capable of adsorbing bromine on its surface and acting as a bromine gas electrode, such as a platinum electrode. Fig. 1 of the drawing shows diagrammatically a convenient apparatus assembly.

AB is a resistance element connected with a source of currentC, e. g. a storage battery. A

standard calomel electrode D. of known potential is connected to the negative terminal A. Electrode D is of the usual type, consisting of a stoppered and sealed glass tube containing a layer of mercury at the bottom, covered by, a layer. of a pastemade of calomel and mercury intimately mixed with and moistened by a standard potassium chloride solution, and the tube filled nearly to the stopper with more of the potassium chloride solution. A smaller tube may be sealed into the larger tube as shown and is provided with an extension, convenientlya rubber tube,

terminating in a porous porcelain plug for. 1111- -mersing in the solution to be tested' The form of standard calomel electrode shown is simple and convenient to use, but any other suitable form may be used, if desired. The positive electrode E consists of a piece of platinum wire, preferably smooth, encased at the upper end with an insulating glass tube and having the lower end, in the form of a spiral, exposed for immersing thetest solution. The electrode may also be provided with a propeller and pulley, supported by a suitable hearing, so as to be rotated and to serve as a stirrer. The platinum electrode E, when immersed in a solution containing free bromine, adsorbs some 0! the bromine on its surfaceand functions thus as a bromine gas element A voltmeter V .is connected to terminal A and slidingcontact H in parallel with the test cell. v a

,To operate the apparatus, the solution to be tested, containing free bromine-and bromide ions, is placed in vessel F, switch- S is closed and slidingcontact 11 is adjusted to give a zero reading on galvanometer G. The .voltage reading on voltmeter V shows the difierence in potential between electrodes D and E. Thesingle potential of ,D being known, that of E can be directly calculated by adding the. reading of V algebraically thereto, although for practical purposes the direct reading of V alone is sufiicient to indicatethe condition of oxidation of the test solution.

The potential of electrode Ewill vary with the relative concentration of free bromine and bromide ions present, as already explained. For continuous testing purposes, in order to follow the course of the liberation of bromine during the operation of the process, samples of the solution under treatment may be testedat intervals, or,

preferably, a continuous stream of the solution may be caused to flow through. vessel F, the voltage reading at any moment showing the degree of liberation of the bromine. Under suitable conditions, also, the electrodes may be immersed directly in the solution in the apparatus wherein the oxidation is carried out. Since the observed voltage issubje'ct to a temperature factor, it is necessary to carry out the measurements at about constant temperature-or, if that is not feasible, to provide a temperature compensating device to provide for variations in the temperature of the solution being tested. Such device may conveniently consist in an added resistance of known temperature coeflicient connected in series. with element AB and the source of current C, and immersed directly in the test solution. As the temperature of the solution rises, the resistance of the compensating device increases and correspondingly reducesthe voltage drop along AB to balance the reduction in voltage between electrodes D and E which results from the temperature rise, and conversely'as the temperature of the test solution falls below the standard.

'For continuous reading the electrode system may be connected to a potentiometer combining resistance-element AB, battery C, galvanometer G andvoltmeter V in one self-contained apparatus which automatically adjusts itself to maintain the zero reading of the galvanometer, and continuously indicates or records on a chart the voltage between the test, electrodes. Equipment of mercially available. Furthermore, by combining with the potentiometer suitable mechanical or electrical devices responsive thereto, the inextent provided either way from the desired pointv results in closing a motor operating circuit and starting the motor which in turn is connected by a suitable mechanical drive for opening or closing a valve. Various other means of actuating control valves or of otherwise controlling the supply of materials, however, may be employed, such as will be apparent to'those familiar with the use of automatic control devices.

As a standard electrode we prefer to employ the saturated calomel electrode, consisting of the system Hg/HgCl/ saturated KCl solution containing an excess of solid KCl crystals, the single potential of which is 0.245 volt at 18 C., referred to the standard hydrogen scale. Such electrode is convenient for use in plant control work as it is not subject to variation in concentration and can beused for long periods without change. However, a calomel electrode of lower concentration, e. g. a normal or a N/ 10 calomel electrode, or any other suitable velectrode of known potential may be substituted, if so desired.

In order to liberatebromine in the elemental state from a bromide-containing solution, the latter must have a neutralor acid reaction. If initially alkaline, the solution is to be acidified,

such last-mentioned description is now compreferably with a strong mineral acid, otherwise a portion of the bromine content upon oxidation will be converted to soluble oxidized compounds until the alkalinity is neutralized in this way at the expense of the bromine. In strong saline solutions containing a high concentration of ions bromine may be substantially completely liberated by treating the same with chlorine under conditions of approximate neutrality, corresponding to a hydrogen ion concentration expressed as pH=7. In more dilute solutions a slight acid reaction to counteract the tendency of free bromine to be hydrolyzed is necessary in order to obtain a quantitative liberation of bromine, the degree of acidity required varying with the dilution. In the case of sea water, which contains approximately 3.5 per cent' total 'solids, chiefly chlorides, we have found an acidity between about pH=3 and pH=4 is required for the quantitative liberation of bromine. In a sea water acidified to a lower degree, i. e. having a pH value greater than 4, a portion of the bromine will be hydrolyzed to bromic acid sufiicient to raise the acidity and accordingly lower the pH value to the aforementioned value between pH=3 and pH=4, thereby causing a loss of bromine in the extrac-' tion, as more particularly described in our copending application Serial No. 472,820, filed Aug. 4, 1930.

For any given solution of a bromide containing a definite total concentration of ions from which bromine is to be liberated, e. g. by chlorin- 1 ating, the oxidation potential increases from a characteristic minimum value, whereat theformation of free brominecommenceato a characteristic maximum whereat all of the bromine is liberated in the 'elementafstate and no brdmide ions remain. Further addition or chlorine in excess produces a continued rise of the observed oxidation potential, due to the effect of .free chlorine.

In applying our improved method of control, therefore, it is required first to determine the maximum and, if desired, the minimum values for the oxidation potential of bromine in the solution being treated. This may be done in various ways. For instance, the bromine content of a sample of the solution is determined by chemical analysis according to the usual method; chlorine water 01' known strength is then gradually added toanother sample of the solution in exact amount calculated to liberate all of the bromine, whilemeasuring the oxidation potential of the solution according to the method just described. The minimum value is found when the addition of chlorine is begun, and the maximum when the addition thereof is just finished. The result may be checked by blowing the liberated bromine out of the solution with air until it is bromine-free, adding a weighed quantity or a bromide equivalent to the original bromine content and repeating the above determination. For

- every type of bromide-containing solution there is a characteristic range of oxidation potential values which dependsupon the initial concentration of bromide ions and also upon the collateral eflect of other ions present. Having determined such range 01'. values for any particular type or solution, and having charted thesame, the degree of bromine liberation in subsequent operations may be read directly from the chart when the oxidation potential of the solution is measured, thereby giving a substantially simultaneous indication of the progress of the oxidation.

'The oxidation potential range of bromine in sea water acidified to pH=3 is shown in the proximately average curve which would be subject only to minor variations in working with sea water from any region not materially, affected by fresh-water dilution or conditions of abnormal evaporation, as in landlocked bays, etc. The first traces of free bromine in the solution being tested cause an immediate rise of the oxidation potential, as measured by the difference in voltage of the saturated calomel electrode and the bromine electrode, to about 0.88 volt. lfis the liberation of bromine continues and the concentration of bromide ions diminishes the voltage curve breaks sharply between about 0.88 and 0.91 volt and then continues to rise uniformly. .We have found that 0.97 volt is the point whereat substantially all of the bromine has been released in the elemental state. Translating these figures to the standard hydrogen scale byadding the value for the single potential of the standard saturated calomel electrode, i. e. 0.245 volt at 18 0.,

single potentials of the two standard electrodes and about 60 parts bromine permillion, and

which has been acidified to pH=3. The values hold approximately for other degrees of acidity between pH=6 and pH='3. The actual figures for any solution would be dependent upon both the bromine content and the content of otherions present. If the bromine content were to be increased without materially altering the total solids, the minimum value given above would be lowered in proportion, but the maximum'value, whereat all of the bromine is liberated in the free state, would remain substantially the same. Conversely, decreasingthe total brmiine content would raise the lower value without changing the upper value. Theeffect vof the presence or other ions is to depress the single potential of 1 bromine, so that for solutions having the same bromine content as sea water buta higher content 01 total solids the bromine single potentialwould be lowered, and for with a lower percentage of total solids the mum mile 190- '1 the composition.

with reasonable accuracy regardless of the source of such sea water, excluding of course cases where-local fresh water dilution materially alters In the extraction of bromine from sea water, therefore, the same is preferably to be acidified to a pH value between about 3 and 4, and the acid solution is then to be treated with chlorine until the oxidation potential thereof reaches a value of approximately. 1.215 volt on the standard hydrogen scale. Under practical conditions the final value may vary between about 1.20 and 1.23 volt. The efiiciency a nd economy of the process depends upon the proper control of the chlorination within the limits as stated particularly under conditions of continuous operation suchas obtained in commercial practice. If the oxidation potential falls below the stated values, the degree of chlorination is not suflicient to liberate all of the bromine. On the other hand, if the oxidation potential exceeds the preferred range, a material excess of chlorine will have been used, which means not only a waste of chlorine, but also added expense for purifying the bromine from such excess of chlorine. lnas-r much as the total amount of bromine present is only about 60 parts per million, requiring theoretically about 27 parts of chlorine for the liberation thereof, it isv apparent that a variation of only a few parts per million in the'amount of chlorine used can produce either a relatively high percentage loss of bromine or of chlorine. The

permissible variation is so small in proportion to the total volume of sea water treated that it has been found practically impossible to maintain an effectual control of the chlorination by other means than by the herein described electrometric method. By controlling the chlorination to maintain the oxidation potential of the chlorinated sea water ata value between 1.20 and 1.2;; 'volt referred tothe standard hydrogen scale, it has been found possible to secure a. maximum liberation of bromine with a minimum consump tion of chlorine. The liberated bromine may then be separated from the aqueous solution in any suitableway, as by blowing out with a voluminous -current of air, and the so separated bromine vapors may then-be absorbed in an alkaline solution or other suitable absorbing medium, according to any of the known methods.

The herein disclosed method of control of bromine liberation is equally applicable when, instead of being chlorinated, the bromide-containing solution is electrolyzed or oxidized by any of the other known methods for liberating bromine therein. In whatever manner the oxidation is performed, therefore, the liberation of bromine may be directly measured by means of the oxidation potential indication determined as herein described in complete detail.

A similar method of procedure may likewise be followed for measuring the liberation of iodine when oxidizing an iodide solution in analogous manner. The values for the single potential of 5 iodine in contact with iodide ions are roughly one-half those for bromine for equivalent cone centrations. For any particular set of conditions, however, the exact values may be'readily obtained by the method herein disclosed, and such values'employed to give an indication of the process of iodine liberation. Owing to the fact that there is a measurable difierential between the oxidation potential values for bromine and iodine, our improved method may also be adapted for indicating the, liberation of iodine and bromine consecutively from the same solution;

Other modes of applying the principle of our invention may be employed instead of the one explained, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims or. the equivalent of such stated step or steps be employed.

We therefore particularly point out and distinctly claim as our .invention:- v

1. In a process of extracting bromine from seawater wherein the same is acidified and bromine is liberated by additionof chlorine thereto, the method of controlling such chlorination which comprises producing an electromotive force vary-- ing in response to variations in theconcentration of free bromine in the acidified solution and reg. ulating .the addition of chlorine in accordance with such. electromotive force to maintain the latter at approximately 1.215 volt referred to the standard hydrogen scale.

2. In a process of extracting bromine from-sea water wherein the same is acidified and bromine is liberated by addition of chlorine thereto, the method of controlling such chlorination which comprises producingan electromotive force varying in response to variations in the' coneentra-' tion of free bromine in the acidified solution and regulating the addition of chlorine in accordance with such electromotive force to maintain the latter at a value approximately between 1.20 and 1.23 volt referred to the standard hydrogen scale. I

3. In a method of recovering bromine from a solution containing a minute amount of a bromide on the order of the bromide content of sea Water and also containing a high proportion of chlorides relative to the bromide, in which the solution is oxidized to liberate bromine therein, the step which consists in controlling the oxidation in accordance with the potential offree-bromine in the solution so as to maintain such potenioo solution is chlorinated to liberate bromine therein, the step which consists in controlling the introduction of chlorine in accordance with the potential of free bromine in the solution so as to maintain such potential at a value between about 1.20 and 1.23 volt referred to the standard hydrogen scale.

\ JOHN J GREBE.

RAY H. BOUNDY.

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