Indicator Trcrnsition Ranges in Tertiary Butyl Alcohol SIR: The use of tertiiry butyl alcohol as a solvent for the potmtiometric titration of acids of various functional types has been illustrated by Fritz and Marple (1). For roiitine analj ses, however, a n indicator end point i5 preferred t o a potentiometric titration. Previously the inability to reproduce potentials when titrating acids xith quaternary ammonium hydroxide titrants in organic solvents has hindered the determination of indicator transition ranges. However, a n electrode system recently developed (3) makes it possible to reproduce titration curves in tertiary butyl alcohol very well. It now becomes feasible to titrate a n acid (or acid mixture) potentiometricalk; and to choose a suitable indicator for ihe routine titration of that acid (or mixture). We have determined the iiual transition ranges of a number of indicators in tertiary butyl alcohol under conditions approximating an actual titration. A mixture of sulfuric and malonic acids, when titrated with tetrabutylammonium hydroxide, serves as a suitable buffer over almost all of the potential range available ir tertiary butyl alcohol. TThile it is recognized that the use of two basically different types of buffer systems (neutral and anionic) will lead to Some differences in transition ranges of indicators in the weakly acidic range relatire i o those in the 7,
strongly acidic range ( 2 ) , these differences are minor compared to the effects of other factors such as ionic strength. Moreover, the dissociation of tetrabutylammonium bisulfate, for instance, is so low (at 10-2M concentration) that it is really like a neutral buffer instead of an anionic buffer.
Table I.
o-Phenglphenol" Malonic a c i d " Trial Base Trial Base no. reqd., ml. no. reqd., ml. 1 13.9s 1 8.37 2 14.03 2 8.45 3 13.99 3 8.35 4 14.01 4 8.35 5 13.99 6 14.00 8.38 Average 14.00 zkO.02 & O . 05 Std. dev. a Indicator: p-nitro-p'-aminoazobenzene Blank correction: 0.09 ml End point: 13.98 ml. 6 Indicator: o-nitroaniline Blank correction: 0.05 ml. End point: 5.41 ml.
EXPERIMENTAL
The indicators used are all commercially available materials from Eastman Organic Chemicals or HartmanLeddon Co. dpproximately 1 mg. of indicator was used to 40 ml. of tertiary butyl alcohol in a titration cell of the type described previously ( 3 ) . T o this, 2 mmoles of a 1: 1 sulfuric acid-malonic acid mixture was added, and the solution was titrated with 0 . l X tetrabutylammonium hydroxide. The potential was followed with a glass electrode that was used in previous work on the titration of acid. ( I ) , so the potentials reported for the transition ranges can be correlated with previously published data. The limits for an indicator transition were set by the first observable color change, and by the point of no further change. The uncertainty of the transition edges is approximately =tlO mv. Since the color of the basic form of the indicator was generally more intense than the acidic form, titration of the basic form of the indicator with acid to establish the basic end of the transition range n-ith greater accuracy was not attempted.
Figure 2 shows the transition ranges of the indicators and the colors at each end of the range. I n the case of the sulfonphthaleins, a definite green color formed a t an intermediate point in the conversion from the yellow acid form to the blue base form. I n the case of bromophenol blue, the formation of the green color was sharp enough to use as an end point. The change of methyl red from violet to orange takes place in solutions more acidic than sulfuric acid and was determined by perchloric acid titration. The basic edge of the transition range of o-nitrosniline could not be located with certainty since the e.m.t. changes only slightly a i t h an increase in the amount of excess base. Both o-nitroaniline and p-nitro-p'aminoazobenzene function well as indicators for very weak acids. The color change of p-nitro-p'-aminoazobmzene
DISCUSSION
The titration curve for the sulfuric acid-malonic acid mixture is shown in Figure 1. Except in the -100 to -200 mv. region, the potential could be changed by as little as 5 mv. by the addition of an increment of base.
ORANGE
I R M T W M O L BLUE PICRAMIDE 2,4-MNlmODIPHEhYLAMIhE
Indicator Titration Data
1
,
YELLOW
IYE,LLOW OREEN
Yi+]ORbhGE
YELLOWI
ax,
2.4- DIN(TWANIL1N E MILLILITERS
TITRAN:
Figure 1. Titration of sulfuric acidmalonic acid buffer First and third potential iweaks correspond to titration o f sulfuric acid and tetrabutylammonium bisulfate
COLORLESS
2,4-DrmlTROTOLUENE
p-NITRO-
-
IORANGE
BLUE
ORANGE
BLUE
-Go
P'- AMINOAZOBENZENE
e- NITROANILINE
ORANGE
AZO VIOLET
GREEN BLUE
Figure 2. Indicator transition ranges of some common indicators in tertiary butyl alcohol solvent VOL. 35,
NO. 9, AUGUST 1963
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from orange to green is sharp, although the indicator blank is relatively large. I n Table I are shown data for the titration of o-phenylphenol with 0.02M tetrabutylammonium hydroxide using p -nitro-p '-aminoazobenzene indicator. Correction for the indicator blank gives excellent agreement with the results of a potentiometric titration. Comparison of the precision of the two methods is not valid. I n the indicator titrations, the transitions are very distinct, owing to the reaction of a very small amount of dye. The potentiometric end point is, in both cases, subject to a relatively large variation due to the small potential break at the equivalence point. Replicate potentiometric titrations would be subject to errors that are larger than those associated with the
indicator titrations, and hence the precision would be poorer. The potentiometric values were included to show that the indicator end points are almost identical to the inherently more accurate potentiometric end points. The titration of malonic acid using o-nitroaniline was also studied. Here also, the results agreed well with the potentiometric values. Use of indicators that contain sodium or potassium salts, or indicators that are in the form of a sodium or potassium salt should be avoided unless the transition range is more positive than -0.3 volt. 4 t potentials more negative than this, the transference of the alkali ion is so large that the color change interval cannot be established (with any certainty). This does not preclude the
use of an indicator that contains sodium or potassium, since a trial and error method may still be used to establish its usefulness. LITERATURE CITED
(1) Fritz, J. S., Marple, L. W., h A L . CHEM.34,921 (1962). 12) Kolthoff. I. 11..Guss. L. S..J. Am. Chern. Soc: 60. 2518 11938). (3) Marple, L. 'W., Fiitz, J. S., AISAL. CHEM.34, 796 (1962). LELAKD W. MARPLE JAMESS.FRITZ Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Ion-a Contribution S o . 1229. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
Modification of a Thermogravimetric Balance for Pyrolysis Experiments in a Controlled Atmosphere and after the pyrolysis, as has been proposed in the literature. A similar difficulty has arisen in the modification of the newer recording thermogravimetric balances for pyrolysis in a controlled atmosphere. For example, the otherwise satisfactory technique of Vassallo ( 9 ) employs a downward flow of inert gas which may encourage condensation on the lower part of the weight transfer rod-the rod which transmits the load of the sample in its holder to the balance mechanism.
SIR: The quartz spring balance of LVladorsky (3, 4) has been criticized by Jellinek ( 2 ) . He points out the danger, in any suspended system, of condensation of volatiles on the (cooler) support and spring early in the pyrolysis, resulting in too low a measured weight loss. Later in the pyrolysis, the condensate may volatilize, giving too high a weight loss at long times of heating. To show the absence of such condensation effects, it is clearly not sufficient to weigh the support and spring before si I ico ne rubber
I
f
StOS
A similar criticism can probably be made of the more elaborate system described by Stonhill (8). Tn an attempt to overcome this disadvantage, Soulen and RIockrin ( 7 ) have modified the Chevenard balance by passing the weight transfer rod through a sleeve, with most of the outflow gas passing down the outside of the sleeve where condensation may occur without influencing the recorded sample weight. They claim that, in the absence of the sleeve. the thermogravimetric curveq may be seriously in error. While we have no experience with the Chevenard balance, we have recently had occasion to modify a Stanton TRI thermogravimetric balance for a study of polymer pyrolysis in an inert atmosphere (6). Using the technique of Soulen and Mockrin, we found it difficult to center the rod accurately enough to prevent rubbing on the inside of the sleeve and consequent distortion of the weight-loss curve. Instead, an upward gas flon- Tvas used, which has the advantage of carr! ing corrosive pyrolysis
/p
/' tinplate
push f i t
lid
alloy
0
2
4
U
I inch
YITROGEN
balance
Figure 1 . Design of gland for introduction of inert gas to vertical furnace
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ANALYTICAL CHEMISTRY
FLOW
Figure 2. Relationship between apparent weight gain at 20' C. and nitrogen flow rate
