Tris( hydroxymethy1)aminomethane as an Acidimetric Standard JOHN H. FOSSUM, PETER C. MARKUNAS, A N I JOHN A. RIDDICK Commercial Solcents Corp.. Terre Haute, Ind.
droxide and diluted to 100 ml. with boiled distilled water, and 100 mg. of alizarin red S (sodium alizarin sulfonate) dissolved in 100 ml. of boiled distilled m t e r . Reference Buffer Solution of pH 4.70 for colorimetric determination of end point. Mix 50 ml. of 0.1 N potassium hydrogen phthalate solution, 13.1 ml. of 0.1 S sodium hydroxide solution, 3 dro s of the appropriate indicator solution or 6 drops of the mixe8indicator. and 25 ml. of boiled distilled water.
The need for a good acidimetric standard suggested investigation of the properties of tris(hydroxymethy1)aminomethane. It was prepared in a state of high purity with constant composition. Its hygroscopicity was comparable to that of potassium hydrogen phthalate; neither it nor its solutions adsorb carbon dioxide from the air; it can be dried by heating at 100' to 103" C.; its solutions are stable under all investigated conditions of standardization; the pH of its equivalence point is 4.7; and p-sulfoo-methoxybenzeneazodimethyl-I-naphthylamine is the best indicator for determining the equivalence point. Tris(hydroxymethy1)ami n o m e t h a n e has many properties of an ideal primary standard. It is easily obtained, purified, dried, and preserved in a pure state, has low hygroscopicity and relatively high equivalent weight, undergoes stoichiometric reaction, and has a negligible indicator blank.
PURlFICATIOIi O F TRIS(HYDR0XYMETHYL)AMINOMETHANE
Dissolve 1200 g r a m of tris(hydroxymethy1)aminomethane in 2400 ml. of distilled water in a 4-liter beaker by heating the mixture to 60' C. Add 50 grams of Yorite and keep the mixture a t 50" to 60" C. for 30 minutes with constant stirring. Filter the mixture, while hot, through a fritted-glass filter funnel of fine porosity, containing a 0.6-cm. (0.25-inch) mat of filter pulp, into a 4-liter suction flask to remove the Norite. Repeat the treatment. The filtrate should be nearly colorless. Transfer the solution to a 4-liter beaker, add 2 or 3 glass beads to prevent bumping, cover with a ribbed watch glass, and concentrate, a t a slow boil, to a volume of about 1300 ml. Equip two 4-liter beakers Iyith mechanical stirrers and add 2000 ml. of purified methanol to each. Add half of the concentrated solution to each portion of methanol by pouring the solution slowly down the side of the beaker. Continuously stir the methanol during this addition and during t,he crystallization period. Allow the mixture to cool for about 30 minutes, then place the beakers in an ice-salt-water slurry and roo1 the mixture to 3' to 4' C. Remove the mother liquor by filtration through a fritted-glass filter funnel of coarse porosity. Wash the product once on the filter by slurrying with cold methanol and remove the surplus methanol by rapid suction. Recrystallize twice more, using the same technique blt omitting the Sorite treatment. For the second recrystallization, dissolve the amine in 1000 ml. of redistilled water, concentrate to a total volume of 1000 ml., and pour into 8OOO ml. of methanol. For the third recrystallization, dissolve the amine in 800 ml. of redistilled water, concentrate t,o a total volume of 800 ml., and pour into 2500 ml. of purified methanol. Wash the product obtained from the third recrystallization in the funnel by slurrying twice with cold methanol. Air-dry the final product between several sheets of filter paper. Grind the air-dried product to pass a 50-mesh screen. Place the screened amine in Petri dishes to a depth of 0.6 cm. and dry in a vacuum oven for 12 hours a t 60" C. a t not more than 10 to 15 111111. pressure, or, preferably, in a vacuum desiccator over phosphorus pentoxide for 24 to 36 hours a t not over 5-mm. pressure, maintaining the vacuum during the entire drying period. The dried tris(hydroxymethy1)aminomethane melts a t 171.1' * 0.2' C. (corrected) when heated a t a rate of 6" C. per minute. The methanol used in purification of the tris(hydroxymethy1)aminomethane in this investigation was Commercial Solvents Corp. commercial grade methanol purified by distillation through a 25-plate Penn State type column a t a reflux ratio of about 1 to 1. The water used for the second and third recrystallizations was distilled from alkaline permanganate. The water still was provided with an adequate spray trap.
H E R E is an apparent need for a good primary-type standard
T i o r the standardization of dilute solutions of strong acids. Most substances now in use fail to meet the requirements of a good standard because they have a low equivalent weight, are difficult to prepare and maintain in a high state of purity, or are hydrates or hygroscopic. The purpose of this paper is to suggest the use of tris(hydroxyniethy1)aminomethane for the standardization of dilute solutions of strong acids. To establish a material as a primary standard, the work of several independent workers or groups is required. By suggesting the use of tris(hydroxpmethyl)aminomethane as a standard for strong acids, the authors hope to create sufficient interest so that the material will be further tested by methods other than those presented in this paper. For this reason the methods used in the present investigation are described in some detail. Typical data arc presented to indicate the accuracy obtained by the authors. EQUIPMENT
Volumetric 50-ml. buret, calibrated by Sational Bureau of Standarda. pH Metcr, k e d s and Sorthrup KO.7661--11 assembly. REAGENTS
Hydrochloric Acid Solution, 0.1 S, prepared from reagent glade concentrated hydrochloric acid. Sodium Hydroxide, 0.1 5 , prepared from a filtered 50% solution of C.P. sodium hydroxide. Tris(hydroxymethyl)aminomethane, purified as described below. PotasPium Hydrogen Phthalate, Xational Bureau of Standards standard sample 84c, effective neutralizing power 100.057c. Sodium Carbonate, purified by the method of Waldbauer, McCann, and Tuleen ( 5 ) . p-Sulfo-o-?rlethoxybenzeneazodimethyl-l-TaphthylamineIndicator Solution. Triturate 100 mg. of the indicator in a mortar with 2.59 ml. of 0.1 S sodium hydroxide solution and dilute to 100 ml. with boiled distilled water. Ethyl Orange Indicator Solution. Dissolve 100 mg. of the indicator in 100 ml. of boiled distilled water. Mixed Indicator. Mix in equal parts 100 mg. of bromocresol green triturated in a mortar with 1.45 ml. of 0.1 sodium hy-
COMPARISON OF STAVDARDS
TIis(hydroxymethy1)aminomethane was compared with two commonly used standards ( 2 ) ,sodium hydroxide solution and sodium carbonate, by standardizing a hydrochloric acid solution R ith each. The sodium hydroxide solution was standardized potentiometrically with potassium hydrogen phthalate, Sational Bureau of Standards standard sample 84c. The hydrochlor'c ac'd solution was standardized with the standard sodium hydroxide solution potentiometrically, and with sodium carbonate according to the method of Kolthoff and Sandell ( 4 ) . The standardization with tris(hydroxymethy1)aminomethane
A\7
491
ANALYTICAL CHEMISTRY
492
Table I. Standard
HKCBH~OI Sodium Hydroxide, N
NaOH
0.10127 0.10126 0.10129 0.10130 0.10131 0.10131
0.10075 0.10075 0.10068 0 10071 0.10071
0.10093 0.10084 0.10084 0.10088 0.10093 0 10088
0,10080 0.10072 0.10073 0.10076 0,10077
0.10129
0.10072
0.10088
0.10076
0.000017
0.00002
0.00003
0.00002
Mean Mean dev.
A sample of tris(hydroxymethy1)aminomethane was kept on the laboratory shelf for 2 years in a screw-cap jar firmly closed with a metal cap containing a plastic-impregnated liner. At the end of the test period there wm no detectable change in composition. Three portions of the amine were stored in a desiccator for 3 days over phosphorus pentoxide, and weighed each day. KO change in weight was detected. Portions of the amine, heated in a laboratory drying oven at 103" C. for five 2-hour periods, were weighed a t the end of each heating period. -4large sample was treated in the same manner and analyzed potentiometrically with hydrochloric acid after rach heating period. Typical results are given in Table 111.
Standardization of Solutions NazCOs
(CHo0H)aCSHs
.
Hydrochloric Acid, N
Duplicate titrations were run only on the 0-hour and 10-hour material of sample 4. 4s the analysis indicates the purity remained constant, the loss of weight may be due to partial volatilization of the compound. A sample was heated at 110" C. for 10 hours, weighed, and analyzed. The material after heating was a light cream color speckled with small brown spots. The data in Table I11 indicate that the amine can be dried for 2 to 4 hours a t 100" to 103 C.
was carried out by the following procedure: Approximately 0.5 gram of the amine was accurately weighed into a 250-ml. tallform beaker. The sample was dissolved in 50 ml. of recently boiled distilled water and titrated potentiometrically with the hydrochloric acid solution. The results of standardizations are given in Table I. The pH at the equivalence point of tris(hydroxymethy1)spinomethane and hydrochloric acid under the conditions of standardization was determined mathematically and graphically to be 4.70. The neutralization curve of the amine and hydrochloric acid shows a clean sharp break a t the equivalence point. All standard solutions were corrected for change in volume due to temperature change.
HYGROSCOPICITY
The hygroscopicities were determined for tris(hydroxymethj-I)aminomethane and several materials commonly used as standwds. The sample of tris(hydroxymethy1)aminomethane was t'he same material used for the titrimetric studies.
Potentiometrically,
SMX4 Indicator,
Ethyl Orange Indicator,
99.92a 100.00 99,99 99.96 99.95
99.95 99.92 99.97 100.00 99.91
99.98 100.04 99.94 100.02 99.97
%
0
b
%
%
Mixed Indicator,
%
99.96 99.93 99.93
Mean 99.96 99.95 99.99 99.94 p-Sulfo-o-methoxybenzeneazodiinethyl-l-naphthylamine, Calculated from d a t a in Table I.
SELECTION OF 1NDICATORS
Several indicators whose pH range included the equivalence point of the titration were selected for test. The indicators were screened by adding a few drops of their solution to a solution of the amine, and titrating potentiometrically with hydrochloric acid. Three indicators, p-sulfo-o-methoxybenzeneazodimethyl1-naphthylamine, ethyl orange, and a mixed indicator consisting of equal parts of 0.1% solutions of bromocresol green and sodium alizarin sulfonate, gave a distinctive color a t a pH of 4.70. The three indicators selected from the screening tests were evaluated as follows:
Tris(hydroxymethy1)aminomethane (0.5 gram) was accurately weighed into a 250-ml. Erlenmeyer flask and dissolved in 50 ml. of recently boiled distilled water. Three drops of the indicator solution (6 drops of the mixed indicator) were added and the amine was titrated with standard hydrochloric acid. The end point of the titration was determined by matching with the color of the reference buffer solution. The strength of the standard acid was 0.10072 N (Table I). The results given in Table I1 were obtained. p-Sulfo-o-methoxybenzeneazodimethyl-l-naphthylamineis preferred because it gives the sharpest color change a t the equivalence point and because it gives good warning as the end point is approached. THERMAL STABILITY
The stability of tris(hydroxymethyl)aminomethane was determined at room and a t elevated temperatures.
Mallinckrodt's rimary standard grade potassium hydrogen phthalate and A.8.S reagent grade potassium chloride were recrystallized twice from conductivity-type water. The salts were dried, ground to pass a 50-mesh screen, redried in an oven overnight a t 105' C., and stored in a vacuum desiccator over phosphorus pentoxide in vacuo until ready for use. hfallinckrodt's primary standard grade benzoic acid was recrystallized twice from neutral 9570 ethyl alcohol which had recently been fractionally distilled. I t was dried, ground to pass a 50-mesh screen, redried, and stored in a vacuum desiccator over phosphorus pentoxide in vacuo until ready to use. The hygroscopicities were determined a t 25' * 0.5" C. in desiccators containing a salt in contact with its saturated solution m described in the International Critical Tables (3). The sample containers used for determining the hygrogco ici ties (30 X 50 mm. weighing bottles of 30-ml. capacity) were %or: oughly cleaned, dried, and placed overnight in the desiccator containing the appropriate humidity. During all periods of exposure in the humidity chambers, the covers of the weighing bottles were placed in a manner to permit circulation of air in the sample container. The weighing bottles were removed from the desiccator, immediatelv stoppered, allowed to stand in air 10 * 1 minutes, and weighed. A4pproximatelp 1-gram samples of the materials being studied were placed in the weighing bottles, and spread to a uniform thiclrnws. The weighing bottles were
.
Table 11. Purity of Tris(hydroxymethy1)aminomethane as Determined with Several Indicators
Table I l l .
Stability of Tris(h.ydroxymethy1)aminomethane at 103" C.
Sainple No.
(Sample weight 2.0150 grams) Heating Weight 1 . 0 ~ ~ ~ - .knalysis, Time, Iluurs Slg. % 70 2 0.8 o 040 ... .l
6 8
*
in
1.5 2 3 2 5 3 2
0 0 0 0
074 114 124 159
0
2 4
6 8 10
Table IV.
...
...
...
...
...
99 99 99 99 99 99
9; 97 99 91
98 91
Stability of Tris(hydroxymethy1)aminomethane at 110" C.
Heating Period, Hours
Weight of Sample, Grains
.Inalvsis.
0 10
2 0409 2,0237
99 95 98 82
%
493
V O L U M E 2 3 , NO. 3, M A R C H 1 9 5 1 Table
V.
Hygroscopicity of Several Standards
(10-day tests) Gain in Weight, % Relative Potassium Humidity, Tris(hydroxymethy1)hydrogen Benzoic % aminomethanea phthalate6 acid 0.19 0.06 0.00 31 0.00 0.17 0.07 51 0.00 0.19 0.17 71.2 0.01 0.27 0.20 91 .4verage of 3 determinations. b Average of 2 determinations.
Potassium chloride
0.08 0.13 0.14 0 47
stoppered and weighed, then laced in the proper desiccator, and placed in the thermostat. TR ese bottles were weighed daily for 10 days, using the technique described for taring the weighing bottles. Representative data arc shown in Table V. The hygroscopicity tests were carried out in May, during which time the air temperature and humidity varied widely. The adsorbed moisture on the weighing bottles is the cause of the major error in the low adsorption range. Errors as high as 0.5 my. on unfilled weighing bottles were not uncommon when these bottles were weighed on days of widely varying humidity. The hygroscopicity of tris(hydroxymethyl)aminomethane compares favorably with that of the common primary standards Lvhich were tested over all humidity ranges that might be encountered in 1:bboratory work. CONCLUSION
Tris(hydroxymethyl)aminomethane fulfills many of the requirements of a good standard. I t is commercially available a t a moderate price; it can be prepared in a high state of purity with
constant composition; and it has a favorable equivalent weight, 121.136. The amine is nonhygroscopic a t usual laboratory humidities and compares favorably in this respect with potassium hydrogen phthalate. Tris(hydrosymethy1)aminomethane and solutions of this salt do not adsorb carbon dioxide from the air. It can be used directly as a primary standard for strong acid solutions by a simple and rapid procedure. Solutions of the amine are stable under all investigated conditions of standardization. The equivalence point can be easily detected either potentiometrically or by use of the proper indicator. Tris(hydroxymethy1)aminomethane has the disadvantage that it cannot be heated above 100” C. indiscriminately. It is a weak base and has the inherent disadvantage of this class of compounds as primary standards. Its dissociation constant has been reported recently by Bates and Pinching ( 1 ) as 1.202 X lop6 at 25” C. This value is comparable to that of potassium hydrogen phthalate. which iq 3.9 X at 18” C. LITERATURE CITED
( 1 ) Bates, R. G., and Pinching, G. D., J . Research Natl. Bur. Stand-
ards, 43,519 (1949).
F.,and Lundell, G. E. F., “Applied Inorganic Analysis,” 9th printing. pp. 137-8, 141, Kew York, John tT‘iley & Sons, 1946. (3) International Crltical Tables, Vol. IV, New York, McGraw-Hill Book Co., 1928. (4) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis.” revised ed., p. 548,New York, Macmillan CO.,i945. (5) Waldbauer, L.. McCann, D. C., and Tuleen, L. F., IND.ENO. CHEM.,ASAL. ED.,6, 33B (1934).
( 2 ) Hillebrand, W.
RECEIVED M a y 4, 1950.
Presented before the Division of Analytical Chemistry a t the 117th 11eeting of the . I v E H I c . * s CHEVICAL SOCIETY, Houston, Tex
Determination of n-Paraffins in Gasoline, Oils, and Paraffin Waxes WOLFGANG LEITIIE’, Carl Borschweg 1, Linz, .4ustriu Chemical processes with hydrocarbons of 5 arious boiling ranges (gasoline, oils, waxes) frequently require knowledge of their contents of n-paraffins and isopamffins, which has required tedious or expensive cxperiinents; an easily workable chemical method for providing this information fills a definite need in hydrocarbon analysis. The method described in this paper is based on a simple chemical reaction with antimony pentachloride, followed by an indirert determination of the n-paraffins which have not been affected by this reagent, while the isoparaffins are converted into insoluble tarry matter. A n accicrac) of almut 3 to 6’3, of total sample may be
T
HE commercial value of hydrocarbon mixtures sometimes
depends largely on their contents of straight carbon chains. In some cases, such as the preparation of detergents, unbranched hydrocarbons in the source material are preferred; on the other hand, n-para&is in motor gasolines are h:irmful because of their low octane rat>ing. In spite of the considerable importarice of this special arialytical question, no simple chemical method of gmeral applicability for the determination of straight-chain piratfiris in hydro1 Present address, Oesterreichische Stiokstoffwerke Aktiengesellschaft, Lins. Austria.
reached. The method applies also to other substances such as olefins, alcohols, fatty acids, etc., after they have been converted to hydrocarbons b y usual methods. There are many possibilities of applying this simple and cheap analytical method in scientific and commercial hydrocarbon processes, as in most cases the contents of branched or normal hydrocarbon chains in the sourcc material, in the intermediates, and in the final products reflects largely on the efficacy of the process and on the quality of the products. I t is hoped, therefore, that petrochemistry will draw considerable advantages from this new method.
carbon mixtures has been known, as there is but little difference in the chemical properties of n-paraffins and isoparaffins. This analytical field is chiefly covered by physical methods, such as t,he applicat’ionof infrared absorption spectroscopy ( I ). Some years ago Schaarschmidt and Marder ( 3 ) found a difference in the reaction rate of n-paraffins and isoparaffins with antimony pentachloride, and they based a rough and qualitative method for distinguishing n-paraffins from isoparaffins on this fact. Under the conditions outlined by these authors a quantitative distinction and separation of n-paraffins in hydrocarbon mixtures of various molecular weight are not possible. This.
