ANALYTICAL CHEMISTRY
1306 .
~
Table I.
~~
Comparison of Potentiometric and Photonietric ' Methods for Chloride in Rocks Chloride Content, % Photometric Potentiometric ~
Sample Granite Diabase Feldspar Clay Granite Hornblende gneiss
0 . 0 0 8 , 0.009
0.022.0.022 0.007, (0.010) 0 . 0 0 6 , 0.008 0.004 0.023
0.0079, 0 , 0 0 8 4
0.021,0.020 0.0057, 0.0064 0.0057. 0,0062 0.0038 0.021 ~-
and precision depcnd upoii the rcproducitility of thr e.ni.f of the cell and the accuracy with which the e.1n.f. is measured. Under the experimental conditions described aliove. 10-4 X chloride solutions were titrat.ed \vit,li an accuracy and p i ~ i s i o n better than 1%, 5 X 10-5 -Ychlol,ide solutions with a precision of 275, and 10-5 N solutiolis with a precision of 10%. As an example, the change of t8hee.1n.f. during the titration of 5 x 10-6 .Y chloride in the recommended supporting electrolyt,e is shown in Figure 1. The change of the e.1ii.f. from the beginning of the tit,ration until 100% excess of silver was present \vas only 45 rnv. Locating the end point from the iiiaximuni in A E / A c is not possihle in this instance. Traces of chloridc were determined in 0.1 S solut,ions of a nuinher of elect,rolytes wit,h the ahove accur:rcy. Thc support,ing electro1yt.e was 0.5 ATin potassium nitrate and 0.1 S in nitric acid. In general, it is recomniended that each worker cletermiiie bhe apparent equivalence potential hiinself under the selected experimental conditions. The chloride content o f IIississippi River water (almut 5 X 10-5 .\-in chloride) was dt~tcwnincdiiy making 100 nil. of water 0.5 S in potassium nitrat,e and 0.1 .\-in nitric acid (0.1 .\*potassium nitrate alone can also be uwd as supporting electrolyte). The average of 10 determinations indicated a chloride content of 1.83 i 0.03 nig. per liter. I n orc1r.r to test the accuracy, two 2lit,er samples of R-at,cr were evaporated t,o 100 nil., supporting
electrolyte was added, :ind the vhloride was titrated. This concentrate was about 10-8 AYin chloride, which can be titrtaed with a few tenths p e r cent accuracy. In this way a chloride content of 1.81 mg. per liter mas found.
Chloride in Rocks. 1 finely powdered sample (0.5 to 1 grain 1 in a platinum crucible with five times its weight sodium carbonate. After cooling, the melt is extracted with hot water, the suspension is filtered, antl the residue is washed several times with hot ~ a t e r . .I drop of nicthvl orange and chloride-free 7 .T nitric acid is added t o the filtratr until the solution is red and so much more nitric acid that the acid concentrati:n is about 0.1 S after solution to 100 1111 -After cooline to 25 . the solution is titrated with silver nitrate to :in equiva'ience potential of 0.2710 + 0.0004 ( f - 2 5 O C.). . Even C.P. aotliuni r:iri)oiintc rontains t,races of chloride; therefore, it hlank with thc renKc~ntis run by the above procedure.. The difference Iwtween thc timount of silver used by the sample and the hlank gives thc chloride content. Obviously, an exact knowledge of the apparent equivalence potential is of no consequence in this a n a l j , as long as the temperature is the same (preferably within a few tcnths of a degree) in the t,itration of the sainplc and the hlank. IS fused
In T:iI)I(>T sonw rrrults :ire (lonlp:+redwith those ohtainrd by the photometric method rrrciitly described by Iluroda and Sandell (5). Both nir~tliotlsgive results of the same order of accuracy. ACKNOWLEDGMENT
Acknowledgment is made to the Graduate School of the University of Minnesota for a grant which enabled the authors to carry out t.his work. LITERATURE CITED
(1) Furman, N. H., antl Low, 0. IT.,Jr., J . .4m. Chem. Soc., 57, 1585 (19 3 5 ) . (2) Kolthoff. I. Ji.,and Laitinrn. H. .I.,"pH and Electrotitrations," 2nd ed., Ten, Tork, John T i l e y t Sons, 1941. (3) Kuroda, P. K., and Sandell. E. B., AXAL.CHEY.,22, 1144 (1950). (4) Taylor, J. K., and Smith. E. R., J.Research Natl. Bur. Stnndnids, 22, 307 (1939).
RWEIVED December 22, 1950:
Argentometric Amperometric Titration of Traces of Chloride With the Rotated 'Platinum Electrode as Indicator Electrode I. 51. KOLTIIOFF AND P. K. KURODA School of Chemistry, t?'nit.ersity of Minnesota, Minneapolis M i n n .
I
N PRELIMIKART work 1,witiiien and Rolthoff (3) s h o ~ e d that good results could he obtained in the amperonietric titration of silvw with chloride or vice versa with the rotated platinum electrode as indirntor clrctrode, if some gelatin \vas added to prevent depolarization of the clertrotlr h?- thc colloidal silver chloride particles arid to ohtniri a smooth deposit of silver on the electrode in the presence of a n escess of silver salt. Laitinen et al. (Z) reportcd Inter thkt cliloridtr in concentrations greater than 0.005 A' rould be titrated with :in accuracy of 0.5% or hetter. However, with 0.001 S chloride solutions they found results which were 5 to 9% low antl in 0.0005 .Y solutions almut 15% low. A considerable improvement was ohtained in these dilute chloride solutions when the medium was composed of a 50-50 inisture of water and acetone. A great practical advantage of the reaction between chloride
and silver is that piecipitation equilibrium is attained very rapidly and hardly :my tendrnrv for the formation of supersaturated solutions is notired during the titration. This is true even in the piesence of small aiiiounts of gelatin. Therefore, on the baris of the solubilitv product, satisfactory results should be obtained in aqueous nirtl~uiii,even a t chloride concentrations as small as 0.0001 S,proyided that the reagent (excess silver) line is not drawn until aftri the addition of such an excess of silver that the solubility of silvei chloride becomes negligibly small. If this precaution is not considered, the reagent line has the arong dope and lon rwults are found, as reported by Laitinen et 01.
As an illustration, Ict u s con~idcrtitration curve I ohtained in the titration at rooin temperature of 0.0001 N chloride in 0.1 N potassium nitrate Qolution containing 0.02% gelatin with 0.01 N silver nitrate (Figure 1 ).
1307
V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1
end point would be found 10 to 15% low, and the reproducibility would be poor. It is clear that the lowest conceritration of chloride, which can be titrated amperometrically, i i determined by the solubility of silver chloride. This solubility decreases very much n-ith decreasing temperature, as is shown in Table I. The data in Table I were obtained by graphical iritc~rl)olationof the values given by Owen ( 4 ) .
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Table I.
So1ubilit)-Product of Silver Chloride a t Different Ten1perat ures
8
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Ternp.. (1. Solubility riroiliirt X 10'0
5 6
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0.26
13 0.71
25
1.78
32
4.14
45 8 '37
4
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0.6 1.0 1.4 1.8 10 - 2 N SILVER NITRATE, ML.
0.P
9.4
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Figure 1. Titration with 0.01 X Silver Nitrate of 0.0001 N Potassium Chloride in 0.1 N Potassium Nitrate hlixture 0.02% i n gelatin 28' C. I. Experimental l i n e 11. Calculated titration line
Thus, i i i the titration of very small concentrations of chloride, ~nucahI)ettn titration line^ ma!- be eypected a t temperatures in the nriglhorhood of 0' C. th:in a t room temperature. This is actually stion-n to be true in the present papcr. The titration of chloride a t sniall concentrations was investigated a t various temperatures, a t varying concmtrations of gelatin, and in the presence of different electrolytes. EXPERIMEYTAL
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The apparatus used was similar to that described by Laitinen et a l . ( 2 ) , except that a potassium nitrate-agar qalt bridge was w e d for connection of the electrolysis cell with the outside saturated calomel electrode The electrolyte for the salt bridge was l4
1
13 -
1P 11
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Figure 2. Titration in 0.1 ,T Potassium Nitrate Containing 0.029" Gelatin a t 27' C. I. 0.OOOS
11.
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chloride
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0.001 Ichloride 1
L-sing a solubility product of 1.8 X 10-10 for silver chloritlt., theoretical titration curve i i calculated which is represented by curve 2 in Figure 1. The experimental and theoretical curves coincide when the ewes< of silver is about 80 to 100% In order to get good results, the reagent line should be drawn through points corresponding to an escess of silver greater than 80 to 100%. If the reagent Iinc were drawn through point< corresponding to an excess of silver of tietween 20 and 70%, the
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0 1 1 IO-*
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3 4 5 SILVER NITRATE, ML.
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Figure 3. Mixture 0.0005 4in Chloride, 0.01 V i n Barium Nitrate, and 0.005% in Gelatin I.
11.
Titrated a t 30' (:. Titrated at 8' C.
Amperometric titrations with the rotated platinum electrode as indicator electrode give rapid and accurate results even a t high dilutions. With very simple equipment chloride can be satisfactorily titrated with sil\er nitrate a t high dilutions, if t h e reagent line is drawn after enough excess of silver has been added to suppress almost completely t h e solubility of silver chloride. Oxygen need not be removed unless t h e solution contains free acid. The solubility of silver chloride decreases markedly with decreasing temperature. For t h e titration of lo-' to 5 X Nchloride solutions it is recommended t h a t t h e titration be carried out close to ice temperature.
ANALYTICAL CHEMISTRY
1308 prepared by heating 30 grams of potassium nitrate, and 3 grams of agar with 100 ml. of water until the solution was clear and free of air bubbles, After each titration the rotating platinum electrode was cleaned with nitric acid, and washed with an ammoniacal sulfite solution and finally with water. A microammeter (30 pa. range; Weston Electrical Gorp.) was used and the current was read with an accuracy of 0.05 pa.
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Even 0.0005 and 0.0001 S chloride solutions could be titrated a t room temperature with an accuracy of 1 to 2% in the presence of neutral electrolytes. The presence of free mineral acids caused some interference because of a relatively large residual current in the solution before addition of silver nitrate. For example, with solutions that were 0,0001 AVin chloride, 0.1 A- in nitric acid, and 0.02% in gelatin, the residual current a t 23" C. was found to vary between 6 and 20 pa. This current appeared to be mainly due to a reduction of oxygen. When air was removed Iyith nitrogen, the residual current decreased to less than 0.2 pa. after 10 minutes. In 0.1 S sulfuric acid instead of 0.1 S nitric acid similar observations were made. I f t e r removal of oxygen the residual current a t the beginning of the titration decreased to 0.2 pa. Solutions which were 0.001 to 0,0001 S in chloride and 0.1 to 0.5 S in nitric acid or 0.1 n sulfuric acid were titrated successfully in nitiogen ( +C 1% accuracy), the silver nitrate not being added until the residual current had decreased to 0.2 pa. 01 less. Sitrogen was pasqed through during the titration except dui ing the measurement.
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Figure 4. Titration of 0.0001 :V Chloride in 0.01!N Barium Nitrate
0.6 1 .o 10 - 2 N SILVER NITRATE, ML.
0.P
Solution 0.02% in gelatin I. A t 2 2 O C. 11. A t 5OC. 111. A t 0' C.
1.4
Figure 5. Titration at 0 " C. of 0.00005 N Chloride
Chloride-free C.P. reagents and conductivity water were used. The chloride content of the gelatin was determined potentiometrically after ashing M ith sodium carbonate and titrating the acidified solution to the equivalence point ( I ) . The chloride content was found to be 0.02'%. In most of the experiments the concentration of gelatin was 0.02%. The chloride introduced with this amount of gelatin corresponds to 0.01 ml. of 0.01 S silver nitrate per 100 ml., which was used as a correction. Unless otherwise stated, the experiments were carried out in air-saturated solutions. EXPERIMEKTAL R E S U L T S AYD DISCUSSIOY
Examples of titration lines of 0.001, 0.0005, and 0.0001 aY chloride a t room temperature are given in Figures 1 and 2. The results in the titration of 0.001 S chloride in the presence of various electrolytes are given in Table 11; the titration is precise to 1%. The end point is taken as the point of intersection of the properly constructed reagent line n-ith the value of the residual current of the solution before addition of reagent.
Solution 0.02 % in gelatin I. I n 0.01 IV KNOB 111. I n 0.01 N Ba(MOd 11. In 0.1 N HNOa IV. I n 0.1 N KNOl
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Table 11. Titration of 100 Ml. of 0.001 1. Chloride (0.029% in Gelatin) with 0.01 N Silver Yitrate at 26-27' C. Electrolyte Present 0.1 N K N O I 0 1V . KiSOa 0 1 N MgSO, 0 05 ,V Ba(NOs)t
AgSOa a t E n d Point. bZ1. 10.00 i. 0 1 10 00 i 0 1 10 05 0 1 10 00 * o 1
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1.0 1.4 1.8 2.2 N SILVER NITRATE, ML.
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Figure 6. Effect of Gelatin on Titration of 0.0001 V Chloride in 0.1 N Potassium Nitrate at 27' C. Concentration of gelatin I. 0.001% IV. 0.05% 11. 0.01% v. 0.10% 111. 0.02% VI. 0.15%
V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1 High residual currents were also obtained a t 0" C. in the presence of mineral acids. Upon removal of oxygen with nitrogen, these currents decreased to less than 0.1 l a . Titration at Low Temperatures. That the titration of very dilute chloride solutions should be more accurate and precise a t low temperatures than a t room temperature is illustrated in Figures 3, 4,and 5 . 18 16
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Figure 7. Titration of 0.0001 'V Chloride in 0.1 .V Potassium Nitrate I.
Without gelatin; at 4 gelatin added to 0.02% concentration
11. O.OZ70 gelatin present from beginning of titration I l l . Calculated titration line in absence of gelatin
1309 chloride in 0.01 J barium nitrate containing 0.02% gelatin was calculated a t different temperatures. The following values were found: LS.+~CI X 1010: 0.3 a t O", 0.5 a t 5", 4 to 5 a t 28". Effect of Concentration of Gelatin. The diffusion current of silver ions in neutral medium decreases with increasing concentration of gelatin. This is clearly seen from the slope of the excess of reagent lines in Figure 6. Thus, the accuracy and precision decrease with increasing concentrations of gelatin. In general, a concentration of 0.02oj, of gelatin is satisfactory in the titration of 0.001 S or more dilute chloride solutions. At room temperature the solubility of silver chloride calculated from the current a t the equivalence point in the presence of 0.02% (to 0.05%) gelatin is of the order of 50 to 80% greater than that calculated from the solubility product a t the same temperature in the absence of gelatin (see Figure 7 ) . Apparently, the difference between the two values is not due to a supersaturation effect caused by the gelatin, because the current a t the equivalence point remained unchanged even after a few days of standing. If the gelatin was addcd a t the equivalence point (Figure 7 ) , the current became practically equal to the value calculated from the solubilit,y product. On the other hand, when the gelatin was added when 80% of the chloride had been titrated, the current a t the equivalence point was about the same as when the gelatin had been present from the very beginning of the titration. I t seems that either the extremely small colloidal particles of silver chloridc formed in the presence of gelatin have a greater solubility than coarse silver chloride or the colloidal silver chloride has a slight depolarizing effect on the electrode. ACKVOW LEUGMEYT
.St 0" and 5' C . a 0,0001 chloride solution in 0.01 to 0.1 LV nitrate solution (0.02% gelatin) could be titrated n-ith an accuracy and precision of 1 to 2% when the excess of reagent line was drann aftcr 25% excess of silver had been added. The nitric acid when titration gave equall\- good results in 0.1 varried out in a nitrogen atmosphere. In neutral medium in t'he presencc of air, a 0.00005 S chloride solution could be titrated a t 0" C. with an accuracj- and precision of 3Oj, (Figure ti), when the reagent line was dravm after more than 50% excess of silver had been added. The exact temperature is immaterial, as long as it is below approximately 5' C. From wrious tit ration lines the solubility product of silver
Ackno\i ledgnient is made to the Graduate School 01 the university for a grant which enabled the authors to carry out this work. L I T E R i T U R E CITED
( I ) Kolthofi, I. If.,arid Kuroda, P. K., ANAL. CHEM.,23, 1.304 (1961). ( 2 ) Laitinen, H d.,Jennings, W. P., and Parks, T. D., IND.ENG. CHEM.,A s ~ L .ED., 18,355 (1946). (3) Laitinen, H. .1.,and Kolthoff, I. M., J . Phys. Chem., 45, 1079 (1941). (4) Owen, B. B., J . Ani. Chcm. Soc., 60, 2229 (1938). RECEIT L D February 19. 1951.
Rapid Method for Determination of Betaine €1. G. W..ILKER, J R . , 4 N D ROBERTA ERLCVDSEY Western Regional Research Laboratory, Albany, Calif.
T H.4S been known for many years that betaine [carboxy-
I methyl trimethyl ammonium salt] constitutes one of the principal noncarbohydrate impurities in sugar-beet processing
liquors, and recent interest in feed-supplemrnt use and by-product recovery has made the estimation of this compound of especial interest to the sugar beet industry. At present, no Association of Official Agricultural Chemists ( 1 ) method is available for betaine, and the present practice (4,5,8) follows essentially the procedure first described by Stanek (9), which involves precipitation of the betaine with potasRium triiodide (periodide method), followed by titration with thiosulfate or determination of nitrogen by the Kjeldahl method. The method is subject to serious error, herause other naturally occurring nitrogenous substances are also
precipitated as complex iodides, especially in acid medium. Thus the preliminary removal of interfering materials, including sucrose, makes the method laborious. Phosphotungstic arid as a precipitant followed by Kjeldahl analysis has been used (6), but the method is nonspecific and somewhat time-consuming. Strack and Schwaneberg (10) suggested t h a t compounds resembling betaine could be determined gravimetrically as betaine reineckates in acid solution, but gave no quantitative analytical data for the determination of betaine itself. The use of Reinecke salt for the determination of choline (3, 11) and other substituted amino compounds ( 2 ) is well established. I n the authors' experience, however, the gravimetric or colorimetric determination of betaine as the reinmkate has not been successful because no
