September 1. 1913. 



THE INDIA RUBBER WORLD 



619 



Sources of Error in Determining the Acetone Extract in Rubber 



Compounds. 



By Professor H'. Jones. 



SOME years ago the total acetone extract obtained from a 

 rubber compound, less the free sulphur, was considered 

 as belonging to the rubber in the compound, and as a result 

 we often had an abnormal increase in the percentage of extract 

 above that contained in the rubber used. Whether this was fair 

 to the manufacturer or not, seemed not to be considered. How- 

 ever, as the examination of rubber compound progressed, and 

 parat}ine began to be used in the compound, it became quite clear 

 that this method must be modified, that not only the free sulphur 

 should be deducted, but all paraffine and mineral oil as well. 

 This is now the general practice and gives a much more accurate 

 composition of the compound. In making these determinations 

 we lind it quite necessary to watch certain sources of error, some 

 of which are still overlooked by many very careful chemists. 



To start these determinations, of course the rubber has to be 

 cut fine, not necessarily to pass a certain sized sieve, but should 

 be cut thin, so as not to make the time of extraction unduly 

 prolonged. 



The extracting apparatus should be one in which the extrac- 

 tion is made at near the boiling point of the acetone, and where 

 the acetone passes through the rubber at this heat. All joints 

 should be ground glass. We generally use one gram of the dried 

 compound in making the extract. 



In placing this in the extractor, we may use either of two 

 methods. First, we may use a Gooch crucible to hold the rub- 

 ber, or we may use a paper thimble or filter paper. This at first 

 sight might seem to be immaterial, but we do not think so. 



If we use a Gooch crucible, we will find that in case the com- 

 pound contains lampblack, some of this black is carried into the 

 extract. This being black can be plainly seen. We find, too, that 

 a small amount of mineral filling is also carried into the extract. 

 This being white does not show, and is quite likely to be over- 

 looked. 



If a paper is used, we can get the extract free from any lamp- 

 black or mineral. The only objection to the paper is. that while 

 they are ether-extracted, they still contain much resin, which is 

 given up to acetone, and we have found that before using a paper 

 it is absolutely necessary to extract it with acetone until nothing 

 more is dissolved from it, and that this takes often from two 

 to three days' extraction. 



If we use the crucible, it is quite necessary to look for mineral 

 in the extract, and deduct its weight from the total, before pro- 

 ceeding further. 



The second point to be observed is the acetone itself. We 

 rarely, if ever, buy acetone that does not give a residue upon 

 evaporation, and it is therefore the practice to redistil it. This, 

 however, must be done within a short time of its use. as it is 

 found that acetone does not keep. 



Acetone after careful redistilling, so that it leaves no residue. 

 will, after keeping a short time, develop a yellow tint and again 

 give a residue. This is more pronounced if it has been kept in a 

 clear glass bottle in strong daylight. If it is kept in a dark 

 bottle, there is very little change. We think it better in all 

 cases to test the acetone immediately before using. After taking 

 these precautions we will get a correct percentage of total 

 extract. 



This extract will contain free sulphur, paraffine, mineral oil, 

 and organic extract, due to the rubber, and which is saponifiable. 

 In separating the extract into these items, we are again liable to 

 error. 



The first thing we do is to saponify with alcoholic potash. 

 This is done in the usual manner, by evaporating over steam. 



Upon adding water, and extracting with ether, we obtain an 

 ether solution of the paraflSne and mineral oil, while the organic 

 extract remains in the water solution as soap, together with the 

 free sulpluir, which is probably taken up as sulphide of potash. 



If now we evaporate the ether solution, we will get the com- 

 bined paraffine and mineral oil ; then by deducting this weight, 

 plus the free sulphur, from the total extract, we obtain the 

 organic. 



We have found in many cases when the reverse of this has 

 been done ; instead of weighing the mineral fats, the soap solu- 

 tion has been acidified and extracted with ether, evaporating this 

 to get the organic, then by taking this organic plus the sulphur, 

 from the total extract, to give the mineral. 



W'e learn that parties using this method have generally found 

 the mineral or paraffine much less than what was used, and they 

 could not account for it. The reason for this is, that they got 

 the organic figure too high, so making the mineral oils too 

 low. This is readily accounted for by the use of alcoholic potash, 

 which has become only slightly yellow upon keeping, when there 

 is found in it what appears to be aldehyde resm, w'hich being 

 saponified, is weighed with the organic from the rubber, making 

 this figure too high. 



The free sulphur we determine from the soap solution, after 

 extracting the paraffine, in the following way : To the solution 

 we add a small excess of nitric acid and a few drops of bromin. 

 We then evaporate this down to one or two cubic centimeters, 

 add carbonate of soda in large excess, transfer to a platinum 

 dish and fuse, to get rid of organic matter; dissolve in water and 

 hydrochloric acid, and precipitate the sulphur in the usual way 

 with barium chloride. We thus get all determinations from one 

 extraction. 



Recapitulation : 



The errors to be guarded against in these determinations are : 



1st: In the condition of the acetone used. 



2nd : Resin in the paper thimble or filter paper. 



3rd: In using a Gooch crucible, the liability of carbon or 

 mineral matter going into the extract and being weighed up as 

 extract. 



4th : In weighing the organic extract instead of the mineral. 

 The condition of the alcoholic potash is liable to cause errors. 

 In all these cases the liability is to make the organic extract too 

 high. 



We have found that in following these methods, we at times 

 get an extract which might appear to be abnormally low ; that is, 

 we find the organic extract to be less than that contained in the 

 rubber used, while the general idea has been that we should in 

 all cases get more ; the increase being accounted for by the vul- 

 canization, which is credited with increasing the organic extract 

 in the compound. 



This has led us to a long series of tests, to determine why in 

 some cases we get less and in others we get more extract than 

 was contained in the rubber used. We find that if the compound 

 is properly made there is no increase in the organic extract, 

 while as it is usually made, and with the same ingredients, there 

 is a considerable increase. This increase does not take place 

 during vulcanization, as is thought, but during the mixing on the 

 rolls, so that the maximum of extract will be found after mixing 

 and before vulcanization. 



