AIDS FOR THE ANALYST Precise Electronic Titration Instrument. Kenneth E. Harwell, Texas A. and 31. Research Foundation, and Department of Oceanography, College Station, Tex. often requires the analysis of great R numbersinofoceanography samples of sea water for the determination of a defined term which i. universally used ESEARCH
salinity. “Salinity” is in oceanographic Iv(>rli. The salinity value is intended to represent the total amount of dissolved salts per kilogram of ocean water, but because of the complexity of ocean water the term is of necessity empirical iii natuie The salinity value has been defined by an international commission ( 1 ) as “the total amount of solid material in grams contained in 1 kg. of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized” (8). Salinity values aie ordinarily calculated from chlorinity values by the equation of Knudsen (8). Salinity = 0 03
+ 1 805 X chlorinity
The ,.chlorinity” is also a defined term; its definition is “the mass in grams of ‘atomic weight silver’ required to precipitate the halogens in 0.328533 kg. of sea water” (1). The chlorinitv value is determined by titration of the water samples with a standard solution of silver nitrate. A high degree of accuracy is necessary in these analyses, sinw minute changes in salinity produce profound effects in the behavior of the oceans. Silver-silver chloride electrodes and :I sensitive potential indicator appear to provide the most precisca means available for locating the end point in this titration (3). The instrument described herein was designed to meet the requirements of high precision and rapid operation. It comhines a higher degree of precision, sensitivity, and stability than i. ordinarily found in coinnic1cial titration instruments, with thc added advantages of simplicity and economy of construction. It is well known that a m d a l electrode in equilibrium with :I solutioii of its ions nil1 produce an electrical potential which is related to the metallic ion concentration in the solution ( 2 ) . Electrode potentials mav be used to indicate the concentration changes as they occur a t the end point of reactions during titrations. To utilize these electrodc potentials in a practical method of analysis, an instrunicnt is needed which will not onlr measure but will continuously indicnte the cuiqting potential. I t is also
imperative that the measuring instrument dran no current from the electrode; otherwise the potential will not be indicative of the existing ion concentration. The vacuum tube voltmeter seems to be the only type of instrument capable of meeting these requiremen ts. DESIGN
The design objectives for the present instrument were good stability, a wide voltage range, and a higher sensitivity than is ordinarily found in commerci:il titration initruments of the continuously indicating type. Abalanred or bridge-type vacuum tube voltmeter circuit (Figure 1) was used. Two similar triodes were placed in adjacent arms of the bridge. Potential from the electrodes was placed on the grid of one triode and a reference potential was placed on the grid of the other triode. Bridge balance was obtained when the electrode and reference potentials were equal. The balance indicator was a sensitive microammeter connected between the plates of the two triodes. Power was supplied to the circuit from a simple voltage-regulated source. The stability of the instrument result* primarily from the bridge-type circuit. Since changes in temperature and voltage tend to affect each half of the circuit equally, they are largelv canceled. Other factors coptributing to stability are the voltage regulator tubes md the use of a dual triode, 6SC7, having a single cathode. The scnqitivity results primarily from thc use of a sensitive meter. The reference potential is obtained from the volt:ige drop across n. precision wire-rvound resistor, Rg, and is regulated by the two voltage-regulator tubes. A ten-turn Helipot potentiometer, E,, is used to metisure the refercnc.e potential which is applied to the grids. The Htalipot allows the reference potential to be read with a precision of 1 part in 1000. As the reference voltage is obtained from voltage regulator tube., its value may vary 2% with a severe fluctuation in line voltage. Honevcr, highly accuiute measurements may be made with the instrument if an accurately known reference voltage is used. S o current is drawn from the electrode., since the measuring giid is maintained negative a t all times. The grid bias is obtained from the voltage drop across the cathode resistor, RH. A few microamperes of grid current will flow unless the cathode is biased positive 0.5 to 0.8 volt. With this bias no grid current can he detected. s Yo
Figure 1. Diagram of Electronic Circuit CI,Cz.
0.01-mfd., GOO-volt condensers Ca. 10-mfd.. 450-volt condensers LI. Filter choke, 16-h, 30 ma. R L IO-turn Helipot, 10,000 ohms, i O . l % linear Rz. 2.7-megohrn, 0.5-watt resistor Ra. 0.27-megohm. 0.5-watt resistor Ra. 68,000-ohm. O..j-watt resistor Rs,Ra, Rs. 50,000-ohm. 10-watt wire-wound resistors
Ri. 5000-ohm wire-wound potentiometer Re. 250-ohm, wire-wound precision resistor, I R C WWI RID. 10,000-ohm, wire-wound IO-watt resistor RiI.
1000-ohm, wire-wound, 10-watt resistor
TI. Power transformer, 110-volt a-c: 6.3-volt, 2.5 ua.
616
2-amp.; 55O-volt, CT. 50 ma. 0-50 microammeter, 41/z inches s q u a r e
amp.; j-volt,
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4
617 voltagc O I I thc othci, grid rcniains constant, the illate currmts will be unbalanced and the meter will indicate the amount of voltage change. The meter may he calihrsted to read in volts or other units. B typical calibration curve using this method of operation is shonm in Figure 4. When greater accuritcy is desired, thc unit may be used as a null point instrument. The plate currents are balanced as usual; the unknown l~otentialis placed on one grid and the Hclipot is turned until plate current halunce is q n i n restored (zero meter reading). The Helipot reading is a measure of the unknov.-ii voltage. The Helipot clial may be c:tlibr:ttrd l>y any voltage measuring raliinstrument of greater :twurar\.. .\ tvpir:iI .. 1)r:ition riirvr is shoLvii ill Figure 3 .
YNSITIYIT"
5 *I
mwi
TERMNUS
-
1
1
ZERO, BALANCE, R ,
l i g i i r c 2.
\
PILOT LIGHT
1
I
Electronic. Ti [rat ion Instrirmen t
'Hie t \YO t iiod(v of iiiort USCi t u l w do not h a w sufficiciitly similar char:lcteristim to allow the circuit t o h r halanccd : usu:ill>~ orily one tub? in a group of five or six \vi11 be usable. h \wll b:rlanced tube is desirable but not necessary. Thc c~~niponent values given in this paper are such that only 1 ~ ~ 1 1 I):tlanred tul)cs will he selected. If plate current balanrc c;innot I)e o b t a i n d w i t h thc tuhes available, the tolerance of tlic instrunient may be increased by increasing the value of potcntiomr.trr 127. Current hal:tncc may I)e obtained with a11.1. BSCT tubr if the value of K , or l i s is changcd sufficiently to compensa,t,e for (,he difference in pl:tto resistance of t h r two triodes. The instrumcwt itself providw thct liest ine:i~is f o i , trsting and .seIrctiiig 1):tlanccd tuhcs. Snitch SI\;is :I l\vo-gaiig, fivc\-])o>itionsrlcctor ~a.itc!h. The front gang is used to roritrol the sriisitivity of the microaniincter (luring preliminary :itljustmentr. T h r rear gang of SH,-, is used to control the stirring motor in thc titration beaker. A real danger exists that t l i v l)c~rsoiidoing a titration will remove thr titrrtioii I)c~ikcrfrom l i l t . rlec~trodesLvhile the meter is a niasimum This action will usually damage the niirroa,iiiinetrr. kely that the lieaker would \)c removed \\-hilt t,he motor is running. the nirtvr is protected by reducing its sensitivity simultaneously with switching off the motor. An outlrt sorket I'or the stirring motor is provided on the back of the case. The i.ange switch, SW,, is also a great conveniencc, to the analyst bec4:iu.e it makes possil~loa progrcssivc inrreasr i n wnsitivity :I- the IPI iti point is n pi )ro:ic:hrd,
I'ERFOR\f ANC E
The input resistnncc, ivas found to bo greater than 2 X lo9 ohms, this value k i n g thc ul)per limit of t h r rcsistancc-mc:tsuririg instrument used.
I
I
I Figure 3.
Chassis Arrangement
OPERATIOS
The circuit diagram is shown in Figurc 1. I4gures 2 iiiid 3 show the arrangement of the controls on the instrumeiit I):uicl and of thr parts on thr rhwsis. The operation of the circuit) i follo\vs: The two grids of the dual triode, 6SC7, are connected to the same potential through switch SW,. Thc plate-balancing potentiometer, R7, is adjusted until the microammeter indicates zero current, u t maximum sensitivity. Under these conditions the plate currents are equal when the gi,id potentials are identical. When S W , is thrown into the other position, one grid is connected t)o the electrodes and the other grid remains connected to the reference potential. The reference potential may be adjusted, with R I , until the plate current balance is restored; then the unknown potential is equal to the reference potential. The grids may again be connected together through SWd as a further cherk on the platc current balance at the new grid potentials. The instrument may be used a s i~ direct indicating meter or as a null point instrument. Sormally K ; is adjusted for plate current balance (zero ineter reading) when the two grids are a t identical potentials. If the voltagr. on thc' mcxasuring grid is changed while the reference
POTENTIAL I N VOLTS
Figure 4.
31eter Calibration of Electronic Titration Instrument
The volt:tyr range is 0 to 1.5 volts. The voltage sensitivity, using the meter as the indicator, is 1.91 mv. per division, Figure 4. By readjusting the reference voltage in increments this order of sensitivity may be had over the entire range of 0 to 1.5 volts. The Helipot senyitivity is 1.54 mv. prr division, Figure 5 APPLICATIONS
dlthough the iiistrunient was designed for a specific application, determiriation of halide ion conrentration using the silver-
ANALYTICAL CHEMISTRY
618 silver halide electrode, its characteristics make i t clearly applicable to other electrode systems, or to any situation in which low voltages must be precisely measured and no current is to be drawn from the system. I n general, the end-point potentials are found (using the null point method) by plotting the conventional ( 4 - 6 ) curves of voltage reading against milliliters of silver nitrate added. Thc point on the curve having the maximum slope is taken as the end point.
r
1000
--,
d 2001-
= oI w/ 0
Figure 5.
1 02
I 04
1 i I 06 08 10 POTENTIAL IN VOLTS
..,.,.
I 12
14
The silver electrode is simply a piece of silver wire sealed in one end of a glass tube. The reference electrode consists of a similar piece of silver wire immersed in a concentrated solution of potassium chloride. The potassium chloride solution is contnined in the enlarged upper portion of the reference electrode; electrical contact is made with the sample solution through an agar-salt bridge which is contained in the lower part of the elertrode. The silver wire or btrip may be cleaned by the following procedure: Wash in concentrated ammonium hydroxide to remove corrosion and silver salts, wash with water, immrrw in a solution of nitric acid ( 1 part of concentrated nitric acid, 1 part of water) until a clean surface is obtained, and wash thoroughly with distilled water. The voltage from electrodes cleaned in this manner will not be reproducible during the first few titrations. The electrode potentials will drift until an equilibrium has been established between the silver and a layer of silver chloride on its surface. If desired, a thin coating of silver chloride may be formed on the electrode surfaces previous to titration by placing the electrodes in a sodium chloride solution, connecting the electrode to the anode of n battery, and passing a small amount of current through the electrode. The electrodes may be sealed in the glass tubes vith ordinary sealing wax, but one of the ne\\ c'r thcmioplastic or thermosetting plastics is to be preferred.
I
16
ACKNOWLEDG.ME3T
The author wishes to express his appreciation to the Bureau of Ships for sponsorship of the research project (Contract NObsr57244) of which the present work is a part. A considerable part of the initial work on the instrument was
Potentiometer Calibration of Electronic Titration Instrument
Once the end-point voltage has been found, one merely sets the Helipot to the end-point voltage of the desired ion and titrates to zero meter reading. The following experiment is described to illustrate the precision obtained with the instrument: Samples of deep sea water (15.00 ml.) werc carefully measured
into the titration beaker with a precise ipet. Sufficient distilled water was then added to cover the eyectrode tips and stirrer paddle. The standard silver nitrate solution was 2uantitatively added from a Knudsen automatic buret (9). The nudsen buret waa used in preference to a conventional buret because it allows a greater precision in reading. A set of values obtained for seven successive titrations of 0.22N silver nitrate is: 19.950, 19.945, 19.950, 19.950, 19.945, 19.945, and 19.950 ml. The standard deviation value (7') for these results is f0.0026 part per thousand of halide as chloride. The sensitivity a t the end point is such that it is very difficult to withdraw sufficiently small samples of silver nitrate from the buret to balance the meter precisely (the width of a buret calibration line being roughly equivalent to 10 divisions on the meter scale). The deviations found in the above results w e primarily due to limitations in the reading and operation of the buret. 4b 18 Copper Flexible Lcod Although the electrometric method hm the ability to eliminate essentially all error due to mislocation of the end point, errors associated with weight and volume measurements and temperature changes remain undiminished.
I
,OOt
2001 I80
1 185
I 190
i 195
1 200
i i 205
...." 210
215
M L OF AgNO,
Figure 6. Titration of Sea Water with Silver Nitrate
7
Section O f Rubbrr Stopper SILVER-SILVER CHLORIDE ELECTRODES
This laboratory has experienced several difficulties with the use of commercially manufactured silver-silver chloride electrodes. The electrodes described below have been used with satisfactory results; they are easily and economically constructed and give reliable performance. Plans for their construction are shown in Figure 7 .
Soturoted K C l Soluti
Reference Electrode
Flexible Copper L e a d Wire
Figure 7 .
Sealing
Wax Or Ploitlc
Reference Electrode and Rleasuring Electrode (lower)
Ac
V O L U M E 26, N O . 3, M A R C H 1 9 5 4 done a t the University of Texas, Dcpartment of Chemistry, under the direction of Len% F. Hatch. LITERATURE CITEI) (1)
(2)
(3) (4j
(5)
(6) (7)
(8) (9)
Forch, Knudsen, and Sarensen, K u l . L h a s k e Iyide/iskab. Sclskub. Skrifte?, .Vaturvidenskab math. dfdel., 12, No. 1. 151 (1902). Glasstone, Samuel, “Textbook of Physical Cheiiiistry,” 2nd ed., New York, D. Van Nostrand Co., 1946. Hindman, J. C., and .Inderson, L. J., and Moberg, E. G., J . Marine Research, 8,No.1,30-5 (1949). Kolthoff, I. M,, and Furman, N. H., “Potentioinetric Titrations.” 2nd ed., New York, John Wiley & Sons, 1931. Kolthoff, I. &I., and Laitinen, H. A., “pH and Electi.otitratiotis,” 2nd ed., New York, John Wiley & Sons, 1941. Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” rev. ed., New E’ork, Macmillan Co., 1943. Snedecor, G. IT.,“Statistical Mcthods,” Amcs, Iowa, Iowa State College Press. 1946. Sverdrup, H. V., Johnson, M, IT., and Fleining, R. H., “The Oceans,” p. 51 and references contained therein. Keu. York, Prentice Hall, Inc., 1942. IVoods Hole Oceanographic Instit,ution, Xoods Hole, “Deteril~inationof Chlorinity by Knudsen Method,” M a s . , 1946.
C o s ~ r R i s r i i o sNo. 28 froin the Dcpartment of Oceanography, Texas A . and M. Colley?.
Capillary Specimens for X-Ray Diffraction Powder Analysis. F. \Ill,itthc\\ . F, (‘entral Research Laboratory, Canadian Indu\trw, 1,td , llcllasterville, Quehee, Canada. T H E mounting
619 free time and stored ready for uiie when :L specimen is to be mounted. Suitable glass or quartz capillaries can be purchased ( 5 ) but are readily drawn from Pyrex 707 (Corning Glass \Torks), a special formulation of glass which is less absorbing t,o x-rays than standard borosilicate or soft, glass. ii 15-mm., thin-walled glass tube has been found a coiivenient stock from which to drciIv capillaries \Yhirh should be 0.025 inch in outside diameter (measured with a S o . 22 B&S wire gage) 311d with a all t,hin enough to crush b e t m e n the fingers. Plaslir capillaries may be made from polystyrene or cellulose acetate h y a technique described by Beu and Classen ( 2 ) . To mount a q)ecinieii, a glass capillary wit11 tanipcd plug is inserted in the holc in the policeman and vacuum is :tpplied. A s the end of the c:ipill:try is placed near the powdered specimen, it is readily drawl into the capillary and is caught on the cotton plug. The tube niny be sealed in a fine flame if efflorescence, deliquescence, or rract,ion with the atmosphere is a difficulty. In one lsborator,v ( 1 ) it has been found more convenient to loave the wiqi oil the capillary and turn it end for end catching the cotton with the capillary in the hole in the policeman. When the specimen is in pl:rce, the cotton may be removed. The whole procedure c:m rentlily be carried out in a dry-box fitted with rubber gloves. Thrcc stages in thc prcparntioii of a sl)ccBimcn nre shown in thc rlraiving. LITERATURE CITED
Sational Researrh Counc,ii. O i i t : w i o , (’xnnda, priwtc communication. 13eu, K. E., and Classen, H. H.. Rei,. S c i . I / r s t ? . , 19, I T ! ) (1948). Gibson, G., and Bicek, E. .J., ANAL.CHEM..20, S84 (1948) (inisspelled Gihons in original article). Hagelston, P. J.. Dunn, H. W., Re,. Sci. In.str., 20, 873 (1049). Thetmal Ryndicntc, J . Sci. Instr., 28,289 (1951).
(1) Bai.iics, W. II.,
(2) (3)
(4, (5)
of x-ray diffraction powder specimens in glass or
7 plastic capillaries has advantages in that the specimen ip encloxtl and no organic solvent or binder is rcquired. The qpecimen i z t h w straight and mounts readily on the chuck 01 d d l e wed in nioqt American designs of powder cameras. The di.advantage remains that the capillaries :ire difficult to prepare ant1 fill. Methods of doing this are described b y H,agelston and Duiin ( 4 ) :ind by Gibqon and Bicek ( 3 ) .
Apparatus for Fractional Sublimation a t Normal or Reduced Pressures. Kerner Hausmann, Rockefeller Institute for Xedical Research, S e w York 21, K,Y. Riiber described a veisatile, vertical vacuum b u b h i m I-vtor1900, consisting of a vaporizer and condenser connected by a
ground-glass joint [Ber.,33, 1655 (19Oo)l. To prevent ~ublimate from falling back he interposed an asbestos plate abovc the sublimand. The apparatus was heated in an sir bath. This subliniator is extremely ~ i m p l cto Get up, yet very cffectivc’. The apparatus described here consists of the same basic p:trts, except for the asbestos plate, which is replaced by a coarse eintercd-glass plate and a different type of heater. The di:tgrani qhon c the Fell known electrically heated micro
COTTUN PLUG
Thermometer
I (/--
Ti7kPk-D COTTON PLUG
Vacuum
-Sublimate
A tevliniquc which greatly simplified the filling of capillaries has been in u w in this laboratory for home years, although originality for thr method is not claimed.
Sintered
.4rubber polireman as used in analytical chemistry is put on a glass tube 6 inches long, and the rubber tip is cut off to leave a rubber cap on the tube. Through this cap a hole is made with a coarse needle, and into it the capillary is inserted. Suction is applied to the glass tube from a vacuum line with a bypass stopcock with which the vacuum can be readily adjusted. If an opening is made in the glass tube which may be readily closed with the index finger, very effective control of the vacuum is possible. -4cotton plug is made by rolling a tail on a wisp of absorbent cotton between the thumb and forefinger. The tail of the wisp may then be readily drawn into the capillary by full vacuum The vacuum is released and the wisp cut with sharp scissors. The cotton may then he lightly tamped to a plug with solid glass pokers inw-ted from either end. Capillaries can be prepared in
-
Hot
stage
+
Prn
