I X D US TRIAL AND ENGINEERING CHEMISTRY
August, 1926
(4) Borax solution (18.9 grams per liter) in the presence of sodium sulfate (I50 grams per liter) and sodium carbonate (10.6 grams per liter): Polyhydroxy compound Mannitol Glucose
Borax found Grams/liter 18.9 18.8
( 5 ) Borax solution (18.9 grams per liter) in the presence of sodium carbonate (10.6 grams per liter), sodium bicarbonate (20 grams per liter), sodium chloride (150 grams per liter), and sodium sulfate (150 grams per liter) : Polyhydroxy compound Mannitol Glucose
Borax found Grams/liter 18.9 18.9
Amount of reagent necessary t o titrate 1.0 gram of anhydrous sodium tetraborate : Reagent Mannitol Glycerol Glucose Levulose Honey Invert sugar
Initially added 3 grams 10 cc. 30 grams 10 cc. 10 cc. 40 cc.
Totally added 5 grams 90 cc. 45 grams 10 cc. 10 cc. 80 cc.
In the case of levulose and honey the first end point obtained after adding initially 10 cc. of the agent was taken as final value, because these two compounds give end points which are dependent on the concentration of the levulose or honey present in the solution.
Titration in Presence of O t h e r S a l t s
When borax is titrated in the presence of other alkalies, as sodium carbonate or sodium bicarbonate, these mixtures do not show a definite end point when neutralized with 0.1 N HC1 against methyl orange indicator. Since the accuracy of the following alkali titration is dependent on the accuracy of the neutralization of the mixture, it was necessary to eliminate this source of error as far as possible. For this purpose the alkalinity of a certain amount of the borax solution and the alkalinity of certain amounts of the other salts were ti-
821
trated separately using 0.1 N hydrochloric acid with methyl orange as indicator. From the data thus obtained the total alkalinity of the mixture of borax with the other salts was calculated. The mixture was now titrated with 0.1 N hydrochloric acid and methyl orange indicator to the calculated end point. The color obtained by this titration was taken as a standard. For further experiments color charts were made, showing the exact color at the dead neutral point with varying concentrations of borax and carbonates. Conclusions
Borax, whether present alone or in mixture with other salts, may be determined accurately by using either mannitol or glucose. The largest difference was found in the presence of carbonate and sulfate, but even in this case it did not exceed the limits of the experimental error. Although it is possible to obtain fairly accurate results with levulose and honey, if they are added in proper concentration and the first change of color is taken as final end point, their use cannot be recommended since the results are dependent on the concentration of the added agent. The results obtained with glycerol and invert sugar are somewhat less satisfactory than those with mannitol and glucose. Though a larger quantity of glucose (about ten times) is required than of mannitol, this is no disadvantage, as a large background of white material is helpful in distinguishing the end point. As commercial glucose (Cerelose) may be obtained a t a cost of a few cents a pound, while the price of mannitol is around $6.50 a pound, the advantage of glucose may readily be seen. Commercial glucose in the pure form now obtainable is therefore recommended to replace mannitol in standard borax determination. The errors in the determination of borax in the presence of other salts are due to errors in the initial neutralization of the alkalinity, and not to the boric acid titration itself. This source of error may be eliminated by means of alkalinity tests and the establishment of color standards. ~
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Estimation of the Unsaturated Content of Petroleum Products‘ By A. W. Francis AR’CHLR D. LITTLE,INC , CAMBRIDGE, MASS.
HE estimation of olefins in gasoline by means of sulfuric acid involves the uncertainty of the optimum concentration of acid which will remove the bulk of the olefins and practically none of the aromatics. There is also an error due to loss by vaporization, which may be considerable. Iodine and bromine have been used for this analysis, but neither has been quite satisfactory. The reaction with iodine is so slow that a large excess of iodine and a long reaction time must be used. The conditions of time and temperature have been standardized empirically, and probably merely attempt to balance the unreacted olefins against the substitution reaction in saturated compounds. With bromine there is still greater difficulty in avoiding substitution, and methods of correction have been employed, such as the measurement of the amount of hydrogen bromide evolved when a nonaqueous bromine solution is used, but these are cumbersome and unreliable. It is also difficult to keep the bromine solution of uniform strength, whether water or an organic solvent is used.
T
1
Received April 8 , 1928.
To overcome these difficulties, a method has been devised which employs a standard solution of potassium bromide and potassium bromate, which holds its strength indefinitely, and in acid solution generates bromine KBr03
+ 5KBr + 3HzS04-3KzSOa
f 3Hz0
+ 3Br~
the rate depending upon the strength of the acid. By making the solution only slightly acid the bromine can be generated slowly; and by vigorous agitation this is consumed by the double bonds fast enough to keep it a t very low concentration and thus avoid substitution reactions. This is important for reliable results. The fact that the gasoline does not mix with the reagent is no disadvantage, and may be desirable in avoiding substitution, but it does necessitate vigorous agitation. It makes no difference in the titration whether dibromides or bromohydrins are formed, as may be seen from the following equations: RCHBrCHBrR’
RCH=CHR’
+ Brz + HzO7
+ HzO
LRCHBrCHOHR’ f HBr Bromide and bromate have been used previously for es-
INDUSTRIAL AND ENGINEERING CHEMIXTRY
822
timation of unsaturation. Frank2 used a carbon disulfide solution of the bromide and bromate, and Utz3 a chloroform solution of the bromide, titrating with an aqueous bromate solution. Weger4 and Graefe6 and Routela6 also used bromide and bromate. It was usually considered necessary to have a solvent for the gasoline; but this is not required and may even be detrimental. By the direct titration with the standard reagent two errors are involved: (a) since the solution must be strongly acid, bromine is generated in high concentration a t the point of entrance of the reagent, and may cause substitution; (b) since the flask is open all the time, there is
Vol. 18, No. 8
cubic centimeters of saturated potassium iodide solution are added, and the iodine liberated is titrated with thiosulfate solution, using vigorous shaking near the end of the titration, to extract the iodine from the oil layer. The calculation is as follows: 0 08 N = V D (T1N1-T*N2)
where N is the bromine number in grams per gram of sample, V and D are the volume and density of the sample, TIand N 1 the titration and normality of the bromide-bromate, and Tz and Nz the titration and normality of the thiosulfate. The bromine number itself is fairly satisfactory for comparison of gasolines, but for the actual percentage of unsaturateds, which is often desired, the mean molecular weight, M , of the olefins is required. For a narrow cut of a distillate this can be estimated from the mean boiling point and is a p proximately 54 plus half the mean boiling point in degrees Centigrade. This relation is based upon the boiling points of the pure normal olefins. Several cuts of 2.5 per cent each of a highly unsaturated cracked gasoline were titrated and the percentage of olefin was estimated in this way. The per cent unsaturateds is U = MN/160. Results
0
10
2D
W
40
50
FZRCENT DISTILWE
60
70
BO
90
IOU
danger of losing some of the more volatile portion from the sample, and as will appear later, this portion has the highest unsaturation. These errors are largely eliminated in the method here presented. Reagents Half normal bromide-bromate. Fourteen grams KBrOa and 50 grams KBr are dissolved for each liter of solution. This solution is best standardized by titration of a weighed sample of aniline dissolved in acid. Six equivalentsof bromine are consumed. Fifth normal sodium thiosulfate. Fifty grams. Na&08.5Hz0 are dissolved for each liter of solution. This 1s standardized by titration of the iodine produced when a definite volume of the bromide-bromate solution, to which potassium iodide has been added, is acidified. Either of these solutions can be of any other convenient strength. Potassium iodide. A small volume of saturated solution is prepared. Ten per cent sulfuric acid.
Method
A slight excess (preferably not more than 1 cc. as estimated from a trial titration) of the bromide-bromate solution is measured into a small Erlenmeyer flask, and the sample of oil, 3 to 50 cc., depending upon the unsaturated content, is pipetted in. The solution is quickly acidified with about 5 cc. of 10 per cent sulfuric acid, and the flask is stoppered. It is shaken for 1 minute as vigorously as may be necessary to keep the color a pale yellow. If the color is dark yellow in spite of violent shaking, too much bromide-bromate has been added, and the analysis should be considered only a trial titration. I n any case, in order to complete the liberation of bromine, 15 cc. more of acid are added and the shaking is continued for another minute. I f the solution remains completely colorless, a little more bromide-bromate solution is added. The final color should be light yellow. One or two 2 Lunge, Chem. tech. Untersuchungsmethoden, 2nd ed., Vol. 111, p. 755. Julius Springer, Berlin. 8 Petroleum Z., 2, 44 (1906). 4 Chem. Ind., 28, 26 (1905);Petroleum Z., 2, 101 (1906). 6 2.angczu. Chem., IS, 1584 (1905). 6 Chem. Zcnlr., 1912, 11, 638.
The results are shown in Table I and Figure 1,in which the bromine numbers and percentages of olefins are each plotted against the per cent of distillate. The higher unsaturation is, as usual, in the lighter fractions. The mean bromine number of the fractions agreed with that of the whole sample. From the mean percentage of olefin of the several fractions the mean molecular weight of the olefins in the whole sample was estimated as 116-that is, between octene and nonene. The use of this figure for gasolines of average boiling range will lead to results which are not greatly in error. Table I-Distribution of Unsaturated8 in a Typical Cracked Gasoline a s Estimated by Bromide-Bromate Solution Mean boiling point Olefin Bromine -OLEFINFractiona Sp. gr. O C. Mol. wt. No. G./cc. Per cent 0.49 68 55 80 1.35 1 0.724
2
0.766 0.787 0.792 lo 0.801 25 l9 0.805 30 0,810 38 (last) 0.828 0 Each 2.5 per cent 6
89 98 106 125 140 155 189
98 103 107 117 125 134 153
1.04 0.86 0.83 0.78 0.73 0.65 0.35
0.49 0.45 0.44 0.455 0.47 0.44 0.27
64 57 55.5 57 5s 54.5 32
of sample.
A few typical results are shown in Table 11. I n the latter part of the table known mixtures of a straight-run and a cracked gasoline were made up and titrated. The “calculated” figure assumes that the analyses on the pure gasolines were correct. of Unsaturated8 in Pure Compounds and Known Gasoline Mixtures (Results of analysis by bromide-bromate) Found Calcd. Per cent Per cent Pure trimethylethylene 100.6‘ 100 Hexene (containing some pentene) as hexene Neutral white oil (Nujol) 0.04 0.0 ( A ) Straight run No. Texas gaso0.90 line 0.86 47.1 ( B ) Cracked gasoline 47.5 Known mixtures of A and B: A B Per cent Per cent Table 11-Estimation
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ig: 1
{
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36.9 30.3 19.2 12.60 9.31 7.43 7.43
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