1215
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8
The indicator current method is interesting for industrial routine work where a reproducibility of *0.2’3(0 is sufficient and where simplicity of execution and saving of time are important factors. SUMMARY
A condenser is connected to a potentiometric titration cell. The charging or discharging current of this condenser presents a sharp maximum in the neighborhood of the end point of a titration. End points arc less sharp when the resistance of the measuring circuit increases. An electronic device for the application of the described method and other potentiometric methods is described. Comparison with the classical potentiometric method showed that the reproducibility of the results is within *0.2mc in many cases. The titration error, always by excess, varies according to evperimental conditions from 0.2 to 0.6%. Figure 6. Variations of Indicator Current in Titration of Ferrous Sulfate Solution by Potassium Permanganate Solution Electrodes, graphite-platinum Yolume of analvzed solution. 100 nil. Concentration b f reagent, 0.i N R a t e of addition of reagent, 0.021 ml./sec. Capacity of condenser of measuring circuit, 200 microfarads Correct end point, 9.80 mi.
nate solution, when the graphite-platinum system was used as elrctrodes. The indicator current method is not ss accurate as the classical method, but end points are reproducible within *0.2% in many caws. The indicator current method is applicable only Then both the establiqhment of the electrode potential and the reaction are fast cnough. The indicator current method is applicable only when a sharp value of A E / A v corresponds to the end point. Therefore it is not applicable in very dilute solutions.
ACKNOWLEDGMENT
The author wishes to thank Pierre Van Rysselberghe of the University of Oregon for his help and advice during the preparation of this paper. He also wishes t o thank Paul Erculisse and .4ndrB Juliard of the University of Brussels, Belgium, for discussing some aspects of this work. LITERATURE CITED
(1) Baker, H. H., and Milller, R. H., Trans. Electrochem. Soc., 76, 75
(1939). (2) Delahay, Paul, Anal. Chim.Acta, 1 , 19 (1947). (3) Delahay, Paul, Bull. soc. chim. Belg., 56, 7 (1947). (4) Kolthoff, I. M., and Furman, N. H., “Potentiometric Tltrations,” New York, John Wiley &- Sons, 1926.
RECEIVED February 18, 1947. P a r t of work described was carried o u t a! the University of Brussele, Belgium. T h e contents of this series of articles constitute part of a thesis presented in partial fulfillment of the requirements for the Ph.D. degree a t the L-niversity of Oregon, 1948. Patent pending.
Electrical Differential Method of Measurement in Electrot itrations Application to Conductornet ric Titrat ions PAUL DELAHAY, University of Oregon, Eugene, Ore.
4 conductivity cell is fed at constant current by an alternating current-regulated power supply. The voltage on the terminals of the cell is applied after rectification to a condenser connected in series with a microammeter. The end point of a titration is indicated by a reversal of the current floliing through the meter. A laboratory device and experimental results are described. End points are reproducible within *0.1 to 0.2Yo even in dilute solution (loe3 mole). The duration of a titration is only a few minutes.
T
HE application of the indicator current method t o conducto-
metric titrations is shown schematically in Figure 1. An alternating current-regulated pox-er supply feeds in GG‘ a conductivity cell connected in series with a variable resistance, R. R is so regulated that the impedance of the conductivity cell is only a few per cent of R. The current flowing through the cell is then practically constant, even if the conductivity of the analyzed solution varies. Variations of the conductivity of the analyzed solution are thus indicated by corresponding variations of the voltage, E , on the terminals of the conductivity cell. E is applied after rectification to a condenser connected in series with a microammeter, AM. If
the conductivitv of the analyzed solution is minimum a t the end point of a titration, E is then maximum at the end point. Condenser C is charging before the end point and discharging after the end point. The end point is thus indicated by a reversal of the indicator current flowing through M. VARI4TION OF INDICATOR CURRENT
It has been shown (1, a, 3) that the indicator current can he calculated by means of the approximate formula i =
c -dE dt
1216
ANALYTICAL CHEMISTRY
There C is capacity of the condenser of the measuring circuit, E is e.m.f. applied to t,he condenser, and t i s time. Let us calculate the variations of the indicator current in the case of the titration
A B (reagent')
T
+ CD = A C + B D
(2) where A C is either a slight'ly soluble or a slightly dissociated compound. For the sake of simplicity we make the following assumptions: The conductivity, x, of the analyzed solution is expressed by the formula 1: =
k
::C i l i Z i
Se
(3)
where ci is the concentration of the considered ion, Ii is its niobility, and z, ir its valence. The rate of addition of the reagent is constant. The volume, v, of reagent added up a t time t is thus u =
where E is the maximum value of the rectified voltage applied to the condenser of the measuring circuit. After the end point it lvould be found by similar calculations:
(1 -
(1)
fft
Figure 3. Compensation of Leakage Current Flowing through Condenser of Measuring Circuit
where a: nil. per second is the rate of addition of the reagent. The solubility or the dissociation of compound AC is negliyible. The variation of the volume of solution during - the titration is negligible. The current flowing through the cell is constant. By applying Formula 3 one gets for the maximum value of the voltage applied to the condenser of the measuring circuit :
T . Output transformer of amplifier of voltage on terminals of Conductivity cell Selenium rectifier 4000 ohms, 5 watts; 50 ohms, 1 watt 20,000 t o 200,000 ohins according t o quality of condenser C P. Potentiometer, 1000 ohms, 2 watts C. ZOO-microfarad electrolytic condenser, 460 volts .If. 10 microammeter with zero in middle of scale Arrow 1. Direction of leakage current of condenser C Arrows 2 and 3. Direction of compensation current
Se. RI. Rz. Re.
before titration
a t the end point (v = n ) , k being a proportionality constant. Before the end point
(7)
Combining Formulas 1 and 8, one gets the strength of the iridicator current before the end point G G' il = cE x
(10 Experimental Verification. The actual variations of the indicator current during the titration of a 0.005 Asilver nitrate solution by a 0.068 .Y sodium c h l o r i d e solution are shonn in Figure 2. The measurements were carried out with the e l e c t r o n i c device that device v a s replactd
described belox. The microammeter of by a current-recording device. Curve I, the zero current line, is not a straight line becausr o f the fluctuations in the functioning of the electronic amplifier and oscillator. Curve I1 represents the variations of the indicator current during the titration. Because of the solubility of the silver chloride, the indicator current varies progressively froin ampere. the strcngth i, = (1.0 * 0.3) 10-6 t o t 2 = (3.0 * 0.6) I n view of the assumptions made during the calculations and of experimental errors, there is a fairIy good agreement between t h e experimental indicator current and the theoretical values i l = 0.82 10-6 and it = 1.7 10-8 ampere. The ionic mobilities used in this calculation were taken from Kolthoff and Laitinm ( 5 ) . LABORATORY DEVICE
\
.
,
Le
7.'6
'
- .
I
'v,."
Figure 1. Principle of Indicator Current Method in Conductometric Titration
Figure 2. Variations of Indicator Current During Titration of 0.005 N Silver Nitrate Solution by 0.068 N Sodium Chloride Solution
Terminals connected t o a&.-regulated power supply Variable resistance Constant resistance Selenium rectifier Condenser Microammeter
Calculated indicator currents i i , 4.7' microamperes; i2,0.82microampere
GG'.
R. T.
Se.
C. M.
..
- -_,
... ..
Importance of Voltage Regulation of Alternating Current Power Supply Feeding C o n d u c t i v i t y Cell. A fluctuation i n thc voltage of the alternating c u r r e n t po\yer supph feeding the conductivit! cell acts upon the detector circuit in the samr manner as a variation in the ronductivity of the analyzed solution. During titrations the variations of conductivity a w generally rather small, let us say 10 to 50%. If a titration is to be peiformed with an accuracy of O.2Yc, variations of 10 50 - = 0.02% t o = 100 0.2 0.2
1217
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8
r-r
Figure 1. Schematic Diagram of Laboratory Device for Conductometric Titration by Indicator Current Aiethod Power transformer, 700 volts. center t a p , 200 ma. Output transformer, ratio 1/1 Filter choke, 200 ma. ? h r . Choke of 10 henrys, 200 ohms, 50 ins. If. M c r o a m m e t e r with zero in middle of scalr. .rnsitivity 0.2 inicroampere per division 1.1. 900 ohms, 40 ivatts G0,OOO o h m , 1 watt I:, r3. r i . 900 ohms, 40 watts ri. 40,000 ohms, 1 v a t t r-. 30 ohms, 1 watt i:. i ' ? ~ 15 000 ohms, 5 matts (for C.L. 6) 0 i 2 5 o i m s . 1 x a t t (for C.L. 6). 170 o h k P.. VI. ,'IO. 60,000 ohms, 1 watt ),I. 40,000 ohms, 1 watt I 20,000 ohms, 1 watt C'IX, 3,000,000 ohms, 1 u%tt 1.14. 1,000,000ohms, 1 watt 115. 1,000,000ohms, 1 x a t t ,+. 500,000 ohms, 2 wart3 , ;. 200,000 ohms, 2 watts TI.
7'2. C'hi.
I
in the conductivity of tliv wlutiuii should tle dett3c4tcdat the rnd point The voltage of the alteriiating cwrrnt pon-c'r auliply should thus he regulated a t 0.01 to 0.055{. Usefulness of Amplification of Voltage Applied to Condenser of Measuring Circuit. The indirator cui,rriit increases with the c:ipacity, C, of the condenser of the measuring circuit and with thc voltage, E , applied to that condenser (see Formula 1). Consccluvntly i t seems useful to increase both C and E. However, ( ' slioriltl not exceed a few hundred microfarads; for higher values I I C ~ , thc time iconstant of the measuring circuit hecomes too long a i d the iiidicated end point is riot correct (1, 2 , 3). On the other haiid, thr voltage applied to the conductivity cell should not es~ e c d1 to 2 volts. For higher values, the conductivity ccll docs not work properly on account of flwtuatioris. Consequmtly 1 h r ~riily\yay to increase the iiittxriaity of the indicator current is tu aniplify the voltage applied to t h c coiiductivity cell. T h i s amplified voltage is then applied, after rwtification, t o the ?ondeiiser of the measuring circuit. Schematic Diagram. PRINCIPLE OF ELECTROSIC DEVICE.An .electronic oscillator feeds the conductivity cell. The voltage on the terminals of the cell is applied after aiiiplificatiori arid i'ec tificatiori to the condenser of the measuring eircuit. Both oscillator and amplifier are fed by a regulated direct current power supply. COAfPZSSATIoS O F LE.4KAGE C U R R E S T FLOWVISG THROL-GH ER O F MEASL-RISG CIRCLTT. If a voltage of 20 to 30 0.10';
100,000 ohms, 2 watts 60,000 ohms, 2 watts 30.000 ohms, 5 watts r??, 1.23. ~ 9 4 ,r16. 10,000 ohms, 5 watts r ~ . 1,000 ohms, 1 Tratt P::, , , I . 50 ohms. 1 watt 4,000 ohms, 5 w a t t s r28, r s , rso. rlii and raa should give sensitivirie.; 5.10 7 a n d I O - d a i u y . div. ro iueter .lf m. 2 000 000 ohms 1watt nj. 20,OOb t o 200,060 ohms, 2 watts. ar,cordin:: to (luality of condenser Co CI, C2. 32 microfarads (electrolytic, 5 2 5 m l t s ) C1, Cs. 50 microfarads (electrolytic, 50 volta) CL Ca, Cs. 25,000 cm. (paper) Cr. 1 microfarad (paper, 1000 volta) Cs. 200 microfarads (electrolytic 5 2 5 volts) CI?,CI;. 4 microfarads (electrolytic. 450 volt.) Philii,.; rectifier .l.Z.4 or .imerican rectifier 5C4-I; ' ti. Phili1)s voltage regulator 4337 or .iiiirric,nn t l i l > v V R 90tr, 13, f a , 1 6 , 16.
n e , rw. rn, r n .
9r'
0"
t:,!s. .Se.
Philips ainglitier
rllhr
C.L. 6 or iiiierican tube 6LIti-(;,
S:pleniiiiri rectifier
v o 1 1 ~i* applied to t t i c * c ~ ) r i d t ~ r i ~ofc ~thr i ~ iiieawriiig circuit, the loiiliagc~ current of thr condenser interferes rvith the reading. This I c d a g e current should thus he compensated by nleans of the circuit shown in Figure 3. The output voltage of the amplifier of the device is applied to the resist anccxs, R, and IZa, caonriected in series. The voltage on the terminals of Rl is applicd to the comdewer, C. The leakage current ha.;, list u xrruic 1. Thtb lealiage current is then compensated by an inverse currtwt (:wro\vs 2 and 3) regulated by means of the poteiitioiiieter, P. The coinpensation remains correct even if the coiiductivity of the auttlyzed solution varies. The device s h o r n iii Figure 4 was de~ O ~ I P L E TDIAGR.LX. E cigiied for European Philips radio tubes, the only ones available, at thv timc when this set wa5 built. Anicrican tubes can be used ivith two slight modifications indicated i i i the text.
Direct Current Regulated PoWer Supply. The output voltage o f a classical full-wave rectifier unit is regulated by means of a
tn-o-stage gas t,ube stabilizer. Tubes \'It 90 can be used instead of Philips tube 4357, and the American full-nave rectifier 5U4 instead of tube -1.Z. 4. The current flowing through tubes h, ts, f,, t5, and ts should be regulated at 25 millianiperes a-hen both oscillator and amplifier are operating. Oscillator. The phase-shift oscillator (4) feeding the conductivity cell is operating a t about 200 cp. Resistances r11 and t l z should be regulated in order to get a pure sine n a v e when the oscillator is operating a t full power.
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8 Table I.
Potentiometric Conduct,ometric E n d Point E n d Point Jf1. 'W. 24.90 i 0 . 0 2 24.88 * 0 . 0 3 2 4 . 7 0 * 0.02 24.95 f 0 . 0 4 2 . 5 x 10-3 10-3a 24.95 * 0.08 2 4 . 2 5 * 0.04 S;olutiun prepared by dilution of 2 . 5 X 10-3 mole solution. ~.
.~
~
Reproducibility
% kO.1 10.1 *0.2
r -
200
z/z vo
"
nd[
~~
1 l A f k '
]'-I
%
Experimental Error
Theoretical Error $0 -0.12 -1.2 -3.0
with the ordinary conductometric method. The volume -(1.0 of reagent added a t the end - ( 2 . 8 10 . 4 ) point does not need to be small in comparison with the volume of the analyzed solution. An ordinary buret can thus be used. The temperature of the analyzed solution does not need to be controlled with accuracy. If a titration has to be carried out above room temperature, an ordinary gas burner or an electrical heater can be used. The indicator current method is applicable only when the conductivity of the analyzed solution is maximum or minimum in the neighborhood of the end point. The ordinary method is applicable to all cases.
410
(0.0
*
0 2) 0.2)
8
In the case of Titration 2 the relative titration error is given by
E
The application of the method does not require all the precautions which are essmt'ial
Titration of a Silver Nitrate Solution by Sodium Chloride Solution
Concentration of A g S O s Equiv./l. 2 5 x 10-2
I'
1219
(11)
2 6 - IC $. LA Khere V Ois the initial volume of the solution to be titrated; n is the volume of reagent added up to the end point of the titration; L is the solubility product of compound AC; Or is concentration of reagent gram equivalents per liter; and la, LE. and IC are ionic mobilities of ions A , B , and C.
ACKNOWLEDGMENT
The theoretical titration errors indicated in Table I have been calculated by means of this formula. The agreement with the esperimental values is good.
The author wishes to thank Pierre Van Rysselberghe of the University of Oregon for his help during the preparation of this paper.
c o l v c LUSION s
LITERATURE CITED
The indicator current method is faster than the ordinary method of conductometric titrations. As the end point is indicated by a reading on a meter, the duration of a titration is reduced to a fe\y minutes. End points determined by the indicator current method are reproducible within *0.270 even in dilute solutions (10-3 mole!. The negative titration error becomes fairly large (several per cent) in dilute solutions.
(1) Delahay, Paul, -43.~~. CHEX.,40, 1212 (1948). (2) Delahay, Paul, Anal. Chim. Acta, 1, 19-32 (1947). (3) Delahay, Paul, BLLZI. SOC. chim. Belges, 5 6 , 7-35 (1947). (4) Ginsto, E. L., and Hollingsworth, L. M.,Proc. Inst. Radio Engrs., 29,43 (1941). (5) Kolthoff, I. bl., and Laitinen, H. h.,"pH and Electrotitrations," p. 119, Iiew York, John Wiley & Sons, 1941. RECEIVED February 18, 1947. Part of work desrribed was carried out a t University of Brussels, Belgium. Patent pending.
Determination of Moisture in Some Protein Materials Use of Karl Fischer Reagent ELIZABETH A. RIcCOMB Western Regional Research Laboratory, iilbany, C a l i f . Fischer reagent has proved adaptable for the determination of moisture in protein materials such as chicken feathers, dried egg white, gelatin, hog hair, gum gluten, soybean protein, and zein.
D
URISG the course ofinvestigations on some protrin materials
a t this laboratory, it was necessary to have a rapid and accurate moisture method by which samples could be analyzed in a comparatively short time. The rapid volumetric method developed by Fischer ( 4 ) has been found satisfactory for determining moisture in food materials such as starches, pectins, egg polvders, fruit powders, and some dehydrated vegetables, as described by Johnson (6). Kaufman and Funke ( 7 ) and Richter (Sj have also applied this method to foodstuffs such as fats, marmalade, and cocoa. In view of the application of this method to such products, Fischcr reagent has been investigated for the determination of moisture in casein, chicken feathers, dried egg white, gelatin, gum gluten, hog hair, peanut kernels, soybean protein, and zein. This paper presents the results of this investigation and describes the conditions necessary to obtain optimum values. The procedurr. recommended for treatment of the protein materials to ~ S W I ' C complete removal of the water requires from 5 to 60 niinutec: aiid i.s more rapid than other methods now in use for such materials. Fischer moisture values on all the materials studied, with the exception of casein and peanut kernels, agree with values obtained by drying t o a constant weight at room temperature in vacuum over anhydrous magnesium perchlorate. The
Fischer method in general gives results which are as good as, or better than, more time-consuming oven methods. REAGENTS AND APPARATUS
Absolute Methanol. I t was found necessary to dry the commercial absolute methanol, which contained 0.1 to 0.5% water. The alcohol was dried with magnesium as described by Gilman and Blatt ( 5 ) . Fischer Reagent. The Fischer reagent was prepared by a method similar to that described by Smith, Bryant, and Mitchell ( 9 ) . To 84.7 grams of resublimed iodine, dissolved in a mixture of 269 ml. of reagent grade pyridine and 667 ml. of dried absolute methanol, were added 64 grams of sulfur dioxide gas. To avoid appreciable heating, the sulfur dioxide was added slowly at the rate of about 40 grams per hour. This solution deteriorates rapidly when first prepared, but becomes more stable and suitable for use in 3 to 4 days, Standard Water. The standard water solution was prepared by adding sufficient distilled r a t e r to dried methanol to give a solution containing approximately 2 mg. of water per m1.- The ratio between Fischer reagent and the standard water and the correction for the moisture content of the alcohol used were determined each time a series of determinations were made. ilfter the ratio was determined, the exact water value of the standard water solution and the Tv-ater equivalent of the Fischer reagent were determined by titrating weighed amounts of reagent grade crys-
