plastic cement. If silicone grease is employed, regreasing will be Only infrequently required. Because the major function of the stopcocks is the isolation of the chambers during the short filling operation, slow leaks between the chamhers are of no consequence. The O-rings effectively prevent leakage to the exterior* On the Other hand, if One Or more of the chambers is to remain empty during an experiment, the closed stopcock should be supplemented with a stopper of the type described above for use in the absence of stopcocks.
LITERATURE CITED
(1) Alm, R. S., Williams, R. J. P., Tiselius, A., Acta Chem. S c a d 6 , 826 (1952). (2) Bock, R. M., Ling, N*-S.,ANAL* CHEM* 26, 1543 (1954). (3) B ~ H. G., ~ ~ i~ ~ ~et~h~ i i~, .~ ~ c t 16, a 245 (1955). (4) Busch, H., Hurlbert, R. B., Potter, V. R., J. Biol. Chem. 196, 717 (1952). (5) Gherkin, A., Martinez, F. E., Dum, hl. S., J . Am. Chem. SOC.75,1244 (1953). (6) Desreux, V., Rec. trav. chim. 6 8 , 789 (1949). (7) Donaldson, K. o., Tulane, v. J., Marshall, L. M., ANAL. CHEM.24, 185 (1952). (8) Drake, B., Arkiv Kemi 8 , 1 (1955).
(9) Lakshmanan, T. K., Lieberman, S., Arch. Mitchell, Biochem. Biophys. K., Gordon, 45, 235 M.,(1953). Haskins, F. A., J . Bid. Chem. 180, 1071 (1949). ~(11)h Parr, ~ c. ~ W., . Proc. Biochem. SOC., 324th Meeting, xxvii (1954). ('?{&$' K* A'1 ANAL' 28, 1451
( l ~ ? a ~ ~ ~ ~ ~ ' , ~ ; l Fedl f i e ~ ~ ~ ~ RECEIVED for review November 19, 1958. Accepted February 9, 1959. Presented in art before the Federation of American Eocieties for Experimental Biology, Philadelphia, Pa., April 1958.
Automatic Derivative Potenti ometric and Spectrophotometric Titrations of Organic Acids H. V. MALMSTADT and D. A. VASSALLO' Deparfrnenf of Chemistry and Chemical Engineering, University of Illinois, Urbana, 111.
b Nonaqueous titrations of carboxylic acids, sulfonamides, imides, mercaptans, phenols, and enols can b e performed rapidly and accurately with a single titrant-solvent combination and automatic derivative potentiometric or spectrophotometric end point termination. The acids are titrated in acetone solvent with tri-n-butylmethylammonium hydroxide titrant. The applicability of many electrode pairs and indicators to potentiometric and spectrophotometric procedures, respectively, was investigated, and the most suitable ones applied to the automatic titration of a variety of acids. The end point reproducibilities are within 0.01 mi. of titrant, the buret reading error, and the results show that the automatic derivative procedure is an accurate method for determining percentage purity or neutralization equivalents of organic acids.
R
literature has contained several reports on the titration of acids in relatively inert solvents (1-4, 6, 7). Most titrations of organic acidic materials are currently performed by recording the titration curves ( I ) or by a manual potentiometric procedure (4). Investigations in this laboratory of the automation of titrations by a derivative technique (10-15) demonstrate that a wide variety of acids can be titrated rapidly and accurately using acetone solvent, tri - n butylECENT
-
Present address, Polycheniicals Department, Experimental Station, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.
862
ANALYTICAL CHEMISTRY
methylammonium hydroxide titrant, and either automatic derivative potentiometric (10) or automatic derivative spectrophotometric ( I S ) end point termination. Many electrode systems were studied to determine which was most suited for automatic derivative potentiometric end point detection in acetone solvent. A platinum (10% rhodium) indicator electrode us. graphite reference electrode connected directly to the input of the commercially available SargentMalmstadt automatic titrator (15) was suitable in titrating automatically all except the very weakest acidic materials, such as phenol. Such materials can be titrated automatically using azo Violet indicator and the derivative spectrophotometric end point detection method. Other indicators were studied in acetone solvent and the most suitable were selected and tested using automatic derivative spectrophotometric end point termination. The same indicators are suitable for manual visual end point detection in acetone solvent. The precision of the automatically obtained results was generally rrithin 0.01 ml. of titrant, the reading error for the buret used. The automatic titration procedure is rapid and accurate for determining percentage purity or neutralization equivalents for organic acids. Fritz and Yamamura (4) used acetone and Bruss and Wyld ( I ) used methyl isobutyl ketone (4-methyl-2-pentanone) as solvent for the titration of organic acidic materials using quaternary ammonium hydroxides as titrants. hlalmstadt and Vassallo described the use
of acetone-water solvent for the quantitative detennination of perchloric-acetic acid mixtures ( I S ) and sulfuric-hydrochloric and sulfuric-nitric acid mixtures (19). Acetone was selected for this study because of its availability, low cost, purity, and general solvent characteristics. APPARATUS A N D MATERIALS
TITRATOR. Automatic derivative potentiometric and spectrophotometric titrations can be performed using a modified titration stand ( I S ) for the Sargent-Malmstadt titrator, but the new commercially available SpectroElectro titration stand (E. H. Sargent &- Co., Chicago, Ill.) is more convenient for switching to either potentiometric or spectrophotometric end point termination, with simple polarity, electrode and filter selection, and an easy method for the insertion and removal of the titration vessel. The photoconductive detector circuit was used for the spectrophotometric procedure and the platinum (10% rhodium) us. graphite electrode pair for the potentiometric procedure. RECORDER. Model G-10 Varian (Varian Associates), 100-mv. recording potentiometer. BURET. A 10-ml., self-zeroing, Teflon stopcock buret (Fischer & Porter Co.) was filled with titrant from an elevated bottle through the side arm which was fitted with a Teflon stopcock, and all interconnections were made with either Teflon or polyethylene tubing. BURETVALVE. Gum rubber tubing with a pinch-off solenoid is commonly used for starting and stopping the delivery of titrant, but the tubing is attacked by the nonaqueous titrant and requires frequent replacement.
Table 1. hqueous PH Interval
Indicator
0.01N
Perchloric Acid
Indicator Colors"in Various Acetonic Buffers Perchloric Nitric Acidb Acidb Acetone
Benzoic heid
-4nthranilic Acidb
Excess O-NOz Phenolh Phenolb Base
Glass vs. Calmel, Mv. +685
+580
+200
-150
-250
-350
Azo Violet Y Y Y Y Y 9.3-io.5 Thymolphthalein C C C C (! 8.3-10.0 PhenolDhthalein C C C C C 8.0-9.6 r 0 Thymdl blue Y Y Y 12.0-14.0 hcid Fuchsin r r r r r Bceto redc Y Y Y Y Y 9.8-i1 . 6 1-Naphtholbenzein g Y Y Y Y 6.0-7.6 Bromothymol blue Y Y Y Y Y Azo Violet I Y s Y Y Y 4.8L6.4 Chlorophenol red 3' Y Y Y P 3.04.6 Bromophenol blue b Y Y Y Y 3 . 8-5.4 y Bromocresol green b Y Y g 3.7-5.2 Alizarin Red S V V Y IO, 0-1 1 . 0 Aniline Blue i9l b b 0 b b b r Methyl blue b r r r X'eutral red 6.8;s. 0 Y Y 4.66.2 r 0 0 Methyl red Y Y r 3.1-4.4 Methyl orange 0 Y Y Y V C 3.0-5.0 b r C Congo red 1.2-2.3 r Metanil yellow Y Y Y Y C 0.0-2.0 c C Malachite green HCl g g V V V 0.1-3.2 V b Methyl violet V v V 0.8-2,6 V Crystal violet b b, blue; e, colorless; g, green; 0,orange; p, pink; r, red; v, violet; y, yellow. 0 Color code. b Half neutralized. Diazotization product of p-acetoaniline and o-cresol.
g
However, neoprene tubing, now supplied with the titrator, has the necessary flexibility and chemical properties. The recently described automatic stopcock twister (9) replaced the tubingsolenoid system in this work. ELECTRODES. Nickel, Chromel, tungsten, rhodium, copper, silver, palladium, platinum (I 0% rhodium), tin, iron, titanium, antimony, and platinum wires or rods sewlPd in glass tubes; a general-purpose Beckman glass electrode (Beckman Instruments, Inc.) ; a Sargent calomel elcctrode modified with saturated mc.tlianolic potassium chloride to replace the aqueous solution; and a graphite electrode made by stripping a few inches of wood from one end of a 9 I-I pencil. These electrodes were used in various combinations as described. INDICATORS. Commercially available indicator materials were made up as 0.1% solutions in absolute methanol. SOLVENT.Reagent grade acetone. ORGANICACIDS. Various reagent grade and C.P. acids analyzed as received. TITRAKT. The titrant is tri-n-butylmethylammonium hydroxide in about 80% benzene-20% 2-propanol. Forty grams of silver nitrate were dissolved in 250 to 300 ml. of demineralized water and to this were added 100 ml. of 25% sodium hydroxide, which was prepared from a saturated aqueous solution of sodium hydroxide. The resulting silver oxide precipitate was washed by decantation with ten 100-ml. portions of water, collected on a filter, and washed twice with water, six times with absolute methanol, and finally with one 100-ml. portion of 2-propanol. The moist silver oxide was added to a previously prepared solution of 32.8
grams of tri-n-butylmethylammonium iodide (12) in 90 ml. of 2-propanol contained in a glass-stoppered 300-ml. flask. After the flask had been shaken for 15 minutes, the mixture was poured through a sintered-glass filter funnel into a dark storage bottle. The flask was rinsed with three 25-ml. portions of 2-propanol, and the filtrate in the dark bottle was made up to 1 liter with dry benzene. This titrant, protected from carbon dioxide and moisture by an Ascarite tube, was stable for more than 1 month. Moist silver oxide was used because it reacts much faster than dry material. 2-Propanol, being less acidic, was used rather than methanol (7). ELECTRODES FOR AUTOMATIC DERIVATIVE POTENTIOMETRIC TITRATION
Various types of reference and indicator electrodes can be used with the automatic derivative titrator, even though their absolute potentials shift from one titration to the next. The platinum (10% rhodium) us. graphite and other electrode pairs were tested for applicability in the titration of organic acidic materials using acetone solvent and tri - n - butylmethylammonium hydroxide titrant dissolved in about 80% benzene-20% 2-propanol. No advantage was apparent for any electrode pair over the platinum (10% rhodium) us. the pencil lead reference electrode which is available, simple, small, and reliable for the automatic titration of a variety of acids in acetone solvent and applicable for previously described methods (10, 12, 16). None of the electrode pairs was suitable
- 450
P
r
C C
C
Y r Y
Y g
P r b b V 0
r Y Y Y
r
Y C V V
C
y r
t:
g-b r r b
- 580
-750
r b
V
b r b
r b
Y r
Y
r b .b r r b
b b V
r b
b
b
V 0
V 0
V 0
r Y Y Y r Y
b
r
r
Y Y Y
Y 9 Y r Y
C V
C
V
V
r Y
C V V
V
for titrating phenol itself without amplifying the signal prior to feeding into the derivative titrator. Recorded potentiometric titration curves using various electrode pairs are shown in Figures 1 to 4, for the titration of 5-ml. aliquots of 0.1N benzoic acid in 50 ml. of acetone. The titrant was added continuously a t a rate of about 2 ml. per minute. For the automatic derivative titrations an electrode pair was connected directly to the input of the titrator, except for the glass us. calomel pair, which was connected via a pH meter. The electrode materials (Figure 1) respond rapidly and provide sharp changes of potential a t the equivalence point. All of these materials can be used as indicator electrodes vs. the modified calomel electrode. The electrode materials (Figure 2) change potential rather slowly in the equivalence point region, and these electrodes have possibilities as reference electrodes to replace the calomel electrode and salt bridge. Figure 3 shows the potentiometric curves for platinum (1Oyo rhodium), nickel, and Chromel us. graphite, and it is apparent that the pencil lead acts as a good reference electrode for automatic derivative titrations, yielding titration curves similar to those with the calomel reference electrode. Rather unusual titration curves are obtained using either platinum (10% rhodium) , nickel, or Chromel us. tungsten as the electrode system, as shown in Figure 4. The equivalence points on these curves are marked with an VOL. 31, NO. 5, MAY 1959
0
863
P t (IO l b Rh) VS W
I
l
1
l
I
l
l
2 3 6 5 M I L L I L I T E R S OF BASE
,
6
'
Figure 2. Recorded curves for titration of benzoic acid Colomel reference electrode vs. various materials with characteristics suitable as automatic derivative reference electrodes
1
2 3 6 5 M I L L I L I T E R S OF BASE
6
7
Figure 1 . Recorded curves for titration of benzoic acid
N I V S GRAPHITE
Various indicator dectrodes vs. calomel reference electrode
Most of the present color indicators for nonaqueous titrimetry have been used on an empirical basis. Some answers as to choice of an indicator can be obtained from the literature, but most investigations were done to determine the relative strengths of acids in various solvents (5, 8). Only phenolphthalein, thymolphthalein, and thymol blue hare received much attention as indicators in inert solvents (14). A study was made as to xvhich indicators function in acetone solrent and their applicability in titrating 7-arious acids. Some pseudobuffer systems were prepared which covered a wide range of acidity; 0.LV solutions of different acids: were half neutralized with 0.1N tri-nbutylniethylammonium hydroxide and diluted tenfold with acetone to form buffers n hich were O.OO5N in both acid and salt. The acids used. in order of decrrasing relative acidity, were perchloric, nitric, benzoic, anthranilic, onitrophenol, and phenol. Solutions of 0.01N perchloric acid, 0.01N tri-nbutylniethylammonium hydroxide, and acetone were also used to find the color of each indicator in these solutions. To 10 nil. of each of the above solu-
864
b
ANALYTICAL CHEMISTRY
Figure 4. Recorded curves for titration of benzoic acid
w i 1 CHROMEL VS. GRAPH1
INDICATORS FOR AUTOMATIC DERIVATIVE SPECTROPHOTOMETRIC TITRATION
MILLILITERS OF BASE
from the total volume of titrant used for titration of the acid. The milliliter acetone blank is usually about 0.01 nil. of 0.1.Y titrant per 10 ml. of acetone used, and is most easily determined by the manual titration of a 50-nil. aliquot of acetone to the thymolphthalein end point.
8
arrow. There is no attempt to explain the shapes of these titration curves, but they are suitable for automatic derivative end point termination The potential across the electrodes is fed directly into the derivative titrator and the titration is terminated automatically at the equivalence point (marked by the arrow).
1
Platinum (10% rhodium), nickel, and Chromel electrodes vs. tungsten electrode
t 0
CHROMEL V S W
0
1
2 3 4 5 MILLILITERS OF BASE
6
Figure 3. Recorded curves for titration of benzoic acid Platinum ( 1 0% rhodium), nickel, and Chromel indicator electrodes VI. graphite reference electrode
tions were addrd 2 drops of 0.1% indicator. The colors of the indicators in the various solutions are shonn in Table I. Other indicators 15-hich function in water but did not have a pronounced color change in acetone were eosin (0.0 to 3.0), m-cresol purple (1.2 to 2.8), phenol red (6.8 to 8.4), and Tropeolin 0 (11.1 to 12.7). The choice of indicator for a specific titration nw made with the aid of T'ihle I. PROCEDURE
4 sample containing approximately 0.5 meq. of acid is dissolved in a small quantity of a suitable Eolr-ent, 40 ml. of acetone are added, and it is titrated with standard tri-n-butylmethylammonium hydroxide titrant. Automatic end point termination can be performed by either the derivative potentiometric or derivative spectrophotometric procedure. The samples may be dissolved in many solvents-alcohols, ketones, ethers, hydrocarbons, or chlorinated hydrocarbons, as long as the final titration medium contains a t least 90% acetone. Water can be used, except for very neak acids such as enols and phenols. Acetic acid in regular reagent grade acetone causes a small titration blank which is determined and subtracted
Automatic Derivative Potentiometric Titration. A platinum (10% rhodium) indicator electrode is connected directly to the grounded cathode (black lead in cable) and the graphite reference electrode t o the grid (red lead in cable) of the Sargenthlalmstadt automatic derivative titrator. B sample contained in a small beaker is inserted into the titrator; the delivery of titrant, previously adjusted to a rate of about 2 ml. per minute, and the stirring are started by pushing the automatic titration button; upon automatic termination a t the end point, the volume is read and recorded. If a second end point is to be obtained for a mixture, the button is pushed again and thp buret read after the automatic termination. Automatic Derivative Spectrophotometric Titration. The correct filter and polarity positions are set on the Spectro-electrotitration unit as determined by the indicator t o be used (Table 11). Tn-o drops of a suitable indicator are added t o the sample contained in a small beaker, the beaker is inserted into the titrator, the start button is pushed, the titration automatically terminated a t the end point, and the volume of titrant delivered is recorded. It is important that the correct stirrer be used in the titrator, ~o that the stirring is efficient but that no air n hipped into the solution. CALCULATIONS
The per cent purity of a known acid is calculated using Equation 1, and the neutralization equivalent of an unknown acid by Equation 2 .
7cpurity
=
10 IT'
(VS
(1)
Seutralization f,quivalerit
=
Table II.
w h m *YBis the normality of base,
1 ' ~the milliliters of base from start of titration to the end point, I', the
milliliters of base to titrate impurity in the acetone, TV, the gram equivalent weight of acid, and TT' the grams of sample titrated. RESULTS
The precision for both the automatic derivative potentiometric and spectrophotometric methods is illustrated (Table 111) for a series of 10 titrations each for a relatively strong acid, benzoic, and a weak acid, succinimide. The standard deviation in all cases is about the reading error for the buret used. The percentage purity for each of several organic acids, mostly reagent grade, was determined by automatic derivative end point termination (Table IV). The platinum (10% rhodium) us. graphite electrode pair was used for the potentiometric procedure; for the spectrophotometric procedure azo Violet indicator was used for the acids succiniinide through mercaptobenzothiazole, and thymolphthalein indicator used for benzoic acid and the last four acids in Table IV. Determination of mixtures of mineral acids by automatic derivative potentiometric and spectrophotometric titration has recently been described (la). Mixtures of salicylic acid and methyl salicylate were titrated by the potentiometric procedure in this study (Table V). The accuracy of the results was limited because of the small volumes of titrant used. The reproducibility for each mixture was \\ithin the buret reading errors. LITERATURE CITED
(1 ) Bruss, n.B., S'yld, G. E.A,, A\N.41J. CHEX 29, 232 (1957). (2) Cundiff, R. H., Markunas, P. C., Ibid., 28, 792 (1956). (3) Deal, V.Z., Wyld, G. E. A , , Zbzd., 27, 47(1955). (4) Fritz, J. S., Yamamura, S. S., Ibid., 29, 1079 (1957). ( 5 ) Hammett, L. P., J . i l m . Chern. SOC. 50,2666 (1928). (6) Harlow, G. A., Noble, C. ill. Wyld, , G. E. A., AKAL.CHEX 28, 785 (1956). ( 7 ) Harlow-.G. -4.. Wvld. G. E. A , . Ibid..
Selected Indicators for Various Acid Types
Filter, Polarity Acid Tl-pea Indicator Color Change 11p Position* GOO 2 Azo Yiolet Red-blue Very m eak acids Colorless-Hue 575 2 Substituted acetic aclds Thymolphthalein 575 2 Yellow-blue Substituted benzoic acids Thymol blue Red-y ellow 550 1 Mineral acids Neutral red a Sitrated phenols may be similar in strength to any of these acids. Acidic strength of phenols increases directly with number of nitro groups and depends somewhat upon their position in benzene ring. * Polarity switch position 1 connects output irom photoconductive detector circuit so that a decrease in absorbance at the end point provides a positive grid voltage swing at the input to the derivative control unit. position 2 gives correct connection for an increase of absorbance at the end point for the preselected wave length.
Table 111. Reproducibility of Automatic Derivative Nonaqueous Titrations
Titrant for 5 111. 0.1-1Benzoic Acid, 311. SpectroPotentio- photometric metrica 5 250 5 260 52;i5 5253 5 235 5 260 5 250 5 265 5 255 5 255 5 265 5 250 5 255 5 263 5.250 5.270 5 260 5 250 5.250 5 260 Av . 5 253
5 258
Titrant for 5 n11. 0 . 1 s Surcinimide Acid, All.
I'otentiometric 5 255 5 260 5 255 5 260 5 250 5 250 5 255
