condensation of formaldehyde with 4-(phenylamino) benzenediazonium chloride. The maximum absorption band for these compounds occurred at 375 m l . DISCUSSION
The spectrophotometric determination of diazonium compounds has many advantages over the methods now used. The nitrometer method is time-consuming, the average sample requiring about 1 hour. Care must be taken to liberate all of the nitrogen and to adhere rigidly to the adapted technique. I n the coupling method, the rate of addition during the backtitration is critical. The procedure using standard reducing agents requires less time, but demands the preparation and standardization of reagents. All
these methods necessitate extra time in the use of blank determinations. The spectrophotometric method requires only the initial calibration curve. Once the curve is established, the sample can be run in 10 minutes. The accuracy is comparable to that of the other methods and therefore should make it a practical control method from a manufacturing standpoint. Although the 380-mfi absorption is valid only for the 4-(dialky1amino)benzenediazonium cations investigated, the method should be applicable to other benzenediazonium cations with a calibration curve established a t the proper absorption maximum. ACKNOWLEDGMENT
The authors are indebted to Frederick Von Hessert, Fairmount Chemical Co.,
for furnishing the samples of the diazonium compounds used in this investigation and to the A. B. Dick Co. for making this publication possible. LITERATURE CITED
(1) Baril, A., J . Chem. Phys. 22, 1275
(1954).
(2) Bottel, R. S., Furman, N. H., ANAL. CHEM.29, 119-20 (1957). (3) Elofson, R. M., Mercherley, P. 8., Ibzd., 21, 565 (1949). (4) Knecht, E Thompson, L., J . SOC. Dyers Colou&s 36, 215-19 (1920). (5) Pierce, A. E., Rising, M. M., J . Am. Chem. SOC.58, 1363 (1936). (6) Siggia,
S., “Quantitative Organic Analysis via Functional Groups,” 2nd ed., pp. 123-7, Wiley, S e w York, 1954. (7) Spencer, G., Taylor, F. J., J . SOC. Dyers Colourists 63, 394-5 (1947). RECEIVEDfor review April 2, 1958 Accepted August 18, 1958
Direct Titration of Hydrochloric-Sulfuric and Nitric-Sulfuric Acid Mixtures in Acetone Solvent Auto matic Derivative Potentiometric or Spectrophotometric End Point Detection H. V. MALMSTADT and D. A. VASSALLO Department o f Chemistry and Chemical Engineering, University of Illinois, Urbana, 111. Accurate automatic mzthods are described for direct titration of nitricsulfuric and hydrochloric-sulfuric acid mixtures. Using tri-n-butylmethylammonium hydroxide as titrant and acetone as solvent the proton from either hydrochloric acid or nitric acid i s titrated together with the first proton from sulfuric acid to give one end point. The second proton from SUIfuric acid i s titrated to another end point. Both end points are sharp and can b e determined successively with equal precision b y either automatic derivative potentiometric, automatic derivative spectrophotometric, or manual visual detection. The end point reproducibilities are about 0.01 ml. and a relative precision of 0.2% i s realized, i f a total volume of about 5 ml. is involved in each calculation. Solvent and titrant considerations and characteristics of the end point detection systems are discussed.
T
titration of hydrochloric-sulfuric acid and nitric-sulfuric acid mixtures with morpholine titrant in a n HE
206
ANALYTICAL CHEMISTRY
acetonitrile medium was reported by Critchfield and Johnson ( 2 ) . This method mas complicated by precipitation of morpholinium sulfate after the first end point,. It was recommended that special procedures be used to circumvent this difficulty. More recently, Bruss and Wyld (1) used tetrabutylammonium hydroxide in 2-propanol as titrant and methyl isobutyl ketone as the titration medium with potentiometric end point detection. Stoichiometry is probable with this method, but quantitative results were not presented. It would be difficult, hoxever, to analyze acid mixtures dissolved in significant quantities of water, because only a small amount of water is miscible with the methyl isobutyl ketone. The use of acetone as a solvent for differentiating acid mixtures was reported by Fritz and Yamamura (j), n-ho discussed its many advantages and stated, “its only major limitation is that it apparently enters into reactions with rather strong acids, as evidenced by loss of stoichiometry when titrating such compounds.’’ It has been found
in the present work that stoichiometry can be obtained for the titration of strong acid mixtures in acetone solvent containing small amounts of water, with tri-n-butylmethylammonium hydroxide as titrant. Solvents, titrants, and end point detection systems suitable for the titration of strong acid mixtures were investigated and results are described in this paper. The advantages of automatic derivative potentiometric and spectrophotometric end point detection for rapid and accurate titrations have been discussed (9-11). The automatic derivative spectrophotometric titration of a perchloric-acetic acid mixture in acetone-xater as solvent with aqueous SOdium hydroxide as titrant was described by the authors (I@, along with other examples. When automatic titrations are performed with two or more successive end points, the derivative system requires only that the start button be pushed again after each end point, without the setting of any controls. The application of either automatic derivative potentiometric and spectrophotometric end point detection to the
titration of typical mixtures of hydrochloric acid-sulfuric acid and nitric acid-sulfuric acid in acetone solvent provided rapid and accurate results. Results of equal accuracy for the same samples were obtained by manual visual end point detection using neutral red to indicate the first and thymolphthalein the second end point. Both indicators can be added a t the start, because the color changes are from red to yellow for the first end point and yellow t o blue green for the. second end point. APPARATUS AND MATERIALS
Automatic derivative potentiometric titrations r e r e performed with the Sargent-Malmstadt titrator ( I S ) , with a platinum (10% rhodium) indicator electrode connected directly to the grounded cathode (black lead) and a graphite reference electrode connected to the grid (red lead) of the derivative control unit. The graphite electrode was made from a 9 H pencil which had the wood removed a few inches from one end which was in contact with the solution and also for a short length from the other end to which the grid lead is clipped. The platinum (10% rhodium) electrode was of the usual type of wire sealed in glass. The automatic derivative spectrophotometric titrations were performed with titration assembly (la),consisting of the Sargent-Malmstadt derivative control unit and a modified titration stand equipped to isolate and detect a narrow band of radiation whose absorbance changes rapidly a t the equivalence point. To increase the sensitivity of detection, the barrier layer cell was replaced with a cadmium sulfide photoconductive cell (Model CDS-10, Hupp Electronics, Chicago, Ill.), The CDS10 detector was connected in series with a 2200-ohm resistor and a l.5-volt dry cell. A 0.l-pfd. condenser was connected in parallel with the 2200-ohm resistor t o eliminate alternating current noise, and the voltage drop across the parallel combination m s fed through a reversing switch t o the input of the derivative control unit. For convenience in testing and comparing the end point systems, both potentiometric and spectrophotometric titrations mere often recorded simultaneously and one or the other end point system mas used for automatic termination. This was accomplished by mounting the electrodes in the titration vessel compartment of the spectrophotometric unit (16). Standard laboratory recorders were used in obtaining the titration curves. All rubber tubing in the titrant delivery system had to be replaced with Teflon tubing, to prevent attack by the nonaqueous titrant. No tubing was available which was both inert to the titrant and also suitable for use with the solenoid-operated pinch-off valve. Therefore, the solenoid-operated pinchoff valve was replaced with an automatic stopcock twister (8). The recent availability of flexible, inert, neoprene
tubing should permit use of the solenoidoperated pinch-off valve ( I S ) , eliniinating the more complex stopcock twister. -.' d 7~b-11,self-zeroing, Tefloir stopcock buret (Fischer & Porter Co., Hatboro, Pa.), graduated in 0.05-ml. divisions, was used. Titrant addition through the stirrer shaft, as previously described for the spectrophotometric attachment (la), was not used, because it is more coniplex and docs not provide more efficient stirring than the propeller stirrer and bent delivery tip combination used with the commercially available instrument ( I S ) . It is important to use a stirrer of the correct size, and the delivery tip must be next to the stirrer shaft, just above the propeller blades, to ensure rapid mixing. The buret and delivery tips were interconnected by a short piece of Teflon tubing. The delivery tips were made from various sizes of capillary tubes to control titrant flow a t the desired delivery rates. A small holder mas mounted on the base of the titration compartment to hold the sample vessel in the correct position, and either a 35 X 100 mm. vial or a 100-ml. beaker was used for the automatic titrations. Tri-n-butylmethylanmonium iodide was prepared by reacting 93 grams (120 ml.) of tri-n-butylamine, purified by distillation from potassium hydroxide, with 75 grams (36 ml.) of methyliodide. The reaction is exothermic and sometimes requires cooling. The resulting solid was dissolved in a minimum amount of boiling acetone, cooled to ice bath temperature, and reprecipitated with dry ether. Two reprecipitations from acetone with ether were sufficient. The material is not as hygroscopic as most quaternary ammonium iodides. Tri-n-butylmethylammonium hydroxide (0.10N) was prepared by reaction of 50 grams of the iodide with 40 grams of silver oxide in 140 ml. of absolute methanol according to the procedure of Markunas and CundifT ( 3 ) . The mixture was filtered, and the filtrate was diluted to 1500 ml. with dry benzene. The titrant was stored in a dark bottle protected from the atmosphere with an Ascarite-Drierite tube. KOloss in titer was found over a 6-week period. Solutions of hydrochloric, nitric, and sulfuric acids (0.1000N) in 90% acetone were made by diluting 1.000N aqueous solutions with reagent grade acetone. The indicators consisted of 0.2% neutral red and 0.1% thymolphthalein dissolved in absolute methanol. Other basic titrants, sodium and potassium methoxide, and potassium hydroxide in 2-propanol were prepared
(4). TITRATION CONSIDERATIONS
Solvent. Five-milliliter aliquots of 0.05000M sulfuric acid in 90% acetone were titrated wit,h tri-nbutylmethylammonium hydroxide in 50 ml. of various solvents. The
I 0
I
1
I I 1 I I 2 3 4 5 6 VOLUME OF T I T R A N T , MC
7
Figure 1. Recorded potentiometric curves for 5-ml. aliquots of 0.05M sulfuric acid titrated with 0.1N tri-nbutylmethylammonium hydroxide in various solvents using platinum ( 1 0% rhodium)-graphite electrodes A. B. C.
D. E.
Acetic acid Ethylene glycol Methanol 2-Propanol ferf-Butyl alcohol
F. G
H. 1.
Acetone Methyl ethyl ketone Acetonitrile Pyridine
potential of the platinum (10% rhodium us. graphite electrode system was continuously recorded using a titrant delivery rate of 2 ml. per minute. The titration curves for various solvents are shown in Figure 1. Many of the common organic solvents are capable of differentiating the two protons from sulfuric acid, and there was no precipitation in any of the solvents. Methanol, C, and 2-propanol, D, are capable of differentiating the f i s t and second protons from sulfuric acid. tert-Butyl alcohol, E, is excellent, while ethylene glycol, B is poor. These results could have been predicted by a consideration of the relative acidities of the alcohols (7, 15). The presence of small quantities of alcohols should not interfere with the differentiation. Solvents such as acetone, F , methyl ethyl ketone, G, and acetonitrile, H , show large and sharp potential breaks for both end points. Acetone was chosen for more detailed study, because of its excellent solvent properties, purity, availability, and relatively low cost. In all of these solvents sulfuric acid was titrated t o tlTo equivalence points. In water and acetic acid solvents only one end point is observed, but in water it occurs when both protons are titrated, in acetic acid when the first proton is titrated. The effect of dielectric constant on neutral and anionic acids VOL. 31, NO. 2, FEBRUARY 1959
207
was discussed by Wolff ( I @ , and Harlow and Wyld (6) give an excellent descrintion of the effect of dielectric constaub un acid strengths. Titrant. Various titrants were tested, to find an inexpensive one that would have the desirable titration characteristics. Alkali metal hydroxides, methoxides, and isopropoxides were tested using acetone as the titration medium. Precipitation of salts occurred n-ith all these titrants and the recorded potentials shom-ed breaks which were not stoichiometric. Other workers noted similar results in other solvents (9). The titration breaks occur at some point which depends upon the volume of solvent and concentration of reactants. This is probably caused by a combination of precipitation and complexing. Organic solvents other than acetone shon-ed similar effects. Tri-n-butylmethylammonium hydroxide titrant did not have these objectionable features. The one limitation, rather high cost, is outweighed by its great usefulness in almost any solvent. Impurities. To obtain high accuracy (better than to 1 to 2%) for the determination of acid mixtures, the small quantities of acetic acid in acetone and carbonate in the base must be corrected for. The proton from the acetic acid present in acetone solvent titrates with the second proton from sulfuric acid between the first and second end points. The effect of carbonate in the base for a nonaqueous titration of a strong and weak acid mixture is analogous to that in an aqueous titration. The base has one normality for the complete titration of the strong acid, and a t the first end point each carbonate ion added has reacted with two hydrogen ions. The base has another normality for the titration of the weak acid, for a t the second end point each carbonate ion added has reacted with only one hydrogen ion. There is little chance for loss of carbon dioxide from the solution during titration, because of the solubility of carbon dioxide in acetone (0.0156 mole per 50 ml. a t 20" C.) (14)*
It is not convenient to remove the acetic acid from reagent grade acetone or the carbonate from the base. It is simpler to obtain and apply correction factors for both impurities. The methods are described in the standardization and calculations sections. Strong acids such as perchloric or phosphoric interfere with the first end point, while weak acids such as benzoic or acetic interfere with the second end point. Although methods can be developed for determining these interferences separately, the method proposed is especially for the determina208
ANALYTICAL CHEMISTRY
tion of nitrating and chlorosulfonating mixtures. End Point Detection. The automatic derivative poteni.;t.?kb1 IC system is especially desirable, because of its simplicity when using a platinum (10% rhodium)-graphite electrode system. This electrode system was recommended by Malmstadt and Fett (9) for a variety of aqueous and nonaqueous titrations, and has been adopted for many applications ( I S ) . The electrodes can be connected directly to the input of the Sargent-Malmstadt derivative control unit without any high impedance amplifier such as required for the glass electrode. The platinum (10% rhodium) indicator electrode responds more rapidly than the glass electrode, and this prevents overstepping the end point (9). The graphite electrode provides a simple reference electrode without a troublesome salt bridge. The use of the platinum (10% rhodium) us. graphite electrode system would not be satisfactory in titrating to a fixed end point potential, because its absolute value shifts from one titration to the next. In fact, reproducible potentials m-ould be difficult to obtain with any electrode system in a mixed solvent system containing water and alcohols (6). However, the automatic derivative system is ideal for such situations, because the maximum rate of change of the platinum (10% rhodium) indicator electrode always occurs a t or very close to the equivalence point, regardless of the absolute potential. Automatic derivative spectrophotometric titrations are always a possibility if either one of the reactants or reaction products (10) or an added indicator (fb) undergoes a rather rapid absorbance change a t the equivalence point. Color indicators are known or can be found for most titrations in either aqueous or nonaqueous systems. They can be the basis for either manual visual or automatic derivative spectrophotometric end point detection. TITOindicators, neutral red and thymolphthalein, changed color a t the two equivalence points for the titration of hydrochloric-sulfuric and nitricsulfuric acid mixtures. Both indicators are added a t the start of the titration and neutral red changes from red to yellon- a t the first end point and th-ymolphthalein changes from yellow to green a t the second end point. This is the basis for the manual visual titration of the acid mixtures. The same indicators are the basis for automatic derivative spectrophotometric titration of acid mixtures. Both indicators provide sharp changes of absorbance a t a wave length of 575 mp. Therefore a 575 mp interference filter can be used t o isolate a narrow band of radiation Khose intensity on
the detector increases suddenly a t the first and decreases a t the second end point. The voltage changes from the photo detector are fed directly to the Sargent-Malmstadt automatic derivative control unit (19). Stoichiometry. The stoichiometry of the reaction of strong acids with tri-n-butylmethylammonium hydroxide in acetone was verified by crosschecking hydrochloric, nitric, and sulfuric acid solutions with aqueous and nonaqueous base solutions. Two 0.1000N solutions of each of the acids were prepared from the same 1.000N aqueous acid solutions, one in aqueous solution and the other in 90% acetone. Five-milliliter aliquots of both solutions were titrated with standard 0.1OOON aqueous base in 40 ml. of water solvent. Five-milliliter aliquots of the acids in 90% acetone were added to 40 ml. of the acetone solvent and they were titrated with tri-n-butylmethylammonium hydroxide, which was standardized against benzoic acid in acetone. All titrations checked within a range of 0.3% of the correct values. No significant interaction of the strong acids with 90% acetone was observed over a period of a few hours. PROCEDURES
Standardization. When a 10-ml. aliquot of standard hydrochloric acid in 90% acetone is added t o 40 ml. of acetone solvent (containing small quantities of acetic acid) and titrated with 0.1N tri-n-butylmethylammonium hydroxide titrant (containing a few tenths per cent of carbonate), two sharp end points are obtained which are usually about 0.1 ml. apart. These can be observed best by recording the titration curve, while adding the titrant continuously a t a slow rate in the region of the end points, about 0.2 ml. per minute, but both end points can be determined accurately by any of the three methods of detection described in the previous section. At the first end point each carbonate ion added v a s reacted with two hydrogen ions, and the effective normality, hi1, for the titration of strong acids can be calculated by dividing the milliequivalents of hydrochloric acid by the milliliters, VIS, to the first standardization end point. At the second end point the carbon dioxide produced up to the first end point has reacted with the base to form bicarbonate and the acetic acid in the acetone has been titrated. The effective base normality, N z , for the titration of weak acids (such as bisulfate ion in acetone) can be calculated by dividing the milliequivalents of hydrochloric acid by (VW - V O ) , where VzS are the milliliters to the second standardization end point and
Vo are the milliliters of base required to titrate the acetic acid in the total quantity of acetone used for either the standardization or the unknoivn mixture. For most carefully prepared bases the ratio NJN1 was about 0.995. V o is found by titrating 50 ml. of acetone solvent with the titrant to be standardized. For all bottles of reagent grade acetone tested there was about 0.001 meq. of acetic acid per 10 ml. of acetone, or, in other words, Vo mas about 0.05 ml. for the usual volume of about 50 ml. of acetone used for the reported titrations of acid mixtures. For a 5-ml. titration this amounts to a 1% error unless corrected for. Vo is most easily determined by using continuous mechanical stirring with the buret tip below the solution surface and thymolphthalein visual end point detection. Mixed Acid Samples. Accurately weighed, concentrated nitric-sulfuric acid samples (containing about 10 meq. of acid) are added t o 10 ml. of water in a 100-ml. volumetric flask, and after cooling, the contents are made up t o volume with reagent grade acetone. A 10-ml. aliquot is pipetted into a suitable titration vessel, and 40 ml. of acetone are added, and the solution is titrated with the standard 0.1N tri-n-butylmethylammonium hydroxide. Any of the three recommended methods of detection can be used to determine the two end points. Chlorosulfonating mixtures must be weighed in oleum bulbs, which are then broken in a stout flask containing 10 ml. of water. After the solution has been cooled, the contents are quantitatively transferred into a 100-ml. volumetric flask, using reagent grade acetone for washing the flask and diluting t o volume. Automatic Derivative Potentiometric Titration. A platinum (10% rhodium) and graphite electrode system, connected directly to the input of the automatic derivative titrator, is immersed in a sample solution contained in a 100-ml. beaker. The delivery of the titrant, adjusted to a rate of about 2 ml. per minute, and the stirring are started by pushing the automatic titrator button. Upon automatic termination and reading of the first end point, the 10-ml. automaticzeroing buret is refilled and the automatic start button pushed again. By this method, the buret reading after automatic termination a t the second end point is the volume difference in milliliters between first and second end points, referred t o as V Din Equations 8 and 9. A typical set of data for single automatic titrations of various small quantities of hydrochloric and sulfuric acid mixtures is given in Table I. Better accuracy is obtained by titrating larger amounts which require larger volumes of titrant or the use of a more accurate
Table 1.
Taken
Automatic Derivative Potentiometric Titration of Strong Acids in Acetone Solvent with 0.1 N Tri-n-butylmethylammonium Hydroxide Titrant
H2S04,Meq. HCl, Meq. HS03, Meq. Found Error, Found Error, yo Taken Found Error, yo Taken
0.4500
0 449oa
7~
-0.2
0.2700 0.2705 + 0 . 2 -0.2 0.4500 0.4490 0.4500 0.4490 -0.2 0.2700 0.2720 +O. 7 0.4500 0 ,4 5 1
