I S D r X T R l A L A S D ESGILVEERISG CHEMISTRY
July 15, 1930 (161) (162) (163) (164)
Wade, Doctor's Dissertation, S e w York University, 1930. Wiedemann and Ebert, A n n . Phrsi,k, 33, 241 (1888). Kiedemann, I b i d . , 31, 383 (1887). Williams, Doctor's Dissertation, Yew York University, 1929.
213
(165) Williams and Whitenack, J . Phys. Chem., 31, 519 (1927). (166) Winch, J . Sci. Instruments, 6 (Decernbir, 1973). (167) Zworykin, Phys. Rev., 27, 813 (A) (1996). (168) Zrvorykin and Wilson, J. Optical Soc. A m , , 19, 81 (1929).
Potentiometric Titrations' A Review and a Report of Progress N. Howell Furman FRICKC H E M I C A L
LABORATORY, PRIXCETOX UNIVERSITY, PRINCETON,
N. J
HE literature relating
The outstanding recent trends in the development of Cavanagh (38) has shown to potentiometric titrathe methods of potentiometric titration have been that for certain sjniple types tion methods for the reviewed. These trends appear to be: (1) proof that of reaction-. g . , precipitathe method is capable of great precision; (2) simplificaperiod 1926-30 is approxition of uni-univalent saltslnat,e]y as extensive as that tion of the experimental technic (bimetallic electrodes) ; and for n e u t r a l i z a t i o n s of (3) development of continuous-reading methods; strong acids with strong in thesame field for the period (4) beginnings of the study of potentiometric microbases, v i t h exclusion of car1592-1926. It is therefore titration; ( 5 ) notable progress in the study of all types bon dioside, it is possible to desirable t o summarize of reactions; and (6) the opening up of new fields of briefly the important trends determine the end point by ill this interesting field for the study in solvents other than water. taking from two to four A classified and an alphabetical bibliography of POe. m. f. readings after approfirst-mentioned period. It q-ill be assumed that the tentiometric titration methods are included. priate additions of reagcnt. If Jfo cc. of reagent of norreader is familiar with the essential features of the potentioinetric titration method.* malitp n are equivalent t o the substance to be determined, and \\le will merely recall here that, if a suitable electrode can be if the volume is VO,then the concentration c of the substance found, m-e have an electrical indicator for bhe change in con- is n:lfo/Vo and'if the original activity of the ion is yo,then the cenbration, or activity, of an ionic species, and hence are in a potential of the indicator electrode is: position t o determine the course of reaction, and the end point RT niWnyo E = const. - log of a precipitation, complex formation, or formation of an F Vn undissociated compound. With the aid of the Nernst formula Suppose m cc. of reagent are added (m cc. as the reagent is added in equal sinal1 increments near the equivalence-point (SD) , Received M a y 2 7 , 1930. T h e theory and technic of the method have been thoroughly treated b y Kolthoff and Furman (110). 3 T h e order of treatment is essentially t h a t of d r i c h Muller, and has been used b y t h e author in Chapt. XI11 of Taylor's "Physical Chem1 2
istry."
E =
) ~
RT nMoyo RT nMr - - log _F log Io F V
As a first approximation let y = yo; assume m to be negligible as coinpared \Tith Tro, and lienee that = Then JjT0.
RT F
E = - log
It1
-"A!!
=
RT - log F
+
5)
At constant temperature E is a function of m / X ; and the ratio Jf/mis a function of E. Explicitly,
m
Cavanagh has recorded values of the hyperbolic function, and has shown that the method gives good results in the titration of chloride (0.01 to 0.00% 5)wit11 0.01 N silver nitrate, and in the titration of bromide 10.0002 A:) with 0.01 N silver, or iodide (0.0001 S)with 0.001 S sill-er. I n a second paper Cavanagh (3) has developed a complete equation for a precipitation graph n-hen a uni-univalent type of precipitate is formed. The equation takes into account activity, adsorption, and solubility factors, and is of the general inathernatioa~lforin 9 = siiih By constructing a
r.
214
AXA LY TICAL EDI T I O S
Vol. 2,
KO.
3
pz,
d - -. -_
1.
3.
suitable table, or b y plotting curves for values of the function, it is possible to derive the end point of a reaction from relatively few e. m. f. measurements (two to four). Cavanagh obtained b y his most refined method results accurate to +0.04 per cent in the reactions between halide and silver ions. Lange and Schwartz (121) have shown that the classical method gives results accurate to 0.003-0.007 per cent of the actual chloride content, provided weight burets are used, and that effects of temperature, light, and diffusion from reference electrode are excluded rigorously. Hahn and Weiler (79) have proposed that the end point be determined graphically b y constructing the evolute (locus of centers of curvature) of the titration curve: a straight line asymptotically tangent to the two branches of the evolute cuts the titration curve a t the end point. Kolthoff (109) has criticized the method of Hahn and Weiler on the ground that the construction of the evolute is influenced b y the accuracy of the potential readings and the reliability of the electrode phenomena, and that the classical method (A e. m. f./A cc.) is just as accurate and simpler. Kolthoff suggested that a n alternate way to find the end point is to calculate a n apparent, or “ practical,” equilibrium constant from the titration data, and hence an equilibrium potential. It is interesting that Cavanagh (39) has also suggested the calculation of these two quantities from the titration data. METHOD2. TITRATION TO ZERO DIFFERESCE OF PoTENTIAL BETWEEN ELECTRODES-In its original form this method, which was evolved independently by Pinkhof (111) and Treadwell and Weiss (207, see also 119 and 83), consists in preparing a reference electrode which duplicates the potential which the indicator electrode will have a t the end point. The external circuit is made u p of a galvanometer, resistance, and tapping key. It should be noted that Treadwell (200) was apparently the first to employ a continuousreading circuit consisting of a sensitive millivoltmeter or galvanometer and high resistance in closed circuit between an indicator electrode and the reference electrode which duplicates the end point (2GG).
Camnaah 131) has devised an interesting modification of the method. He constructs a cell in such a wag that the e. m. f . shall be zero at the concentration that the ion IT-hich is to be titrated will have a t the equivalence point. For example, a cell composed of a silver chloride electrode and a hydrogen or quinhydrone electrode will have zero e. m. f. at the end of the titration of halide ion with silver ion, or vice versa, if the hydrogen-ion concentration is kept within a certain limited range. Conversely, the e. m. f. of the same cell will be zero in the titration of acid with alkali if the concentration of rhloride ion is maintained in a suitable 1 range. hllle. Brouchere (32) has employed an extension of the method as follows: Certain metallic i o n s - e . g., cadmium ion -may be determined by using two electrodes of the metal in quest’ion, one being placed in the solution of unknonx
the two electrodes shorv- zero difference of potential. It is obvious that the concentration of the unknown may be derived from the concentration of the known solution and the amount of water added. The method appears to be limited in scope, because the effect of foreign ions and the difficulty of securing two electrodes that show exactly the same potential toward the same solution are factors that would have to be taken into consideration before the method could be of general utility. XETHOD 2.A-Muller (156) opposes a potential equal and opposite to that which the titration cell will develop ab the end point. d tapping key and a galvanometer enable one to detect the end point, which is marked by zero difference of potential between the two systems. The recent papers of Muller and his students, and of other investigators, give data regarding “end-point” potentials. The following methods employ some form of polarization phenomenon. ~ ‘ I E T H3. O DPOLARIZED IXDICATOR ELECTRODE-DutOit and von Weisse (53) converted a platinum electrode into an indicator for a metallic ion-. g., copper-by making it cathode to an auxiliary electrode. d small polarizing current was used. This method has not in general been found very useful or accurate. As far as the author is aware, the only modification of the method which is widely used is that of Wright and Gibson (218),who proposed the use of a cathodically polarized platinum electrode as an indicator in neutralizations. METHOD4. ELECTRODE SYSTEM COSSISTING OF T w o ~ ~ E T A L L IELECTRODES-The C electrodes may be of the same metal, usually platinum, in which case a slight polarizing current is used, or they may consist of two different metals which are slightly attackable-as, for example, platinum and tungsten, The author regards a pair of dissimilar electrodes as being in all essential respects a polarized system. The obvious advantage of such a system is that the rather troublesome calomel half-cell may be eliminated and hence loss of solution by diffusion into the siphon tube is prevented. Metallic systems are also advantageous to use in titrations which must be performed in an inert atmosphere.
I N D U S T R I A L A N D ENGINEERING C H E N I S T R Y
July 15, 1930
continuous-reading (Figure I, 2 ) . With this circuit the endpoint phenomenon is a sudden large deflection followed by a stationary reading (dead-stop end point). A dry cell, 400ohm radio potentiometer, and an inexpensive galvanometer may be used to construct the circuit. Furman and Wilson (73) have developed a simpler circuit which gives a maximum galvanometer deflection a t the end point of most oxidationreduction reactions. (Figure I, 3). Kamienski (9.5) has found that a silicon carbide electrode is a nearly “ideal unattackable electrode,” and has shown that it may be used in conjunction with an indicator electrode in potentiometric titrations. Many other electrode pairs have been studied in more or less detail: platinum-gold and platinum-gold amalgam (68); platinum-platinum black (148); platinum (iridium)-platinum or platinum (iridium)-copper amalgam (162); large and small platinum electrodes (178, 18.5). The use of bimetallic systems is not limited to oxidationreduction reactions. Briinnich (33) studied the use of the platinum-graphite system in neutralizations. The change in potential a t the end point was not very marked. Fuoss (66) found that a number of systems could be used in acid-alkali titrations-. g. ,antimony-antimony-lead, bismuth-silver, anti-
The end-point phenomenon is an abrupt rise or fall in the external e. m. f. of the cell a t the end point, followed by either stationary or more or less rapidly changing e. m. f., depending upon the particular depolarization phenomena for the system in question and upon the relative reversibility of the electrode phenomena immediately before and after the equivalence point of the reaction. The original suggestion upon which this method is based was made by Hostetter and Roberts (89), who observed that a palladium wire showed almost no change in potential during the course of an oxidation-reduction titration. Recently F. Muller (159) has shown that this peculiar behavior of palladium occurs only in hydrochloric acid solution, and has to do with a change in the surface of the metal. Willard and Fenwick (21.5) have made an extensive study of the use of metallic electrode pairs, polarized externally, or chemically, and have shown that their use gives exact end points in many reactions, especially those of oxidation and reduction. Van Kame and Fenwick (163) have studied the end-point phenomena in detail. The original apparatus was rather complicated (Figure I, 1) and necessitated frequent readings of the potential. Foulk and Bawden (61) simplified the circuit and made it
1.
5.
-
ic ,
215
I
I
6.
4;
7.
I F i g u r e 11-Differential
Titration Systems
( 1 ) Retarded electrode (Muller). T h e two platinum electrodes are wrapped around a glass tube; the lower electrode is within a porous porcelain cell. (2) Cox’s differential apparatus. (3) , MacInnes a n d Jones apparatus. T h e electrode a t the left can be isolated from the main body of the solution b y lowering the glass cap surrounding it. (4) Roth’s system. T h e left electrode is within a Jena sintered glass filtering crucible, which can be lifted from its glass rod support by a handle, also of glass rod. ( 5 ) Muller’s apparatus. One electrode is within a capillary tube and the other wrapped around the tube. (6) Differential system constructed from a medicine dropper (Hall, Jensen, and Backstrom), and having the advantages of devices (3) and ( 5 ) . (7) Heczko’s apparatus. A portion of the solution is isolated b y pumice or wood. ( 8 ) Willard and Boldyreff. An extension of the idea of isolating a portion of the solution t h a t is being titrated. T h e electrode in the buret is a reference electrode, and t h e graph of t h e e. m . f.-cc. values is not differential in form,
216
A N A L Y TZCA L EDZ TZON
niony-copper amalgam; copper-copper oxide. Kahlenberg and Krueger (93) examined the utility of a large number of metallic or metallic-non-metallic electrode systems. They found that the systems tungsten-nickel, tungsten-copper, tungsten-silicon, and tungsten-cobalt were especially satisfactory. METHOD 5 . DIFFEREXTIAL TITRATIOS-This method depends upon the concentration-polarization of one of two similar electrodes by some mechanical device n-hich prevents complete mixing of a sinal1 portion of liquid, surrounding one electrode, with the rest of the solution. The graph of the e. in. f. readings as ordinates and cubic centimeters of reagent as abscissas resembles the usual S e. in. f . / S cc. 2‘s. cubic centimeters of reagent graph that is obtained by the classical method (Method 1). Cox (46) first used the differential principle by dividing the solution which is to be titrated into two equal parts (aliquot portions). The two beakers (Figure 11, 2 ) are joined by a salt bridge, and the standard solution is introduced from two burets in such a way that the end point is reached in one beaker slightly before it is reached in the other. I n Figure 11, 1, is shown one of Rluller’s “retarded” electrodes n-hich he makes the basis of a claim for priority in the development of the differential method (158). As first used, t h i j retarded electrode had its lower electrode surrounded with a solution which duplicated the conditions to be expected when the
.
Yol. 2 , s o . 3
simplified technic as was att,ained by Lange and Schwartz (121) by the classical method. Various simplifications of the differential technic have been proposed (Figure 11, 4 to 8). 1lacInnes and Dole have described an improved differential apparatus (Figure 111, 1). With this device the tit,ration is carried rapidly t o an apparent end point without stirring the solution in the reservoir d around electrode E. Upon stirring with the gas lift pump, L , the titration is finished in the normal differential fashion. Clarke and TTooten ($2, see also 1 ; 5 ) have adapt’ed the differential method to the determination of weak acids in very dilute solution. (Figure I I I , 2 ) In the deterinination of 0.001 S acetic acid with 0.001 N barium hydroxide the average deviation from the mean of a series of determinations was iO.8 per cent, and for 0.0004 acid it was 1 1 . 7 per cent. Clarke and Kooten give a fairly detailed development of the theory of the method as applied to the case of weak acids. Continuous-ReadingDevices
K e have stated that Treadwell anti Weiss (207);Foulk and Bawden (61j , and Furinan and Wilson (73) h a x developed continuous-reading niethods based on the use of a high resistance and a sensitive current-indicating device-as, for example, millivoltmeter or -galvanometer. This type of device finds frequent mention in recent communications in the potentiometric field. ELECTROS-TCBE CIFtcvrTs-The best continuous-reading device for rnany purposes, especially when glass electrodes are to be used, is some form of circuit in which the electron tube is used to construct a n e l e c t r o s t a t ic 1-01t m e t e r . Goode ( 7 5 ) first used the audion tube to develop an electro-titration apparatus, and demonstrated its utility ill neutralizations. The plot of galvanometer readings (Figure IT’, 1) against cubic centimeters of reagent gives a graph which is analogous to the usual plot of e. rn. f . readings 1‘s. cubic centimeters of reagent (Nethod 1). It is not’ necessary to p!ot t h e r e a d i n g in many instances. X number of applications of the simple circuit have been described, and many other circuits have been devised (35, 1.5, 21 7 , 94,1 7 0 , 5.4, 1% 1. The author would like to call especial 2 1 attention to the circuits (Figure Is’, 2, 3) Figure 111-Differential Apparatus tvhich Treadwell and P a o l o n i ( , ? ( i d ) (1) hlacInnes and Dole. Inert gas is introduced through G and operates t h e l i f t pump, L . T h e proposed for use in both conductance bottom opening of t h e tube, A , is of capillary dimensions; the opening, H , permits liquid t o reenter the and potentiometric titrations, and to beaker if t h e lift pump is operated too rapidly (2) Clark a n d Wooten. T h e electrode in C is analogous t o t h a t of Muller (Figure 11,5 ) . T h e gas stream is used for stirring and t o exclude air. When A is closed, the solution in C may he changed a t the similar circuits which Callan and will with t h e aid of t h e rubber bulb, B . Horrobin (36) have used. The latter investigators have also used a crystal outer electrode has reached the end-point potential. Another det’ectorinstead of an electron tube. I n constructing an electron tube assembly it is desirable to form of retarded electrode was prepared by wrapping the metal with an asbestos cord. These retarded electrodes were use a circuit that can readily be shifted to use in either conused in the technical preparation of hypochlorite solutions, ductance or potential measurements. The use of a visual indicator in conductance titrations is very convenient. and not for analytical titrations. POTEKTIOMETRIC 1\IICRO-TITRaTIoS-SU3SbeTger~er ( 166), The practical developments of the differential method as an analytical procedure is largely due to lLIacInnes and his working under the direction of Treadwell, vias apparently the co-workers (12.7, 1216, 1 % ) . NacInnes and Dole have shovn first to study potentiometric micro-titrations. (Figure V) that the differential method is capable of high precision. He x a s able to determine 0.5 to 2 nig. of various substances In the analysis of potassium chloride with silver nitrate the with a maximum error of 0.05 mg.; mean error not derived, same precision (10.003 per cent) was obtained with the but apparently not greater than 2 parts per thousand. The ” _
I S D C S T R I A L A S D E S G I S E E R I S G CHEMISTRY
July 15, 1930
217
TREADWELL (1925)
1.
L.
T R E A D W E L L ( 14 2 5 )
E"
P
J.
Figure I lo+++-++,F e + + f , and T i + + - + with Cr+' Reduction of ions of Au, Cu, Ag, Hg, and Bi with C r + + . Applications in analysis of alloys
+
+
+
Successive reduction of C u T + and F e + + + Reduction of C r + f T + f + Vf'+++, ,
a n d Fe'++
223
1 1
1 1
Brintzinger and Oschatz (1927) Brintzinger and Rodis (1927, 1928)
1
Brintzinger and Schieferdecker (1929) Zintl and Rienacker (1926, 1927) Zintl, Rienacker, and Schloffer (1927) Zintl and Rauch (1924) (1925) Buehrer a n d Schupp (1926) Zintl and Schloffer (1928) Zintl and Zaimis (1927)
1
1
with C r + + (applications in analysis of steel)
I& VESTIGATORS Jones and Lee (1922) Crowell and Kirschman (1929, p. 1695) Crowell and Kirschman (1929, p. 175)
1
Bibliography, Classified by Authors (1) Adam, J . S.African Chem. Insl., 8, 7 (1925). ( 2 ) Anderegg and Daubenspiel, Proc. I n d i a n a Acad. Sci., 36, 141 (1925). (3) Atanasiu, Compt. rend., 182,519 (1926). 14) Atanasiu, J . chim. phys., 23, 501 (1926). ( 5 ) Atanasiu, Bul. chim. sac. romdnd s l i i n l e , 30, 1 (1928). (5a) Atanasiu, Ibid., 30, 51 (1928). (6) Atanasiu, I b i d . , 30, 69 (1928). (7) htanasiu, Ibid., 30, 73 (1928). (8) htanasiu, Ibid., 30, 77 (1928). (9) Atanasiu and Stefanescu, Ber., 61, 1343 (1928). (10) Ato, Sci. Papers Inst. Phys. Chem. Research ( T o k y o ) , 10, 1 (1929). (11) Auerbach and Smolczyk, Z . physik. Chem., 110,65 (1924). (12) Baggesgeard-Rasmussen and Schou, Z . Elektrochem.. 21, 189 (1925). (13) Behrend, Z . physik. Chem., 11,466 (1893). (13a) Berry, A n a l y s t , 64, 461 (1928). Benedetti-Pichler, anai. Chem., 73,200 (1928). Bienfait, Rec. t r a y . chim., 46, 166 (1926). , 135 (1922). Bishop, Kittredge, and Hildebrand, J ,A m . Chem. S O L .44, Bodforss, Svensk Kem. Tid., 38, 333 (1926); through C . A , , 20, 1193 (1926). Bottger, Z . phrsik. Chem., 24, 253 (1897). Brann and Clapp, J . A m . Chem. Sac., 51, 39 (1929). Bray and Kirschman, I b i d . , 49, 2739 (1927). Brintzinger and Oschatz. Z . anorg. allgem. Chem., 166, 221 (1927). Brintzinger and Rodis, Ibid., 156, 5 3 (1927); Z . Elektrochem.. 34, 246 (1929). Brintzinger and Schieferdecker, Z . anal. Chem., 76,277; 78,110 (1929) Britton, Analyst, 50, 601 (1925). Britton, J . Chem. SOL.,125, 1572 (1924). Britton, Zbid., 127, 1896 (1925). Britton, Ibid., 127, 2110, 2120, 2142, 2148 (1925). B'ritton, Ibid., 127, 2796 (1925). Britton, Ibid., 127, 2956 (1925). Britton, Ibid., 1926,126; 1927, 147, 422, 425. Britton, Ibid., 1926, 269. Brouchere, Bull. SOL. chim. Belg., 37, 103 (1928). Briinnich, IND. EKG. CHEM.,17, 631 (1925). Buehrer and Schupp, Ibid., 18, 121 (1926). Calhane and Cushing, IND. ENG. CHEM.,15, 1118 (1923). Callan and Horrobin, J . Sac. Chem. Ind., 47, T329 (1928). Cavanagh, J. Chem. SOL.,1927,2207. Cavanagh, Ibid., 1928, 843. Cavanagh, Ibid., 1928,855. Clark, Ihid., 1926,749. Clark, "Determination of Hydrogen Ions," Williams and Wilkins, 1928. Clarke and Wooten, J . Phrs. Chem., 33, 1468 (1929). Closs and Kahlenberg, Trans. A m . Electrochem. SOL, 64,369 (1929) Collenberg and Sandved, Z . anorg. allgem. Chem., 149, 191 (1925). Conant and Hall, J. A m . Chem. SOL.,49, 3062 (1927). Cox, J . A m . Chem. Sac.. 47,2138 (1925). Crotogino, Z . anorg. allgem. Chem., 24, 225 (1900). Crowell a n d Kirschman, J . A m . Chem. SOL.,51, 175, 1695 (1929). Dachselt, Z . anal. Chem., 68, 404 (1926). Daggett, Campbell, and Whitman, J . A m . Chem. Sac., 45, 1043 (1923). Davis, Oakes, and Salisbury, IND.ENG.CHEY.,15, 185 (1923). Drossbach, Z . anorg. allgem. Chem., 166, 225 (19!7). Dutoit a n d van Weisse, J. chim. phys., 9, 578, 608, 630 (1911). Elder, J. A m . Chem. Soc., 51, 3266 (1929). Eppley and Vosburgh. Ibid., 44, 2148 (1922). Ewing and Eldredge, I b i d . , 44, 484 (1922). Fenwick, Dissertation, hlichigan, 1923. Ferguson a n d Hostetter, J . A m . Ceram. SOL.,2, 608 (1919). Fleyscher. J . A m . Chem. SOL.,44,2685 (1922).
i.
Forbes and Bartlett, Ibid.,35, 1527 (1913). Foulk a n d Bawden, Ibid., 48, 2045 (1926). Fresno, del, 2 . Eleklrockem., 31, 199, 617 (1925). Fresno, del, and Valdes, Z . anorg. allgem. Chem., 183,251, 253 (1929). Fresno, del, and Valdes, Anales sac. espaii. fis. puim., 27, 368 (19291; through C. A , , 23, 4161 (1929). Fresno, del, and Vasquez, Ibid., 25, 42 (1927); throxgh C. A , , 21, 1420 (1927). Fuoss, IND.EXG.CHEM.,Anal. E d . , 1, 125 (1929). Furman, J . A m . Chem. Sac., 44, 2685 (1922); Trans. .4m. Electrochem. SOL., 43, 79 (1923); Chapt. X I I I , Vol. 11, Taylor's "Physicil Chemistry," Van Nostrand, 1924. Furman, J. A m . Chem. SOL.,SO, 268, 273 (1928). Furman, Ibid., 60, 755, 1675 (1928). Furman and Evans, Ibid., 61, 1128 (1929). Furman and Wallace, Ibid.,61, 1449 (1929); 62, 1443 (1930). Furman and Wallace, Ibid.,62, 2347 (1930). Furman and Wilson, Ihid., 50, 277 (19281. Gilbert, Ibid.,46, 2648 (19241. Goode, J. A m . Chem. Sor., 44, 26 (1922); 47, 2483 (19251. Gustavson and Knudson, Ibid., 44, 2756 (1922). Haber and Klemensiewicz, Z . physik. Chem., 67, 385 (19091. Hahn and Frommer, Zbid., 127, 1 (1927). Hahn and Weiler, Z . anal. Chem., 69,417 (1926). Hall and Conant, J . A m . Chem. Sac., 49, 3047 (1927). Hall and Werner, Ibid.,50, 2367 (1928). Heczko, Z . anal. Chem., 73, 404; 74, 289 (1928). Heczko, Ibid., 73, 247 (1929). Hedrich, Dissertation, Dresden, 1919. Hendrixson, J. A m . Chem. Sac., 43, 14, 859, 1309 (1921); 45, 2013 (1923); 47, 1319 (1925). Hendrixson and Verbeck, Ibid., 44, 2382 (1922). Hildebrand, Zbid., 36, 869 (1913). Hildebrand and Harned, 8th Intern. Cong. Appl. Chem., 1, 217 (1912). Hostetter and Roberts, J. A m . Chem. Sac., 41, 1337 (1919). Hughes, Ibid., 44, 2860 (1922); J . Chem. SOL.,1928,481. Hughes, Ibid., 1928,491. Jones and Lee, IND.E N G .CHEM.,14,46 (1922). Kahlenberg and Krueger, Trans. d m . Electrochem. Sac., 56, 201 (1929). Kamienski, Bull. Intern. Acad. Polonaise, 1928, 33. Kamienski, Z. physik. Chem., A138, 315 (1928); A145, 43 (1923); Prsemysl Chem., 1927. Kano, Science Repls. TGhoku I m p . U n i v . , 16,713 (1927). Kelley and Bohn, J . A m . Chem. Sac., 41, 1776 (1919). Kelley and Conant, I b i d . , 38, 341 (1916); ( 2 ) 1x0. ENG. CHBY.,8, 719 (19161. Kelley, Spencer, Illingworth, and Gray, I b i d , 10, 19 (19181. Kelley and Wiley, Ibid., 13, 1053 (1921). Kelley, Wiley, Bohn, and Wright, I b i d . , 13, 1053 (1921). Kieferle and E r b x h e r , Biochem. Z . , 201, 305 (1928). King and Washburne, J . Phys. Chem., 30, 1688 (1926). Kirschman and Ramsey, J . Am. Chem. Sac., 50, 1632 (1928). Klit, Z . physik. Chem., 131, 61 (1927). K n a u t h , Dissertation, Dresden, 1915. Kolthoff, Chem. Weekblad., 16,450 (1919). Kolthoff, Rec. tra9r chim., 39, 208 (1920); 40, 363 (1921); 41, 172 343,425, 787 (1922); 43,768,816 (1924); 46, 745 (1926). Kolthoff, Ibid., 47, 397 (1928). Kolthoff a n d Furman, "Potentiometric Titrations," Wiley, 1926. Kolthoff and Hartong, Rec. lrav. chrm., 44, 113 (1925). Kolthoff and Laur, Z . anal. Chem., 73, 177 (1928). Kolthoff and Robinson, Rec. lrav. chim., 45, 169 (1926). Kolthoff and Tomicek, Ibid., 43, 447, 775 (1924). Kolthoff, Tomicek, and Robinson, Z . anorg. allgem. Chem., 150, 157 (1926).
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Boiling Milk in Aluminum Does Not Destroy Vitamin C kluminum cooking utensils have no selective destructive action on the antiscorbutic vitamin of milk, according t o the results of experiments carried out a t Mellon Institute of Industrial Research. Milk has particular importance in the dietary of the infant and child, There has been a growing tendency to boil milk whenever i t is to be used in supplemental feedings, or whenever a supply is of doubtful origin. In thus safeguarding the health of the children against microorganisms and in providing for better assimilation of the proteins, mothers may he assured t h a t when they use aluminum utensils for the preparation of
milk they are not depriving this invaluable foodstuff of its antiscorbutic properties. In the Mellon Institute experiments milk was boiled lightly for five minutes in aluminum or glass containers. Some destruction of vitamin C occurred in each case as a result of the boiling, b u t the metallic utensils exerted no greater action than did those of glass. Another interesting observation is t h a t winter milk from ensilage-fed cows is practically as potent in vitamin C as the best summer milk from cows on pasturage. Full details of the experiments will be supplied by Nellon Institute on request.
