members-e.g., methane, ethane, and methanol-deviate appreciably from the correlation line. On the basis of these correlations, it is non possible to predict, b y a simple calculation, the relative response of coiiipouiids heretofore unknon 11. The striictural features-Le., degree of
branching-must be considered when such a n extrapolation or interpolation is used. ACKNOWLEDGMENT
The authors \\auld like to thank .D.' f Winters for hi5 assistance in the experimental work.
LITERATURE CITED
(1) Rosie, D. 11,
R, L,,
C H E ~29,1263 I. (1957). RECEIVEDfor review Xarch 11, 1958. Accepted September 8, 19%. Conference on Analytical pittsburgh, Chemistry and Spectroscopy, pa., Applied l \ ~ ~ ~ ~ h 1958.
Analysis of Mixtures of Sulfuric Acid with Other Acids by Nonaqueous Titration MlHlR NATH DAS1 and DEBABRATA MUKHERJEE2 Indian Associafion for the Culfivation o f Science, Jadavpur, Calcutta 32, India
b Sulfuric acid can be potentiometrically titrated as both a mono- and dibasic acid in glycolic media with sodium hydroxide or an organic base like piperidine as the titrant. Conductometric titration of sulfuric acid with piperidine in ethylene glycol or a glycol-acetone (2 to 1) mixture also gives two breaks in the titration curve. The potentiometric titration has been utilized for quantitative estimation of several acid mixtures containing sulfuric acid as a common constituent. These include binary mixtures of this acid with nitric, hydrochloric, perchloric, p-toluenesulfonic, phosphoric, salicylic, and acetic acids. Some ternary mixtures have also been titrated. The method has been used for studying the reaction kinetics of sulfonation of cresols, and should be applicable to analysis of acid mixtures encountered in other organic reactions, such as sulfation and nitration.
MUG RE^ n-ith sulfuric acid as a cominon constituent are frequently encountered in the course of many organic reactions. The rwction mixtures obtained during sulfonation. sulfation, nitration and estc.rification are typical and common examples of such mixtures. These mixture< cannot be analyzed b y direct acid-base titration in watc>r. Sulfuric acid. when titrated potrntiometrically in water, gives only one inflection corresponding to the total aridity. Mixtures of sulfuric acid with other mineral acids or common organic acids behave similarly> and hence can not be analyzed by differentiating titration in n ater.
ARIOL-Y ACID
Present address. Jsdavpur University,
Calciitta 32, India.
Present address, Central Leather Resrnrch Institute, 3Indrae 20, India.
Sumerous cases, however, demonstrate t h a t it is often possible to achieve differentiating titration of different types of acid mixtures in nonaqueous or even semiaqueous media. Izmailor and coworkers (fa,I S ) determined binary mixtures of hydrochloric acid with monochloro-, dichloro-, and trichloroacetic acids, respectively, by differentiating titration in ketonic solvents. Shkodin and Izmailov (bo) titrated mixtures of pprchloric-hydrochloric, perchloric-sulfuric, and p-toluene-sulfonicnitric acid mixtures in glacial acetic medium, using pyridine or dimethylaniline as the base. Evans and Davenport 16, 7 ) used butyl alcohol and water-butyl alcohol mixtures to differentiate picric-benzoic and hydrochloric-stearic acids. Acetonitrile was used b y Lavine and Toennies (16) as the medium for the titration of a mixture of perchloric and acetic acids. Critchfield and Johnson (2) used the same solvent for sulfurichydrochloric and sulfuric-nitric acid mixtures, with morpholine as the titrant. Higuchi and Rehm (11) analyzed several acid mixtures b y conductometric titration in glacial acetic acid, and were able to differentiate between the two hydrogen atoms in sulfuric acid. Moss, Elliott, and Hall (17) used ethylenediamine as the titration medium and could differentiate mineral or carboxylic acids from phenols. Dimethyl formamide and pyridine have since been used (5,5, 9, 15) for the differentiation of all these types of acids. Ketonic solvents ha\ e been found to be the most effective differentiating media for the titration of acid mixtures. Using methyl isobutyl ketone, Bruss and Wyld ( 1 ) obtained as many as five inflections when titrating a mixture of perchloric. salicylic, and acetic acids and phenol. Fritz and Yama n u m (8) used acetone for several
acid mixtures. Harlow and Wyld ( I O ) studied the influence of sokents on the resolution of acids by potentiometric titration 11ith qiiatrriiary animonium titrants. Palit (18) reported that. nhen a solvent niivture like ethylene glycolisopropyl alcohol is used as the titration medium, t n o inflections can be obtained -:1 titrating sulfuric acid nit11 sodium hydroxide. This is perhaps the first case reported of the titr a t ion ' of sulfuric. acid a. both a mono- and dibasic acid. Palit also observed that. in the same solvent medium, t h r w infleetions are obtained for a mixture of sulfuric and acetic acids. Kalidas and Das ( 1 4 ) have used ethylene g l p o l isoprop! 1 alcohol as the solvent for several acid mixtures. As glycolic solvents h a r e been found to possess certain advantagrs over many other organic solvents as titration media ( I $ ) , the present investigations were undertaken to find the possibilities of utilizing this method for quantitative analyses of acid mixtures containing sulfuric acid as a common constituent. REAGENTS
The standard alkali solution v\as prepared by rapidly washing beads of sodium hydroxide with distilled water three or four times to remove the superficial coating of carbonate, and then dissolving in a rei-\- small amount of n ater in a stoppered bottle. The solution n-as first diluted with ethylene glycol and then with twice the 1-olunie of isopropyl alcohol. The strength of the solution n-as roughly determined and, if necessary, it \%-asfurther diluted with ethylene glycol-isopropyl alcohol (1t o 2) to make it nearly decinormal. The higher proportion of the alcohol in the mixed solvent helps t o reduce the viscosity of the solution and thus minimize the drainage error. The alkali solution was standardized against potassium acid phthalate disqolred in water, using VOL. 31, NO. 2, FEBRUARY 1959
233
3/
d
i
[///% SODIUM HYDROXIDE r---
p i N]
PIPERIDINE [ O IN]
ml
ml.
Figure 2. Titration of acids and acid mixtures with piperidine in ethylene glycol-acetone (2 to 1 ) 1
ml
1. 2. 3.
4. 5.
Perchloric Sulfuric-perchloric 6, Sulfuric-phosphoric
Acetic Phosphoric Sulfuric
Figure 1. Titration of su!furic acid and mixtures with sodium hydroxide
Curve 1
2 3 4 5
6
Acid or Acid hlixtiire Sulfuric Sulfuric Sulfuric-salicylic Sulfuric-cresolsulfonic Sulfuric-p-toluenesulfonic Sulfuric-acetic
Solvent Ethylene glycol Ethylene glycol-acetone ( 2 to 1) Ethylene glycol-acetone (2 to 1) Ethylene glycol-acetone ( 2 to 1) Ethylene glycol-acetone ( 2 to 1) Ethylene glycol-acetone ( 2 to 1)
phenolphthalein indicator. The alkali was also standardized against a standard solution of hydrochloric acid in butyl alcohol, using thymol blue indicator. Both the methods gave the same result for a freshly prepared solution. Piperidine mas used as a decinormal solution in isopropyl alcohol. It was standardized against a standard solution of hydrochloric acid in butyl alcohol with thymol blue indicator. The strength was also checked potentiometrically. Solutions of sulfuric acid were prepared b y mixing with analytical reagent grade acetone. After the solution had stood overnight, a light brown color developed, but no deterioration in acid strength could be detected over several days. For most of the other acids, both acetone and isopropyl alcohol were used as the solvents. For perchloric acid, however, acetone could not be used, as a dark red color developed on keeping the solution overnight. Hydrochloric acid was standardized n-ith mercury(I1) oxide after conversion to acetate (4), and this acid mas used for standardization of the alkali solutions. Perchloric and nitric acid solutions were standardized with alkali or piperidine solution in glycolic media, using thymol blue indicator. Sulfuric, phosphoric,
234
ANALYTICAL CHEMISTRY
and the organic acids were standardized by potentiometric titration n ith the alkali solution, piperidine also being used for the first two acids, All the solvents were tested for neutrality before use. Potentiometric titrations were carried out with a Cambridge bench pH meter, using a glass electrode in conjunction with a saturated calomel electrode. It n a s found more convenient to use the p H scale rather than the e.m.f. scale. The p H values as recorded on the pH meter are perfectly reproducible; although they may not have any fundamental significance, this is immaterial for titration purpose (19). The term pH has been used throughout the present work to mean the apparent pH as recorded on the pH meter, n-ith the instrument standardized against a n aqueous buffer solution. DISCUSSION
Choice of Solvent Mixture. Y h e n a freshly prepared solution of sulfuric acid in glycol-isopropyl alcohol (1 t o 1) is titrated with sodium hydroxide in t h e same solvent mixture, two breaks a r e obtained. T h e second occurs a t exactly twice the volume of base with respect t o the first, as eypected for a pure dibasic acid. On
keeping the solution of sulfuric acid overnight, the total acidity is found t o decrease, although the first break still occurs at the same point as with the freshly prepared solution. In ethylene glycol alone, unmixed with any cosolvent, sulfuric acid likewise gives two breaks, and on keeping the solution, a similar difference in the titer is observed. However, with isopropyl alcohol alone as the solvent, sulfuric acid gives only one inflection, corresponding to the total acidity; in this case also, the titer decreases nhen the solution is kept for any length of time. The gradual decrease in acidity reported above obviously arises from the fact that sulfuric acid slon1~. reacts with the hydroxylic solvents like ethylene glycol and isopropyl alcohol, presumably forming alkyl sulfuric acid: ROH
+ HZS04
-.c
ROSOJOH
H20 (1)
Thus, dibasic sulfuric acid is converted into monobasic alkyl sulfuric acid, and the total acidity consequently decreases. But the alkyl sulfuric acid, being a very strong acid, is titrated along with the first hydrogen of sulfuric acid, so that the position of the first break is not affected. However, the reaction of sulfuric acid n ith glycol or isopropyl alcohol is slow and does not introduce any difficulty when these solvents are used as titration media for sulfuric acid, as no reaction has been found to occur in course of 2 t o 3 hours. Different cosolvents, like isopropyl alcohol, acetone, and dioxane, have
' 0
I
L
1 0
-
.C
X
5% H20
J-TITRANT:
f
I
\
E
n
Na OH--
1
t
t
*ETHYLENE
I
GLYCOL- ACETONE
1
+
3 n Z
0 U
2
IL
U
u
-ETHYLENE
GLYCOL
e
m
I
PIPERIDINE [O.lfl
--
1
ml.
Figure 4. Conductometric titration of sulfuric acid with piperidine 1 ml.
ml BASE ADDED
-
Figure 3. Effect of water in ethylene glycol on titration of sulfuric acid
bren used in mixtures n-ith ethylene glycol to find the inost suitalde titration medium. Two breaks are obtaincd ill all these solvent mixtures, as also in ethylene g l ~ ~ alone. ol The second l r e a k is always niore predominant than thc first, and is more or less s i n d a r for all the solvent mixtures used. Ilioxane appears to bc the best among these cosolvents, :is judged by the sharpness of the first inflection point; acetone is slightly better than isopropj4 alcohol. Acetone was the principal cosolvent used for the present uwk; isopropyl alcohol was also used to a limited extent. R'ith isopropj.1 alcohol as the cosolvent, the sharpness of the first break is s1ightl:k- improved by using a higher propurtion of ethylene glycol (60 to 70%). Figure 1 presents typical curves for sulfuric acid titrated with sodium hydroxiclc, using different solvents and solyelit mixtures 2s titration media. Choice of Titrant. Sodium h y h o x idc in ethylene glycol-isopropyl alcohol can be used t o titrate a mixture of sulfuric acid with another mineral acid, as wcll as n-ith a n-eak organic acid like acetic acid. Sodium !iydroxide, Iion.ever, tends t o absorb carbon dioxide, forming carbonate, and an alcoholic solution of alkali has a tcndency t o undergo air ouidation, leading t o t h e formation of salts of weak organic acids. These salts of weak
arids behave as bases in glycolic solvents a n d interfere R ith t h e titration. R h e n sulfuric acid is titrated nith ti solution of sodium hydroxide in isoprapyl alcohol, the salt present, presumably sodium propionate, reacts n ith sulfuric acid to form free propionic acid. The first break occurs when the sulfuric acid h
