INDUSTRIAL AND ENGINEERING CHEMISTRY
534
Vol. 20, No. 5
Smooth Platinum Wire for Electrometric Titrations in Neutralization Reactions' Stephen Popoff and Martin J. McHenry THE STATE UNIVERSITY OB IOWA,IOWACITY, 1.4.
HE determination of total acidity or total basicity
was employed throughout. All other solutions were prein the presence of oxidizing agents, metal ions, phenol, pared from the best chemicals obtainable. cresol, and varnish is of special importance in many Theory theoretical and practical problems. Existing methods which employ the hydrogen, quinhydrone, air, or glass There is no doubt that the smooth platinum wire is essenelectrode and the conductivity method are often too cum- tially an oxygen electrode. The electromotive force may bersome or time-consuming, or give rise to errors which be represented by: RT aor are too large to be disregarded. E = E o + - l n ~ nF Q(OH ) The oxygen and the air electrodes have been studied by Furman.2 It is possible to in which the terms have use these electrodes in the their usual significance. presence of oxidizing agents. It is not claimed that An electrometric method, using a smooth platinum Furman mentions the use the electromotive f o r c e s wire instead of the hydrogen electrode, has been tested of the smooth p l a t i n u m given were obtained after and found to give excellent results under conditions wire, but gives no data and equilibrium had been obwhich make other physico-chemical analytical methremarks that its action is t a i n e d . This is neither ods either useless or impracticable. possible nor necessary, as erratic a t times, even in the Sufficient breaks at the end point have been obtained p r e s e n c e of an oxidizing there was no difficulty in to enable one to determine either total acidity or total agent. The smooth platiobtaining duplicates with basicity with precision surpassing that of the most very small variations num electrode was successsensitive indicator method. It is possible to detera m o n g i n d i v i d u a l titrafully used in neutralization mine total basicity or total acidity in the presence tions. M e t h y l o r a n g e reactions by van der Meulen of permanganate, using solutions as dilute as 0.01 N changed color before and and W i l c ~ x o n . The ~ breaks to within 0.01 cc. Tungsten wire in place of platinum phenolphthalein after the a t the end point were about wire gives good results, but the break at the end point equivalent point had been 350 millivolts per cubic cenis smaller. The problem of determining total acidity r e a c h e d . Whenever the timeter. T h e present in the presence of phenol, cresol, and varnish has been wire was treated in the Rame writers have been able to solved satisfactorily. manner the end points were obtain much larger breaks The alkaloids or their salts have been determined at about the same voltage. and have been successful in electrometrically with a precision greater than that I n the presence of permanapplying the smooth platiobtained with other analytical methods. Titration ganate or o t h e r s i m i l a r num wire in a large number curves are given. oxidizing agents the breaks and variety of reactions inwere much larger as the volving n e u t r aliz a t io n , o x i d i z i n g- potentials are For s&plicity and rapidity, the smooth platinum wire offers many advantages over exist- functions of the hydrogen-ion activities. ing methods for determination of total acidity. Determination of Total Acidity Apparatus and Materials Employed It can readily be calculated that in a final volume of 125 The usual potentiometric set-up, containing Leeds & cc., using 0.1 N strong acid and base solutions, there would Northrup student potentiometer, lamp and scale galvanom- be a difference of 0.02 to 0.03 cc. between the methyl orange eter, saturated calomel electrode, and ordinary platinum and phenolphthalein end points if one takes the concenwire, was used in all the experiments. The platinum elec- trations of hydrogen ion to be and for the respectrode was cleaned after each titration by letting it stand tive color changes of the indicators. Under similar con15 to 30 minutes in dichromate cleaning mixture and then ditions but using 0.02 N solutions the difference would be washing thoroughly with water. The end point was found about 0.13 cc., and for 0.01 N solutions 0.25 cc. by the process of interpolation given p r e v i ~ u s l y . ~Air If one uses phenolphthalein as the indicator in titrations free from carbon dioxide was blown into the apparatus involving strong acid and strong base, one would employ throughout the titration. more alkali than necessary corresponding to 0.06 and 0.12 Potassium hydroxide solution was prepared free from cc. of 0.02 N and 0.01 N solutions if the final volume is 125 cc. carbonate by treatment with excess of barium hydroxide Experiments showed the end points with phenolphthalein and subsequent removal of the barium by potassium sul- to occur after the electrometric end points and to correfate. The potassium hydroxide solution was standardized spond to 0.06 and 0.09 cc. of the above solutions. It means, against a B. of S. sample of benzoic acid. The hydro- therefore, that electrometrically one obtains the true end chloric acid solution was prepared from a c . P. solution and points. its concentration checked against the potassium hydroxide. Redistilled water, protected from the carbon dioxide, Determination of Total Acidity in Presence of Oxidizing Agents 1 Receivzd December 31. 1927.
T
* J.
A m . Chem. SOC.,44, 2685 (1922).
a
IND. ENG.CHBM.,16, 62 (1923).
4
Popoffand Whitman, J . A m . Chcm. SOC.,41, 2259 (1925).
The equilibrium existing between the manganate, permanganate, and hydroxide ions, as shown by the equation
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1928
has been the subject of much study. constant,
In the equilibrium
535
When 0.01 N solutions were used and the acid was added to the alkali no breaks were obtained unless permanganate was present.
a2mno4)-- X
OH)a3(~,04)-
any error in the determination of the hydroxide-ion concentration will be multiplied by four in calculating the constant. The precise determination of the basicity in the presence of permanganate is important. CraigG and Ruby6 used hydrogen peroxide to destroy the permanganate and then titrated the total alkali with standard acid. Ruby states that the error is about 0.5 per cent. Schlesinger and Siems' studied various methods of determining the total basicity under the foregoing conditions. They state that there are two sources of error in the Craig and Ruby method when dealing with solutions of low hydrogen-ion concentration. I n the first place, it is very difficult to remove the last traces of hydrogen peroxide from the mixture and these small traces destroy the sharpness of the end point of the indicator. Furthermore, manganese dioxide absorbs alkali to such an extent that the accuracy of the titrations in solutions from which manganese dioxide has been freshly precipitated is greatly impaired. The conductivity method, first suggested by Kuster, Grutter and Geibel,* was used by Schlesinger and Siems with apparent success, but the method is long and sometimes constancy of readings was obtained only after half an hour. Plotting and extrapolation are necessary in conductivity methods. The smooth platinum method was tried for titrations involving neutralization in the absence and presence of oxidizing agents. Titrations using various acids approximately 0.1 ,li and potassium hydroxide were made. Table I gives the data obtained. ACID HzSO4 H2S04
HCzHaOz HCzH:Oz HC1 HCI HC1 HCl H:CzOr
Curve 1-Electrometric
End Points Using S m o o t h Unattackable Electrodes
Table I-Titrations of 0.1 N Solutions OXIDIZING AGENT VOLUXE RATIO 1.0816,1,0812 None 1,0828,1.0816 5 cc. 0.1 N Khin04 1.0689,1.0681 None 1,0695,1.0689 5 cc. 0.1 N KMnO4 0,8945,0.8945 None 0,8945,0,8949 2 drops of 0.1 N KMnO4 0.8956, 0.8951 5 cc. 0.1 N KMnOl 0.8949 1 drop KzCrzO7 1.2351,1,2364 None
Data for the electrometric titration of 0.02 N solutions of acid and base in the presence of 5 cc. of 0.1 N pot,assium permanganate solution are also given. Only the readings taken near the end point are given. Table 11-Titration HCI Cc.
20.03 20.11 20.15 20.19 20 23 20,27
Milliorclts 446 458 460 467 474 488
Data for 0.02 N Solutions HCI
.Millivolts be7 cc.
175 350
Cc. 20.31 20.35 20.39 20.43 20.47 20.51
Milliwolls 504 534 575 607 626 640
Milliuolls ber CC.
400 750 1025 800 475 350
I n the case of 0.01 N solutions, in the presence of 5 cc. of 0.1 N potassium permanganate solution, the breaks near the end point corresponded to 475, 575, 675, 625, and 375 millivolts per cubic centimeter. The volume ratios with and without the permanganate solutions were as follows: CONCENTRATION 0.02 N 0.01 N
iYo PERMANGANATE WITH PERMANGANATE 1.2040 1.2030 1.1963 1.1960
J . Soc. Chcm. I n d . , 38, 96 (1919). J. Am. Chem. Soc., 43, 294 (1921) 1 Ibid., 46, 1965 (1924). 82. onorg. Chcm., 42, 229 (1904).
6
6
1
INDUSTRIAL AND ENGINEERI.VG CHEAVISTRY
536
results. Known samples were prepared by adding definite volumes of standard hydrochloric or oxalic acid solutions to phenols, cresols, and varnishes which were supposedly free from acidity. Water was used as the solvent, together with the minimum amount of ethyl alcohol necessary to insure complete solution. The electrometric method was carried out in the usual way, adding base to acid and using approximately 0.1 N solutions. Typical data are given in Tables I11 to V. Table 111-Total PHENOL WATER ALCOHOL
Grams
Cc.
CC.
Acidity in Phenol POTASSIUM HYDROXIDE Calcd.
Found
HC1
cc.
CC.
CC.
Vol. 20, No. 5
sary. Readings were taken near the end points, using about 0.12 cc. of alkali and the respective end points, calculated by the process of interpolation previously m e n t i ~ n e d . ~ Dilution is of great importance in the successful determination of acidity in the presence of phenol, cresol, and varnish. For example, no breaks were obtained either when 6 grams of phenol were dissolved in 40 cc. of water and 20 cc. of alcohol, or when 2 grams of cresol were dissolved in 40 cc. of water and 10 cc. of alcohol. The amount of alcohol present must be sufficient to prevent precipitation up to the end of the titration; the appearance of a precipitate a t any stage produces an error. It is essential to clean
A C I D I T Y DUE TO HYDROCHLORIC ACID
1.5 3.4 1.9 1.9
20 20 30 30
10 10 10 10
10.74 10.75
10.72
10.72
10.57 10.65 10.63 10.66
10,02 10.03 10.00 10.00
ACIDITY DUE TO OXALIC ACID
1.1 1.1
0 16.16 16.16 16.18 16 15 0 16.20 16.18 0 17.54 17.50 0 1.15 17.50 1 1 0. 0 17.52 _ a Oxalic acid differs from t h a t in first three tests.
1.1
CRESOL
Grams
75 75 75 75 150
Table IV-Total Acidity in Cresol WATER ALCOHOL POTASSIUX HYDROXIDE Calcd. Found
cc.
CC.
CC.
cc.
20 01 20.03 20.06 20.03 20.01
HC1
cc.
ACIDITY DUE TO HYDROCHLORIC ACID
1.7 1.7 1.7 1.0
30 30 30 80
1.0 1.0
80 80
20 20 20 20
10.73
10.75
10.76 13.66
10.65 10.69 13.50
10.5s
10.01 10,03 10.03 15.68
8.62 8.6s
10.00 10.03
ACIDITY D U E TO OXALIC A C I D
20 20
8.76 8.78
Table V-Total VARNISH
Acidity in Varnish Due t o Hydrochloric Acid HYDROXIDE HCI WATER ALCOHOL POTASSIUM Calcd. Found
Grams .~
cc.
cc.
CC.
cc.
CC.
0.5 1.0
25 50
65 85
8.72 8.76
8.66 8.71
10.01 10.06
When titrating a solution of varnish containing oxalic acid the breaks at the end point were small. To determine the end point in this case one must plot a curve. Curves 2 and 3 show the respective breaks near the end points in the determination of total acidity under the conditions specified in Tables I11 to V. Curve 4 shows the whole story during the titration of an acidic solution of
phenol. It will be observed that the breaks a t the end points in the presence of phenol, cresol, and varnish are much smaller than when these constituents are absent. Except in the case of oxalic acid in varnish, plotting of curves for the purpose of determining the end point is hardly neces-
Curve 4-Electrametric Titration of HCI, in Presence of Phenol, with KOH
the platinum nire thoroughly, as dirty wire reduces the breaks, in some cases t o zero. The Fire must stand in cleaning mixture a t least 30 minutes. Quantitative Estimation of Alkaloids
The quantitative estimation of alkaloids has been subject to many investigations. There is much dispute as to the proper indicator to be employed in titrations involving alkaloids. Allenlo says: “Most alkaloids behave as monoacid bases even %Then they contain two atoms of nitrogen. Further, they show many anomalies in their behaviour with the usual indicators.” The dissociation constants given by Weisse and Levy“ are valuable aids in the indicator method of estimating alkaloids. A critical study of the gravimetric, volumetric, colorimetric, refractometric, and polarimetric methods has been made by Hereig.12 Conductimetric methods were first mentioned by Kuster, Grutter and Geibel’ in 1904. Since then many investigators have studied the quantitative determination of alkaloids, chief among which are Dutoit and Meyer-Le3yy, Kolthoff, and Treadwell and Jannet. A variation in the conductimetric method is proposed by Treadwell,l3 who uses a radio receiver tube to detect the null point, The conductimetric methods are interesting but too cumbersome and slow for general use. The hydrogen electrode has been employed by Muller14 and Kranz.lS I n a discussion of Kranz’s work, Kolthoff16 states that since there is reduction a t the hydrogen electrode there can be no equilibrium. The quinhydrone electrode has been used by Wagener and McGill17 who characterize the method thus: 10 “Commercial Organic Analysis,” 4th ed., Vol. VI, p. 181, P. Blakiston’s Son & C o , Philadelphia, Pa. 11 J . chzm phys , 14, 261 (1916). 19 Arch Pharm , 259, 249 (1921). 19 Helu. Chzm. Acta, 8 , 89 (1925). 14 Z . Elektrochem., SO, 587 (1924). 16 J A m Pharm Assocn., 14, 294 (1925). 18 I b z d , 14, 297 (1925). 17 Ibid., 14, 289 (1925).
ISDUSTRIAL A N D ENGINEERING CHEMISTRY
May, 1928
In the majority of cases, results were obtained which were slightly lower than the theoretical values. Where methyl red was added to the solution the end ooint. as indicated bv the color change, invariably was reached -before the electrometric end point, the electrometric results consequently being higher and nearer the theoretical values. It is not possible to say that the samples were 100 per cent pure. There is no absolute criterion for the purity of these samples. It is, therefore, not permissible to assume that the results obtained by the use of the quinhydrone electrode are either more or less nearly correct values than those obtained colorimetrically. However, the results with the quinhydrone electrode are reproducible.
EXPERIMENTAL-The same apparatus and solutions were employed as in the previous work. The alcohol was dis-
537
respectively. Table VI11 gives the quantitative results obtained for a number of alkaloids and their salts. Table VII-Titration of S t r y c h n i n e H y d r o c h l o r i d e KOH KOH Lf illivolts cc. Miilivcils cc. 0.30 67 10.52 82 10.20 10,64 - 105 Wash 50 10.77 - 139 10.30 - 59 10.89 - 160 10.41 - 68 11.01 170 Weight of strychnine hydrochloride, 0.3790 gram; alcohol, 75 cc.; water. 50 cc. E n d point: Calcd., 10.40 cc. Found, 10.35 cc.
-
-
T a b l e VIII-Quantitative
T i t r a t i o n of Q u i n i n e
C u r v e 5-Electrometric
tilled from quicklime, its acidity determined electrometrically. and the proper correction factor employed. The alcohol was used either t o hasten or effect solution of the alkaloid. I n the case of alkaloids three breaks were obtained in the electrometric method, using the smooth platinum wire: the first representing the conversion of the alkaloid to the hydrochloride (when hydrochloric acid was added); the second, the neutralization of exceqs of acid by the base added; and the third, the conversion of the hydrochloride back to the free alkaloid on addition of a base. Potential readings were taken about half a minute after the addition of either acid or base. The dats for one complete electrometric titration of strychnine and for the hydrochloride of the alkaloid are given in Tables VI to 1-111. T a b l e VI-Titration
of S t r y c h n i n e KOH
cc.
HCI
cc.
.If zllzvolts
0.11 11.10 Wash
- 34
11.39 11.52 11.66 11.80 11.94 12.09 21.70
111 129 166 207 220 227 276
EWDPOINT
Calcd.
cc.
cc.
Quinine Quinine Quinine hydrochloridea Quinine hydrochloriden Quinine sulfate" Quinine sulfateo Strychnine Strychnine Strychnine hydrochloride Strychnine hydroch loride Atropine Atropine Atropine sulfatea Atropine sulfate" Cinchonidine Cinchonidine Cinchonidine sulfatea Cinchonidine sulfatea 0 Samples were dried a t about 100' C.
10.40 10.75 9,l5 13.81 12.90 11.23 9.32 11.55 10.35 11.51 9.30 13.85 11.32 11.27 13.30 8.22 11.24 9.48
10.40 10.77 9.25 13,90 13 02 11 30 9.31 11.57 10.40 11.57 9.34
Found
13.89 13.04 13.13 13,34 8.21 11.67 9.91
These results show that the smooth platinum electrode method can be successfully used to determine alkaloids quantitatively. The discrepancies between found and calculated values are comparatively small for those alkaloids which do not have water of crystallization. No special attempt was made to obtain the anhydrous alkaloids, because it is doubtful if one can remove the water of crystallization without decomposition.
.bf Llimlls
39 98
219 211 200 175 111 120 108
- 40
- 43 - 54 - 62 -70 - 77 - 83
-
89 -117 Weight of strychnine, 0.3793 gram; alcohol, 50 cc.; water, 75 cc.; acid a n d base approximately 0.1 N BREAK CALCD. FOUXD
First Second Third
E s t i m a t i o n of Alkaloids
WEIGHT Gram
ALKALOID
cc.
cc.
11.55 10.74 12.40
11.57 10.66 12 41
Curves 5 and 6 show graphically the results obtained in the electrometric titration of quinine and quinine sulfate,
ILI-!I I ! ! I I I I I N I ! ! - k:-il C u r v e 6-Electrometric
T i t r a t i o n of Q u i n i n e S u l f a t e
The precision obtained in the quantitative estimation of alkaloids is gsod. It is not possible to speak of the accuracy of the method, as a t present there is no method for determining accurately the per cent purity of alkaloids. I n most cases the results are close to the theoretical, assuming that the compounds are 100 per cent pure. It is rather difficult to compare the smooth platinum electrode method with other methods, because of incomplete data. Nevertheless, Table IX gives some comparison between the quinhydrone, hydrogen, and smooth platinum electrode methods. The smooth platinum electrode method is a simple, rapid,
538
INDUSTRIAL AND ENGINEERING CHEMISTRY
and precise method for the quantitative estimation of alkaloids. It would be possible to extend the use of the smooth platinum electrode method in the quantitative determination of many aminohydrochlorides in the presence of acid.
Table IX-Comparison ALKALOID Quinine Strychnine Atropine
5’01. 20, No. 5
of Per Cent Errors by Different Methods
SMOOTH
PLATINUM P e r cent 0.20 0.14 0.42
QUINHYDRONE
P e r cent 0:24 1.10
HYDROGEN P e r cenl 1.63 1.26 1.72
Determination of Sulfur Dioxide in Small Amounts in the Atmosphere’ R. J. McKay and D. E. Ackerman INTERNATIONAL NICKELCOMPANY, NEWYORK,N. Y . , A N D BAYONNB, N. J.
The physical and chemical importance of the presE A R L Y a l l of t h e cases erroneous results have ence of sulfur as gases in the air is discussed. Sulfur studies made in this been obtained owing to lack dioxide affects corrosion rates, pathology, and possibly country involving the of attention to necessary previsibility. determination of sulfur dicautions. A modified detailed description is given of the very oxide in the atmosphere have It was found at that time rapid Selby Smelter Commission method for the estibeen with reference to action t h a t m o s t of t h e methods mation of sulfur dioxide in the atmosphere. The on plants and, in a few cases, given in the literature for the method is also applicable, with some changes in conanimals. R e c e n t studies2 determination of sulfur dicentrations of solutions and size of sample, to the have shown that the amounts oxide were inapplicable to determination of sulfur dioxide content of smelter of sulfur dioxide and hydrofield work. The new method flue gases. The gas sample is drawn by the evacuation gen sulfide in the air are varideveloped had two unusual method into a bottle containing iodine-starch solution, a b l e s which rate with the requirements: (1) the need of which oxidizes the sulfur dioxide. The oxidizing solucarrying and using the apmoisture content in determintion is then withdrawn and brought to the same ining t h e corrosive effect of paratus in all weathers; and tensity of blue as a blank, by the addition of standard atmospheres in different local(2) the estimation of very low iodine solution. A number of precautions, the obities. Reference to the milconcentrations of sulfur diservance of which is essential to the successful use of oxide. Later experiencewith lions of dollars yearly lost by the method, are discussed. Emphasis is placed on corrosion seems trite, but in the method has resulted in exact manipulation while drawing the sample. some refinements. A growing view of these studies there is realization of the importance no doubt that the sulfur content of the air is of economic importance in connection with of sulfur gases in the air makes a new description of thefoundacorrosion as well as in pathology. tion method with later refinements seem worth while. Probably there are other modern problems in which this Applicability of Method imperfectly known sulfur content is important. For instance, one of the authors has observed that in cases where The method was developed for, and is particularly applihigh sulfur dioxide concentrations from industrial plants exist in damp atmospheres, fogs are apparently produced by cable to, the estimation of sulfur dioxide in atmospheric air in the sulfur dioxide. Quite sharp lines of demarcation have concentrations varying from 0.05 to 20.0 p. p. m. by volume. been observed in the visibility a t places where change in sul- With alterations in the size of the sample and strength of fur dioxide concentration is rapid. I n the early morning solutions, it is also applicable to, and has been successfully heat dispels a fog more quickly in areas adjacent to a sulfur used for, the determination of the concentrations of sulfur smoke “stream” than in the “stream” itself, and in the even- dioxide, from 0.01 to 10.0 volume per cent, which may exist in ing an otherwise invisible sulfur dioxide “stream” has been smelter flues. The accuracy of the method has been thoroughly checked on observed to become sharply visible as fog and to remain so for hours while fog developed only gradually in its neighborhood. synthetic gas mixtures. The method of making mixtures The conditions under which this phenomenon was observed was described in the work of the Selby Smelter Commi~sion.~ are very different from those in most industrial districts, The exact results of these tests made subsequent to the Selby but since the results are in rough accord with the thermo- Commission’s work cannot be given since they were obtained dynamics of fog formation it seems possible that sulfur con- as part of a confidential investigation. The general statement may be made that, with proper conditions of mixing tent may have a relation to visibility. Several years ago, in connection wikh a study of smelter gases and manipulation of apparatus, results are within gases in the West, a method of quick analysis for these com- 10 per cent of the calculated value. Exceptions to this, of pounds was developed by the Selby Smelter Commi~sion.~course, must be made on concentrations under 0.5 p. p. m., One of the writers had the opportunity of working on this and also under exceptional conditions of direct sunlight and analysis under the direction of the commission. Since then heat. It should be noted that hydrogen sulfide is also oxidized by the analysis has been used in many quarters, and in some iodine solutions and, depending upon the object of the work 1 Received December 27, 1927. * Evans, “Corrosion of Metals,” p. 152, Edward Arnold & Co., 1926: of which the sulfur gas determination is a part, it may be necessary to estimate the hydrogen sulfide separately and make a Vernon, Second Experimental Report to Atmospheric Corrosion Research Committee, British Nonferrous Metals Research Assocn., 1927, p. 122, correction for it. However, for most work in which this
N
e t sq.; Hatfield, Enginrcr, 184, 639 (1922). a Holmes, Franklin, and Gould, Bur. Mines, Bztll. 98, 200 (1915).
4
Wells, Bur. Mines, BuIZ. 98, 192 (1915).
