Bromine - Element From The Sea (1963)

in material study the world's oceans contain at least small amounts of most of the chemical elements here are some of the various chemical species that occur in seawater shown with their relative molecular concentrations in recent times chemists have attempted to extract some of these elements for man's benefit in much the same way as valuable ores or mined on land one of the chemicals that is needed in ever increasing quantities is elemental bromine yet it occurs in seawater in the form of bromide ions in only very Manute concentration 15 ions per million molecules of water how can the element bromine be extracted economically from seawater to answer this question we must first explore some of the properties and chemistry of bromine here is a sample of the element bromine is the only nonmetallic element that is a liquid at room temperature and pressure because of its high vapor pressure bromine evaporates readily even at room temperature forming a colored gas bromine is highly irritating in toxic special precautions must be taken in handling it including rubber gloves and an exhaust system note that the gas is more dense than air how does bromine react with some common elements first let's try the nonmetallic element phosphorus a reaction takes place the product is phosphorus tribromide which fumes at the reaction temperature now let's see how bromine reacts with the element hydrogen we pour out a sample of bromine since we will be dealing with gaseous not liquid bromine in this experiment we can dispense with the gloves a large beaker is heated so that when bromine is poured into it the liquid vaporizes and fills the beaker with gaseous bromine let's turn on the hydrogen and light it in the air the pale blue flame of burning hydrogen is hard to see but look what happens here the flame continues to burn in the bromine vapor hydrogen reacts with bromine forming hydrogen bromide now to another sample of bromine let's add aluminum not much happens at first but once the oxide coating on the aluminum is penetrated a vigorous reaction takes place similar experiments would show that bromine reacts with most of the other elements here the aluminum and bromine combined to form aluminum bromide which when purified gives this solid is this bromine compound soluble in water when water is added the vigorous reaction occurs as the aluminum bromide dissolves here's the purified product from the phosphorus and bromine reaction phosphorus tribromide is a liquid again we'll add water the reaction starts slowly but soon gives off so much heat that bubbles of steam are formed the reaction continues until all the phosphorus tribromide has dissolved this bottle contains pure hydrogen bromide the product of the earlier reaction between hydrogen and bromine when we let hydrogen bromide come in contact with water we see that the water level rises rapidly indicating that hydrogen bromide too is very soluble in water because bromine reacts readily with many substances and because the resulting compounds are highly soluble in water virtually all of the world's bromine compounds are dissolved in the oceans or are trapped in underground brine deposits but what about the solubility of bromine itself when bromine is added to water it settles to the bottom indicating it is more dense than the water now we'll mix the bromine and water with the help of a magnetic stirrer and a plastic coated magnetic stirring bar vigorous and prolonged stirring gives us a saturated solution the brown color is characteristic of aqueous bromine solutions however some drops of bromine remain undissolved this indicates that elemental bromine is not highly soluble in water what are some of the chemical properties of this aqueous solution let's divide the saturated solution in order to check the effect of varying the acidity - this flask we add acid the color does not change appreciably a base is added to the other flask the bromine solution turns a very pale yellow indicating that a reaction has taken place analysis shows that addition of hydroxide ions forms bromide and hypo bromide ions is the reaction reversed when acid is now added yes as the concentration of hydroxide ions is lowered the bromine color intensifies thus we see that bromine can exist in aqueous solution in various forms which are in rapid reversible equilibrium now let's try to obtain elemental bromine from seawater making use of our knowledge of the properties and chemistry of bromine when seawater is first concentrated by a factor of about 50 it will make color changes more visible the absence of any brown color indicates that no great amount of elemental bromine is present when we check the acidity the litmus turned blue seawater is basic since sea water contains hydroxide ions and also bromide ions perhaps the equilibrium we studied earlier exists if so by adding acid to lower the hydroxide concentration we should be able to produce elemental bromine let's find out when acid is added to our sample there is no color change therefore no visible amount of bromine has been produced even though the solution is acidic as indicated by the litmus apparently the hypo bromide equilibrium is not significant in seawater what is needed to change the bromide ions to elemental bromine from the formulas we see that two bromide ions can form a bromine molecule by losing two electrons that is the bromide ions must undergo oxidation to find an oxidizing agent we refer to a set of e 0 values the value for the oxidation of bromine is - 1.06 volts we note that the e zero value of oxygen is minus one point two three volts which makes it a better oxidizing agent than bromine furthermore oxygen is the cheapest oxidizing agent let's find the e zero value for the overall reaction the e zero value for the oxidation to give bromine is minus one point O six volts the e zero value for the oxidation to give oxygen is minus one point two three volts we transpose the lower equation to obtain the e 0 value for oxygen acting as an oxidizing agent it is plus one point two three volts for the sum of these half reactions easy row is plus zero point one seven volts the positive value indicates the reaction should occur but tells us nothing about how fast equilibrium will be reached to test our prediction that bromine should be formed let's bubble pure oxygen through the solution and see if the reaction will proceed there is no immediately visible effect apparently the rate of reaction is too slow to produce any visible amounts of bromine so we again check our table for another cheap but hopefully faster agent to oxidize the bromide ions here is chlorine it is relatively inexpensive as before we calculate the e zero value of the overall reaction the net reaction has an e zero value of plus zero point three ovals although this does not indicate the rate of reaction it tells us that the oxidation of bromide by chlorine should proceed we can check this reasoning by experiment wheel add chlorine to concentrated seawater in an attempt to produce bromine it works the color gets deeper and deeper as more and more bromine is produced the addition of chlorine has resulted in an aqueous solution of elemental bromine now how can the bromine be extracted from this solution let's compare it with a Stoppard flask of aqueous bromine from our earlier experiment a considerable amount of the volatile bromine has accumulated in the airspace above the solution because of the low solubility of bromine and water this suggests that if bubbles of air are forced through the solution they will carry away the volatile bromine a source of compressed air is connected to a dispersion to the air bubbles sweep the volatile bromine out of solution however in order to concentrate the bromine from this dilute gaseous mixture we'll need more controlled conditions let's start with another portion of concentrated seawater in a system designed to handle the gaseous bromine you'll recall that we must first acidify the seawater when the solution is chlorinated elemental bromine is formed compressed air is used to blow the bromine out of the solution if we try to collect the bromine in a beaker of water we note that very little of a bromine dissolves as we should have suspected the solubility of bromine and water is too low to allow us to concentrate it by this method to find an effective method of concentrating the bromine let's consider the solubilities of various bromine compounds in water we note that hydrogen bromide is highly soluble confirming our earlier experimental observation thus a large amount of bromine can be present in water as hydrogen bromide but how can we convert bromine to hydrogen bromide in hbr the oxidation state of bromine is minus 1 the BR 2 therefore must undergo reduction here is the half reaction sulphur dioxide has been found to be the most economical reducing agent here are both half reactions we add them to obtain the equation for the overall reaction then adding the appropriate easy row values gives a positive figure showing that the overall reaction may occur note that an aqueous solution of hydrogen bromide will be produced therefore the gaseous bromine when blown out of the seawater can be concentrated as aqueous hydrogen bromide we have modified the apparatus to attempt the reduction of bromine with sulfur dioxide the compressed air is again turned on and elemental bromine is blown out the sulfur dioxide is added through this tube and mixes with the bromine and water vapor coming down the column the bromine color disappears and colorless hydrogen bromide gas is formed which passes into the water in this receiver after a few minutes the sea water solution loses its color showing that the bromine has been blown out because of its high solubility the hydrogen bromide which was formed should have collected in the water in this receiving flask to check this we again use chlorine to oxidize the hydrogen bromide in the receiving flask the characteristic color of aqueous bromine appears the bromide ions in the hydrogen bromide solution are oxidized by chlorine to produce elemental bromine thus we have succeeded in concentrating the bromine from the seawater in a much smaller volume of solution since our goal is to obtain elemental bromine how do we now remove the bromine from the water in which it is dissolved we could blow it out with air as we did before but to minimize loss of bromine into the air will use live steam produced in this flask the steam condenses as it passes into the solution raising the temperature because bromine is more volatile than water it distills from the solution relatively pure bromine condenses and collects in this proceeding flask on a laboratory scale we have succeeded in obtaining this small quantity of bromine less than one gram from this large volume of seawater over 100 million pounds of bromine are extracted from seawater every year on an industrial scale at the Ethel Dow company's plant at Freeport Texas the seawater enters through a deep water intake channel the water flows past screens which remove small marine life and debris now recall our laboratory procedures as we look at the industrial process for extracting bromine in this large green pipe the filtered water is acidified with sulfuric acid near the top of the pipe chlorine is added to accomplish oxidation of the bromine the chlorinated water is then divided into many small streams from here the water falls as spray through this brick tower as huge quantities of air are forced in at the bottom of the structure to blow out the bromine vapor the air and bromine vapor leaved through this large green pipe the small white pipe introduces sulfur dioxide gas to reduce the bromine vapor the hydrogen bromide which is formed is concentrated in this absorption tank by dissolving in a falling spray of water the concentrated hydrogen bromide solution then enters this tower and chlorine is added to reoxidized the bromide steam is used to distill bromine from the solution the bromine is then condensed to a liquid which can be stored for later use thus the industrial process of extracting bromine from seawater follows our laboratory chemistry starting with acidified seawater chlorine is added to oxidize the bromide ions this yields a dilute aqueous bromine solution air blows out the bromine and sulfur dioxide gas in the presence of water reduces the bromine to bromide highly soluble hydrogen bromide gas is formed and absorbed in water in the final step chlorine gas is used once more to oxidize the bromide ions the bromine is then removed by steam giving us the element bromine