ANALYTICAL CHEMISTRY
1126
The n.ethod is applicable to the analysis of a m-ide variety of ferrous and nonferrous metals and alloys, providing suitable separations are devised for removal of the bulk of the sample without loss of aluminum. EXPERIMENTAL
In order to demonstrate the effectiveness of the cupferronperoxide-oxine separations as a means of removing interference in the aluminon-thioglycolic acid method for aluminum, portions of solutions of various metal ions were transferred to 150-ml. separatory funnels, 1 ml. of perchloric acid was added, the samples were diluted to 50 ml., and the determination of aluminum was performed as in the preparation of calibration curve. The results obtained are recorded in Table I. I n experiments 7,8, and 9 , 3 ml. of cupferran solution mas added andtheextracted solution was extracted with 5 ml. of pure chloroform to free it of
lead metal, 2 grams of antimony metal, 2 grams of tin metal, 2 grams of NBS tin base alloy KO.54-c, 2 grams of NBS solder No. 127, or 2 grams of NBS lead base alloy No. 53-c were added; the mixtures were then analyzed for aluminum as directed in the method described above. A 10-gram portion of test lead or a 2-gram portion of the other samples was carried through the analysis to serve as a blank. The results obtained are recorded in Table 11. “Aluminum Found” represents the weight of alunlinum found in the mixtures minus that found in the blank. ACKNOWLEDGMENT
The author wishes to express his appreciation to Mary E Camphell for her assistance throughout the investigation. LITERATURE CITED
traces of extractable metal ions and cupferron.
(1)
I n order to test the accuracy of the new method for aluminum portions of standard aluminum solution plus, in certain instanrex, 100-microgram portions of various other metal ions were rvaporated to about 2 ml. in an appropriate flask or dish; 10 grams of
RECEIVED for review January 2 2 , 1952.
Chenery, E. AI., A n a l y s t , 73,501 (1948). (2) Gentry, C. H. R., and Sherrington, L. G., Ibid., 71, 432 (1946). (3) Kassner, J. L., and Ozier, hI. A., ANAL.CHEM.,23, 145.3 (1951). (4) Luke, C. L., and Braun, IC. C., Ibzd., 24, 1120 (1952). Accepted May 13,1952
Automatic Karl Fischer Titration Apparatus Using Dead-Stop Principle H. A. FREDIANI, Merck & Co., Znr., Rahway, iV. J . A simple and relatively foolproof automatic apparatus for Karl Fiscliertypc titrations was needed in order to obtain higher precision and greater rapidity in moisrure determinations. A novel electrical apparatus has been developed which utilizes the dead-stop principle, automatically adds the reagent, and differentiates between “true” and “false” or fleeting end points. No vacuum tubes are used. Greater precision is possible because of the automatic addition of reagent and electrical timing of the permanence of the end point. High precision is possible even with laboratorians unfamiliar with the Karl Fischer technique. Relatively complex samples may be titrated rapidly and the possible effect of side reactions is minimized.
T
HE volume of literature on the use of the Karl Fiecher reagent for moisture determinations in many types of samples
amply attests the utility of this technique. The equivalence point involved in these titrations, however, has posed a problem sufficient to prevent maximum use of this method. An experienced chemist, working with relatively clear or colorless solutions, can carry out the Fischer titratio? with a high degree of precision. Because the color change involved is usually from canary yellow to a light amber, practice is required, and the chemist who analyzes infrequent samples usually encounters difficulties. For colored samples, or suspensions of solids, it is often impossible to use the visual end point and recourse must be had to an instrumental method. Although the potentiometric method has been ufied, experience has shown that ( 2 ) the deadstop method serves best for electrically indicating the end point in Karl Fischer-type titrations, as the analysis must be carried out under anhydrous conditions and the usual reference electrodes (salt bridges, calomel, and silver chloride) cannot be used. 81though many ingenious devices have been suggested (4, 6) for automatic potentiometric titrations and a t least three surh instruments are now commercially available, none of these will function for the dead-stop type of titration. The advantages of an automatic titration apparatus, both for laboratories that routinely require many titrations per day and
for ihr laboratory that analyzes infrequent samples by the Karl Fischer technique, are many. The dead-stop technique, which really involves a “polarization” end point, may be employed n herever a sharp transition occurs from the polarization of a t least one electrode to the depolarization of both (or vice versa), this transition coinciding with the end of the chemical reaction being used. Under these conditions a polarized pair of electrodes (10) will have oxygen adsorbed on the anode and hydrogen on the cathode, the former being depolarizable by a suitable reducing agent and the latter by an oxidizing agent. In practice, a very small potential difference, of the same order ot magnitude as the back e.m.f. of polarization, is applied to two phtinum electrodes immersed in the solution t o be titrated. At tlic end point of the titration, when depolarization occurs, a sudden increase of current becomes apparent. Observation of this current provides a far more sensitive indication that the end point h:ts been reached than attempts to measure the e.m.f. involved. The e.m.f. usually is of the order of 10 t o 20 mv. and, because the rapacity of the cell is low, potentiometric measurements must be nmde slowly and carefully. Foulk and Bawden ( 2 ) have aptly summarized the advantages of the dead-stop end point. Gradually increasing excursions of the galvanometer give evidence of the approach of the end point.
V O L U M E 24, N O . 7, J U L Y 1 9 5 2
1127
If the end point is ovemn, the fact becomes immediately apparent. The end point is very sensitive. Equilibrium is reached almost instantly; there is no tedious waiting in the region of the end point.
reagent eliminates personal factors involved and thus permits high precision.
Because manipulation difficulties m e fewer than in the potentiometric method and precision is better than in the visual @), “this technique bas become increasingly popular and at present is probably the most widely used electrometrio method for the determination of the Karl Fischer end point” (5).
In the design of an automatic titration apparatus applicable t o the Karl Fischer titration, the following factors were considered:
DESCRIPTION AND OPERATION OF APPARATUS
An all-glass volume-measuring system was of paramount importance because of the reactivity of the reagent with metals, plastics, and othermaterials. Automatic control of the solution increments WBS desired for utmost reproducibility. Utilization of the dead-stop technique was desirable because of its inherent advantages. Electrical simplicity was desimhle. Rapid response was needed. Ability to titrate directly or to back-titrate with standard water in methanol was desirable. Differentiation between a permanent (true) and a temporary (false) end point was essential. N o dcsien described in the literature. or commerciallv available.
ployed. A powe; relay is included t o actuate a timer d a y (which differentiates between transitory and true end points), the solenoid circuit controlling the flow of titrant, and suitable indicator lights to indicate the status of a titration. The final form now being marketed under a pending United States patent by Beckman Instruments, Inc., South Pmedena, Calif., is illustrated in Figure 1and shown schematically in Figure 2. The three-position circuit selector Serves as follows. Figure 1. Automatio Karl Fischer Titrator
Although the manual dead-stop Fischer titration is capable of higher precision than the visual titration and is more generally applicable (it may he employed in the presence of colored and with insoluble samples), its proper employment involves experience and rigid adherence to manipulatory details for utmost accuracy. As one approsches the end point, galvanometer excursions ocour which, while helpful in indicating ,the nearness of the end point, also introduce the question of proper differentiation between an “excursion” and the true end point. This question becomes more than academic in the analysis of sample8 that are insoluble in the titration medium, where the titmtion involves extraction of the water from the solid into the liquid phase prior to reaction with the Fischer reagent. Typical of such samples are dehydrated foodstuffs (8), dairy products ( S ) , insulating and lubricating oils (l),nnd streptomycin (7). For samples of this type, as well as for some samples that are inducive t o parasitic side reaetionswith the Fischer reagent, rigid adherence to a predetermined uniform technique is a prerequisite for consistently precise results. Rate of addition of reagent, facility in stopcock manipulittious, rand persistency of the end point are all factors influencing such titrations. Equipment designed to titrate sutomat.ieslly with the Fischer
At step one the microrelay is connected directly to the direct current Dower supply through a suitable resistor. bypassing the
ourrent divider thus serves t o adjust the “sensitivity” of the tripping mechanism. Step two then connects the platinum electrodes into the high side of the microrelay circuit, BO that the power relay will be aotivated when the current inereaaes above 40 to 50 @. Thia
of the &r&&y, so that the tripping kechani& is actuated when the current in the circuit decreases below 40 to 50 pa,. This
To c a r r i ~ % ta titration t the power switch is turned on and connection is made to the platinum electrodes. With the circuit selector set to position two (oxidation), methanol is introduced into the titration flask and the potential across the electrodes is adjusted by means of the polarizer control EO that a emall current flows through t h e ojrcuit (5 to 15 MA.as shown on the mioro-
starti t o flow. As the end point is approached, momentary slight excesses of the reagent near the electrodes will cause temporary excursions of the microammeter Earh time this current approaches 50 pa.., the
1128
ANALYTICAL CHEMISTRY
sensitive relay, previously adjusted by means of the sensitivity control to operate a t this value, will make contact and thus close the circuit operating the power relay. When this occurs, the power relay temporarily opens the buret valve circuit, stopping the flow of titrant, and simultaneously starts the electrical clock mechanism in the timer relay. If the current drops rapidly, the sensitive relay contacts will open, the power relay circuit will reopen, the buret valve will operate to cause addition of more titrant, and the timer will reset itself by a clock spring mechanism. At a true end point, the timer will operate for its full preset cycle, a t which time it will depress the microswitch and permanently turn off the current through the solenoid. To start the titration again-i.e., after addition of a fresh sample-it is thus only necessary to reset the titrate knob. As the sensitivity control is merely a shunt for the microrelay, it need be checked only a t rare intervals. The instrument may be turned off and then on again without readjustment of this control.
I
wave selenium disk rectifier to supply the necessary direct current. T h e rectifier-transformer combination has been chosen as more stable and less sensitive to line voltage fluctuations than vacuum tubes. Samples that ordinarily offer little or no difficulty with Karl Fischer titrations may be titrated with the automatic apparatus in 30 t o 60 seconds. I n testing replicate samples of bitartrate, the limiting factor was the time required to weigh out the samples on a torsion balance. Samples that are impossible t o titrate visually and difficult to titrate by manual potentiometric means (such as ether, dihydrostreptomycin, etc.) may be titrated with high precision in 1to 3 minutes. The reproducihility is almost limited by the ability to read the buret.
1 POUWZE CONTRU,’
COOITION OF TIMER Mlm- 8WltCH
J’
I!--__________________BECKMAN Figure 2
AQUAMETER
A q u a m e t e r Circuit
To indicate the status of a titration, three indicator lights have been included in the circuit. One flashes on and remaim lit only while current is flowing through the solenoid and indicates that the titrant is being added. The second has been tied into the electric clock circuit, so that when the timer is operating this stand-by light will so indicate. The third signifies that the titration is complete and one can read the buret. The microammeter on the panel is used for setting the initial polarizing current and adjusting the shunt for the sensitive relay, and also serves to show visually the state of polarization of the electrodes. The adjustable timer is required to differentiate properly between “true” and transient end points. I t may be set to operate a t any interval from 0 to 60 seconds. Experience has shown that for the majority of samples an end point that persists for 30 seconds may be considered permanent; the 30second setting is therefore generally used. However, m hen sluggish samples are encountered, as in titration of suspensions where the water is extracted from the solid phase during the titration, a longer period-Le., 60 seconds-is to be preferred. On the other hand, when interfering reactions are suspected it is possible to set the timer to a shorter period (10 to I5 seconds) to minimize such interference. DISCUSSIOV
For a conipletely automatic instrumerit designed to titrate to a dead-stop end point and to differentiate between true and false end points, this apparatus is remarkably simple and should operate for a long time m-ith little or no mechanical or electrical difficulties. Although a battery was used in the original model to supply the polarizing current to the electrodes and to operate the ponrer relay, current models have been made completely lineoperated by including a stepdown transformer coupled t o a full
.4 semimicrotitration of replicate samples of water in methanol solution yielded 2.54,2.54, 2.53, 2.55, 2.54,and 2.54 ml. of water. The buret was a 10-ml. buret calibrated directly (smallest division) to 0.05 ml. Replicate-samples of dihydrostreptomycin were titrated with the following results: 7.25, 7.28, 7.25, 7.30, and 7.25 ml. of reagent. On these samples the initial teinporary end point occurred a t about 5 ml., so that approximately one third of the titrant was added in gradual increments (automatically) for a period of 1 to 2 minutes. If done manually, the stopcock would have had to be manipulated 50 to 75 times during this part of the titration. LITERATURE CITED
Acker, hI. &I., and Frediani, H. A , , IND.ENG.CHEM.,ANAL.ED., 17, 793 (1945).
Foulk, C. W.,and Bawden, A . T., J . Am. Chem. SOC.,48, 2045 (1926).
Heineniann, B., J . Dairy Sci., 28, 845 (1945). Lingane, J. J., A 4 ~ .CHEM., i~. 20, 285 (1948). Mitchell, John, Jr., and Smith, D. S..“Aquametry,” New York, Interscience Publishers, 1948. Muller, R. H.. and Lingane, J. J., ANAL.CHEM.,20, 795 (1948). Xeuss, J. D., and Frediani, H. A,, presented a t Meeting-inMiniature, North Jersey Section, AM. CHEM.SOC.,Jan. 10, 1949.
Schroeder, C . W., and Nair, J. H., A N A L .CHEM.,2 0 , 4 5 2 (1948). Wernimont, G., and Hopkinson, F. J., IND. ENC.CHEM.,ANAL. ED., 15, 272 (1943). Willard, H. H., and Fenwick, F., J . Am. Chem. SOC.,44, 2504 (1922). RECEIVED for review August 4, 1951. Sccepted April 1, 1952.
