300
ANALYTICAL CHEMISTRY
(98) Pollard, F.. IIcOrmie, J., and Elbeih, I., J . C h r m . Soc., 1951, 466-74, 771-4. (99) Polster, 11..Ch,ena. L i s t y , 43, 228-9 (1949). (100) Poraes, S . ,Pepensks, J., Hendler. S . . and Hoover. S.. Sewaae u?;d I n d . E7&tes, 22, 318-25 (1950). (101) I'iihil, J . , and HornychovL. Chem. L i s t y , 44, 101-3 (1950). (102) Piihil. J ., and KluhalovL, Collection Czechosloc. Chenz.Cornmuns.. 15, 42-51 (1950). (103) Piibil, J.. and LIalLt, I b i d . . 15, 120-31 (1950). (104) Piibil. J..and Svestka, I b i d . , 15, 31-41 (1950). (105) Prodinper. It.., and Kral, H., 2. aual. Che?n.. 133, 100-3 (1951). (106) Quevedo. E.. Ret'. ohrcts. s O n i t . nacion, 14, 99-106 (1950). (107) Kayner, ,J., aiid Logie, D.. J . Soc. CiLem. Z n d , , 69, 309-12 (1950). (108) Reitz, L.. O'Hrien, -4..and Davis, T.. AXAL.CHEM.,22, 1470 (1950). (109) Rliame, G.. FTater a n d Sewage Works, 97, S o . 8. 344-5 (1950). (110) Rolfe, A, Russell, F., and ITilkinsoti. S..J . Applied Chern. (Loridon). 1, 170-8 (1951). (111) Roaenbei,g, L., Mllikrobiologiya, 19, 410-17 (1950). (112) Sakaguchi, T . . J . Pharni. SOC.J a p a n , 62, 404-14 (1942). J., Cole, J . , Overholser, L., ;Irnistrong, A , , and Toe, I.. CHIN., 23, 603 (1951). (114) Fchi.enk, TI-..I b i d . , 22, 1202 (1950). (115) Schroedrr, H., Gesundh. Ing.. 71, 275-9 (1950). (116) Seaman, IT.,and Allen, IT.,Sewage and I n d . W-ustes, 22, 912-21 (1950). (1171 gedivek, I-.,and Vag&, V,,Collectio?a Ctcchosloc. Chew. Coin?~tu?!s.,15, 260--6 (1950).
(118) Shashkin, M.,Zaaodsknya Lab.. 16, 748 (1950). (1 19) Shawarbi, M.. Mikrochemie per. J l i k r o c h i m . Acta, 36/37, 366-9 (1951). (120) Simnrd, R.,Hasegawa, I.. Bandaruk, K., and Headington, C., ;Is.~L. CHEM.,23, 1384 (1951). (121) Pkopintsev, B., and Mikhailovskaya. Gi'dvokhim. Materialy, 14, 108-14 flR48). ~. ~. >- - --,
(122) $Jdes, -1..A I L ~ Y7S6 ,~3-26-55 . (1951). (123) Splittgerher, A,, Muller, K.. Ulrich. E., and Reling, E., Chem.Ing.-Tech., 22, 542-4 (1950). (124) Stone, H., and Eichelberger, R., SAL. CHEM.,23, 868 (1961). (125) Tarns, M.. Cisco, H., and Gariiell. 11...I.A m , Water Works d S S O C . , 42, 583-5 (1950). (126) Teicher, H., and Gordon, L., AKAL.CHEM.,23, 930 (1951). (127) Tompkins, E. ( t o United States of dnierica represented by Atomic Energy Commission), U. d. Patent 2,554,649 (May 29. 1951). (128) Tan Meter, I., and Gerkc, J., Sewoge und I n d . Wastes. 22, 508 (1950). (129) Vsnoasi, R.. Anales asoc. quina. argeriIirm, 36, 75-92 (1948). (130) Verbcstel, J., Berger, d.,and Royet,, I-,,B u l l . centre belge Qtude et document eauz, 8 , 494-500 ,1950). (131) Wells. I., Ax.41,.CHEM.,23, 511--14 (1951). (132) Wilberg, E., 2. anal. Chenr., 131, 405-9 (1950). .I. Chem. SOC..Joprtn. 71, 577-9 (1950). (133) Toshino. T.. (134) l o u t i g . I., and Hiskey, C., -1s.~~. CHEM.,23, 506 (1951). (135) I-oung. R.. Pinckney, E.. and Dick. R.. Po?.oer Eng., 54, S o . 7 , 62 (1950). R L C L I V EDwetnher D 3 , 1951.
[End of Review Section]
TITRATIONS IN NONAQUEOUS SOLUTIONS General and Round-Table Discussions Held by Division of Analytical Chemistry, 119th Meeting, AMERICAN CHEMICAL SOCIETY, Cleveland, Ohio, April 1951
Potentiometric Titration of Salts of Organic Bases in Acetic Acid Salts of Amines, Basic Heterocyclic Nitrogen Compounds, and Quaternary Ammonium Compounds CHARLES W. PIFER
AND
ERNEST G . WOLLISH
Products Control Laboratory, Hoffmann-La Roche, Inc., A-utley, K. J .
T
HE advantage of using perchloric acid for the titration 01 organic bases in glacial acetic acid was first demonstrated bq Conarit and Hall (2, 7 ) . Conant and Werner (3) and Hall (6, 8) determined the strength of organic bases in glacial acetic acid Kith perchloric acid. Hammett and Dietz (9) used formic acid as solvent, while La Mer and Don-nes (15) described conductometric. arid eipctrometric titrations in benzene. Perchloric acid q-as used for the visual titration of amino acids by Kadeau and Branchen ( I ? ) mith crystal violet as indicator. Blumrich and Bandel ( 1 ) applied perchloric acid titrations in glacial acetic acid for the determination of primary, secondary, and tertiary amines, aliphatic bases, and alkaloids. Fritz (4, 6) titrated many organic bases in nonaqueous solvents with perchloric acid in dioxane o r acetic acid. Palit (18) determined alkali metal salts of Keak monobasic organic acids in a mixture of solvents by perchloric acid titration. Titration of Salts of Organic Bases (except hydrochloric, hydrobromic, and hydroicdic acid salts). Plati and Ingbernian (20)reported that organic acid salts of organic bases could be titrated potentiometrically in glacial acetic acid n ith perchloric
aritl. AIarkunas and Riddick (16) investigat,ed the titration rarhosylic a,cid salts in glacial acetic acid and titrated bases m d salts of weak acids. T h e aut,hors found t,hat salts of organic bases with strong acids other than halide acids could also be titrated ~otentio~net.rically. Titration of Halide Acid Salts of Organic Bases. Halide arid salts could not be titrat,ed. [After t,his manuscript had been \\-ritten, two papers by Higuchi and Concha (10, 11) appeared, in which the visual titration of alkali chlorides and hydrochlorides Lvith perchloric acid is described; tmhereaction is driven to cornpietion I)? repeated boiling. IIiguchi and Concha also studied the tiehavior of some inorganic anions in glacial acetic acid. ] The authors' main objective was the development of a procedure to overconic= this obstacle, as most of t,he physiologically active conipc~und~ are prepared as their water-soluble halide acid salts and a method for their direct titration was highly desirable. They \yere able t o titrate halide acid salts of organic bases with perchloric acid by potent,iometric*or visual procedure after converting the salt to the acetate through the use of mercuric acetat e (if
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2 No method for potentiometric titration of halide acid salts of organic bases has been reported, and only limited information on titration of salts of other strong acids with organic bases has been available. The proposed method permits the direct titration of salts of organic bases and halide acids (hydrochloric, h >drobromic, and h>droiodic) when mercuric acetate is added to the solution of the sample in glacial acetic acid prior to the titration with perchloric acid in dioxane. This titration can be carried out potentiometrically or visually, using crystal violet as indicator. Salts of organic bases w-ith strong acids other than halide acids can be titrated without the addition of mercuric acetate. This rapid method has been applied to numerous alkaloids and several Kolthoff and Wi1lni:in ( I S ) had determined the dissociation constants of some inorganic acids, bases, and salts in glacial acet,ic acid and listed t8heinorganic acids in the folloi\-ing order of decreasing dissociation: HCIOl
> HBr > H$Oa > HCI > HKOa
IColthoff and Willman (14) also presented data concerning the basic strength of inorganic acetates and the strength of cations and anions in acetic acid. From these conductance nieasurements it was not,ed that mercuric acetate showed the lowest equivalence condurt,ivity of any salt tested. This observation suggested the idea of adding mercuric acetate to the solutions of halide acid salts of organic bases in glacial acetic acid prior to their tiwation with perchloric acid. It vias thought that the mercuric salt,s formed n-ould exhibit a low conductivity and low. dissociat,ion in accordance xvith Ilolthoff and Killman's postulxtions. The resulting experiments gave support to theae theoretical considerations. Mercuric salts were the only compounds tested that exliihitetl the combined qualities of low conductance and ability to bind halogen anions without affecting the potent,ionietrir titration of the liasic portion of the siilt, using perchloric itt.itl. C'ations of the same group as mercury? such as silver and lead i n t,lie form of aretnte5. were tested because t8hey also can bind the aiiions mentioned above. Hon-ever, silver and lead acet,ate irere found to coiis~nieperchloric acid. The halogens formed v i t l i silver and lead acer,at,e were only slightly soluble in glaicinl acetic acid, while the mercury halides n-ere soluble. Khen i i compuunti ~f k n o m purity was titrated with the substitution of a known quantity of lead acetate for nierc-uric acet,ate (in exceis of the molar ratio). the anmulit of perchloric acid consumed v,w equivalent to the quantity calculated for the organic salt, plus the excess of lead acetate added. This fact eliminated tlie practicability of using these cations, as it would be impossi1)le ts use them in excess of the molar ratio, whereas an exress of mercuric salts had 110 effect upon the potentiom&ic tit,ration. The reaction is believed to proceed in the following manner: Organic base
[gkl] HRr
Hac
-
excess Hac
acetate Of
Orgallic
Hg(CzHa02h
+ perchlorate of organic base
HClOt t-
in dioxane
voltage change METHOD
Apparatus. B e c h a n pH meter, Model G, Fisher Senior Titrimeter, or similar potentiometric titrator. Beckman glass electrode KO.1190-90. Beckman Calomel Electrode, Yo. 1170. Calomel electrodes
301 vitamins. Depending upon the sharpness of the potential break, a precision of 2k0.270 can be obtained for many compounds. In the pharmaceutical industry a large number of compounds are prepared as water-soluble halide acid salts. The basic part of such salts usually constitutes the physiologically active component, for which no direct titration method existed. The proposed method will determine this basic part of the molecule, permitting the rapid assay of many pharmaceutical preparations with elimination of tedious extractions and colorimetric procedures. While the accuracy of the method is comparable to that of conventional methods, its sensitivity permits titrations with 0.01 N perchloric acid in dioxane in numerous cases. were observed to be easily contaminated by mercuric acetate in glacial acetic acid, causing fluctuation of the galvanometer needle. If this condition occurs, the saturated potassium chloride solution inside the electrode should be removed, and the cell should be flushed with distilled water, rinsed with saturated potassium chloride solution, and refilled with saturated potassium chloride solution. Reagents. Perchloric acid, 0.1 X, in dioxane, is prepared by dissolving approximately 8.4 ml. of 70 to 72% perchloric acid in 1 liter of dioxane. This solution is standardized by titration against National Bureau of Standards potassium acid phthalate as described for the sample (16, 21 ). Glacial acetic acid, C.P. hIerciiric acetate reagent is made by dissolving 6 grams of C.P. mercuric acetate in 100 ml. of hot glacial acetic acid and cooling to room temperature. PROCEDURE
A4bample of appropriate size, to require a titration of approxi-
mately 30 ml. of 0.1 N perchloric acid in dioxane is weighed into a 250-ml. beaker. To calculate the approximate weight of sample, the following equation may be applied: Sample weight (grams)
=
30 X 0.1 X milliequivalent weight of compound
This calculation can be simplified by drawing a nomograph from the above formula, plotting milliequivalent weight against weight of sample As it is believed that the sharpness of the end point is dependent upon the electrical conductivity, dioxane being nonconductant, the ratio of acetic acid to dioxane in the standardization and the test should be kept fairly constant. For this purpose the authors generally used a uniform quantity of 80 ml. of glacial acetic acid (including the mercuric acetate solution) for solubilizing the sample. Potentiometric Titration. If the sample does not dissolve readily, the mixture may be heated to boiling. Under these conditions almost all finely powdered samples dissolved completely. If the sample is a salt of an organic base, other than a halide acid salt, the solution may be titrated after cooling to below 75' C. Ho\vever, if the sample is a hydrochloride, hydrobromide, or hydroiodide of an organic base, 10 ml. of mercuric acetate reagent should be added to the acetic acid solution after cooling to room temnerature. The solution is titrated with 0.1 N perchloric aciddioxane solution, using the millivolt scale of a potentiometric instrument. The millivolts, E, are recorded every 0.1 ml.in the vicinity of the calculated end point (Figure 1). The exact end point is determined by plotting a graph of AE/AV us. V (ml.). From the extrapolated curve the milliliters of 0.1 N perchloric acid consumed are obtained (Figure 2). Visual Titration. Various investigators (3,16, 17, 21) advocated the use of crystal violet as visual indicator for acid-base titratidns in glacial acetic acid. This indicator was found to be very suitable for routine titrations of compounds that give a sharp potentiometric break. Procedure. A sample of appropriate size is weighed and dissolved in glacial acetic acid as described under potentiometric titration. If heating is required for the complete solution of the sample, the solution must then be cooled to room temperature.
ANALYTICAL CHEMISTRY
302 In the case of salts of halide acids, 10 ml. of a mercuric acetate reagent are added a t this point, follomed by 0.5 ml. of the crystal violet indicator (0.1% in glacial acetic acid). The solution is then titrated with 0.1 N perchloric acid-dioxane, preferably under mechanical stirring. The correct shade of the end point must be previously determined by visual standardization against the primary standard and the color matched in the titration of the sample. DISCUSSION
Effect of 0.01 N Perchloric Acid as Titrant. As for most compounds the visual end point is not sufficiently sharp when 0.01 N perchloric acid is used, only potentiometric titrations should be conducted a t that concentration.
i
+ENDPOINT
BO-
Effect of Temperature. No noticeable effect on the potentiometric end point was observed when the titration was run either a t room temperature or in hot solutions. Most of the determinations were therefore carried out after cooling to below 75' C., but ~ i t h o u tcontrol of the temperature at a constant level. However, in the case of sensitive substances, such as thiamine hydrochloride, it is advantageous to dissolve the sample in warm glacial acetic acid, cool it to room temperature, and then add 10 ml. of cold mercuric acetate reagent, so as to avoid decomposition.
504Q-
Y L 0 . 1 N HCL04
Figure 2.
Titration of Prostigmine Bromide
Effect of Solvents for Perchloric Acid. In this Rork, perchloric acid in dioxane was used as the titrant. Ji7ith 0.1 S solutions, a titrant prepared with glacial acetic acid instead of dioxane will result in a slight decrease of sensitivity, which becomes very apparent when 0.01 S perchloric acid is used (Figure 3). A comparison of the ratio of dioxane to glacial acetic acid in titrations with 0.01 S perchloric acid is presented in Figure 1.
33
30
35
34
ML 0.1 N H C L O 4
Figure 1. Titration Curves 1. Papaverine hydrochloride 2. Morphine hydrochloride 3. Prostigmine bromide (neostigmine bromide)
Effect of Excess Mercuric Acetate. When a 0.1 Y perchloric acid solution was used as the titrant, quantities up to 3 grams of mercuric acetate could be used without deleterious effects (Table I). However, when small samples using 0.01 S perchloric acid are titrated, the quantity of mercuric acetate used should not exceed2moles per mole of the compound being titrated.
Table I. Effect of Mercuric Acetate upon Potentiometric Standardization of Perchloric Acid Sample Weight Gram
Mercuric Acetate Added Gram
HClOd Consumed
MI.
Calculated Korrnality
.v
Deviation from Mean
s
Anhydrous Sodium Carbonate 38.70 38.80 40.08
0.2024 0.2028 0,2094
0.09868 0.09862 0.09865 Ax-.
0.2041 0.2024 0.1998 0.2044 0,2020
0.3 0.6 0.6 0.6 3.0
.
Ax..
+0.00010 - 0.00009 -0.00008 -0.00001 fO.OOO1O
0.09864
Anhydrous Potassium Chloride 0.2265 0.2247 0.2267
0.3 0.6 1.2
0.09866 0.09864 0.09871
30.65 30.55 30.80
Av.
Potentiometric Titrations with 0.01 N Perchloric Acid 0 Indioxane 0 In glacial acetic acid A . Asterol.ZHC1 B . Pyridoxine.HC1 C. Thiamine. HCI D. Neostigmine bromide
0.09863 0.09874 0.09855 0.09886 0.09863 0.09874
39.00 38.75 38.25 39.10 38.60
+0.00003 - 0.00002 10.00000
Figure 3.
0,09867
-0.00001
- 0,00003 +o .00004
It is evident that the sharpness of the potentiometric break is enhanced with increased concentration of dioxane. This might well be due to the decrease in equivalence-conductance of the glacial acetic acid mixture, caused by the dilution with dioxane. When the total concentration of glacial acetic acid is decreased to less than about 15%, it becomes difficult to conduct a potentiometric titration, as the solution approaches the status of a nonelectrolyte. This effect of greater sensitivity and the ease of handling dioxane prompted its use as a diluent for perchloric acid. As the
303
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2 Table 11. Comparison of Results Obtained with Perchloric l c i d Dissolved in Dioxane and Acetic Acid Normality of Standardization Pot. Vis.
Solvent Dioxane Sample 1 Sample 2
0.1001 0,1001
0.0997 0.0997
Acetic acid Sample 3 Sample 4
0.0993 0.0993
0.0989 0.0989
Asterol Dihydrochloride, % Pot. Vis.
Av.
99.76 99.74 99.8
99.82 99.80 99.8
Av.
99.86 99.87 99.9
99.91 99.93 99.9
potentiometric titration. This prscedure cannot be applied to saits of primary or secondary amines, which would be acetylated and consequently could not be titrated with perchloric acid. Effect of Mercuric Salts. The presence of mercuric bromide, chloride, and iodide in these titrations caused no interference, as the electrical conductivity of mercuric salts is very low.
L
IO
3% In 303-
5%
J
coefficient of expansion of dioxane is greater than that of glacial acetic: acid, which in turn has a coefficient approximately five times that of water, temperature fluctuations tend to have a distinct effect upon the normality fact,or of the titrant. For this reason the perchloric acid titrant should be standardized daily and if considerable variations in temperature occur during the day, a correction for the change in volume should be applied. Results obtained by both potentiometric and visual titration of 2 - dimethylamino - 6 - ( p- diethylaminoethoxy)- benzothiazole (Asteiol) dihydrochloride, using 0.1 LV perchloric acid in dioxane, as well RS in glacial acetic acid, are reported in Table 11. From t#hescresults it is evident that the visual normality factors are slightly lower than those arrived a t by pot,entiometric titrat,ion. However, when t,hese potentiometric or visually determined normality factors are applied t.o potentiometric or visual titrations of t,he sample, t,he results obtained remain within the limit of t,he experimental error. Effect of Solvents in Pharmaceutical Preparations. Solvents commonly used in pharmaceutical preparations, such as et,hyl alcohol, isopropyl alcohol, glycerol, propylene glycol, and Carboa-as showed no interference with the titration. Effect of Water. The presence of water should be avoided, because it impairs t,he sharpness of the end point. .4s is evident from Figure 5, the presence of 1Yc water in a titration of 600 nig. of Asterol dihydrochloride in a total of 100 ml. of solvent caused a slight flatt,ening of the titration curve. Larger quantities of water completely interfered with a sharp end point. In order t,o obtain sharp potentiometric breaks, the amount of water present, should be limited t,o less than 1 yoper t,itration when 0.1 A\-pwohloric acid solutions are used. However, it was posrilile to eliminate as much as 10 ml. of water per sample titrated by adding an excess of acetic anhydride directly to the sample, hcating thc mixture to boiling for approximately 5 minutes, and then bringing the volume to 80 ml. with glacial acet,ic acid. .in cscess of acetic anhydride had no effect, upon the
2 1 -J
=400-
-
I
ML 0.1
Figure 5. Sample.
N
ncLo4
Effect of Water upon Potentiometric Titration Curve
600 m g . of Asterol dihydrochloride in 80 m l . of glacial
acetic acid
Effect of Cations, Almost all cations, other than mercuric compounds, will interfere with the titration, owing to their reaction with perchloric acid. Effect of Organic and Inorganic Acids. It was found that the reaction between the organic base and perchloric acid takes place stoichiometrically and is independent of the amount of free acid (either organic or inorganic) present. RESULTS
The method described was applied to a great number of salts of organic bases. Results obtained on salts of amines are listed in Table 111. Histamine and histidine salts were found to be difficult to solubilize. Gentle heating and prolonged stirring in glacial acetic acid were required to obtain a complete solution. The procedure is also applicable to many antihistaminic compounds.
Table 111. Amine Salts Extraction of Perchloric Acid Base a n d Titration Titration, Q" of Extract. % a Desoxyephedrine (,V,a-dimethylphenethylaniine) hydrochloride A .
100.0 100.1 99.3
99.5 99.8 99.4
P-Ditnethylamino-6-( @-diethylaminoethoxy)benaothiazole(Asterol)dihydrochloride .I 100.0 B 99.9 C 100.0
99 7 99 6 99.8
B
C
Histidine monohydrochloride 99.5 .. 99.8 .. C 99.5 .. Additional amine salts successfully titrated b y perchloric acid method: Guanidine hydrochloride l-p-Am~nobenzoyl-2,2-dlmethyl-3-diethylaminopropanol hydrochloride (Larocaine hydrochloride) @-Diethylaminoethylfluorene-9-carboxylate hydrochloride (Pavatrine) Phenylbiguanidine hydrochloride Quinacrine hydrochloride (Atabrine) Histamine dihydrochloride Hydroxylamine hydrochloride Amphetamine sulfate Ephedrine sulfate l-(3,4-Dimethylphenyl)-2-aniinopropane sulfate (Ro 2-1038-3) .Itropine nitrate Histamine phosphate ( 1 Anhydrous basis. -4 B
38.0
39
40
41
42
43
M L 0.01 N H C L 0 4
Figure 4. Effect of Ratio of Glacial Acetic Acid to Dioxane upon Potentiometric Titrations 0.01 .V pcrchloric acid
ANALYTICAL CHEMISTRY
304 Values found for salts of basic heterocyclic nitrogen compounds are presented in Table IFr. (As papaverine is too weak a base to be titrated in the conventional manner, no other comparable =say values for papaverine hydrochloride could be given.) I t wae observed that sulfates (Figure 6) titrated by this method consumed only 1 mol? of perchloric acid, contrary to the 2 equivalents expected. acid sulfates might be formed in these instances. A sample of quinine sulfate tested required 3 equivalents, as further proof of this fact.
Table V.
Vitamins
Perchloric Scid Titration, Yo*
Thiamine hydrochloride (vitamin BI) A B C D E F
100.1 99.1 99.4 99.4 100.2 100.2
Pyridoxine hydrochloride (vitamin Be) 4001
U.S.P. Thiochrome Assay, %a
A
100 0 100.2 100 0
B
C
99.1 99.0 99.0 Colorimetric Method ( 2 8 ) , % 101 . o 99.0 100.0
U.S.P. Method, % 99.7 99.8 100,2 100.0 C 100.0 99.9 Additional vitamin successfully titrated b y perchloric acid method: Xiacin (nicotinic acid) Anhydrous basis. Niacinamide (nicotinic acid amide) -4
B
bu I N I N E
SULFATE EQUIVALENTS
J3
Table VI.
Quaternary .4mmonium Salts
KHS04 (NO I NFLECTIONI
Perchloric Acid Titration, %"
700 I
4
Ilo
0
2'0
3'0
4 0
Figure 6.
Choline chloride
Sulfate Titrations with 0.1 N Perchloric Acid KzSOd
+ HAC
KHSOi
Xeostigniine bromide (Prostigmine bromide) A 99.5 B 99.6 C 99.9
50
ML 0.1 N HCL04
+ KAc
U.S.P. Method, %"
ii
100.0 100.0 99.9
B C
98.5 98.8 99.7 Precipitation as Reineckate (191, % 100.7 99.7 99.9
(3-Hydroxyphenyl)-ethyldimethyl ammonium chloride (Tensilon)
99.9 .. B 100.1 .. C 99.9 .. Additional quaternary ammonium salts successfully titrated by perchloric acid method: Cetylpyridiuni chloride Triphenyl tetrazolium chloride Tetraethyl ammonium bromide (3-Hydroxyphenyl)-ethyldimethylammonium bromide (Ro 2-3198-1) Dimethylcarbamate of (2-hydroxy-3-cyclohexylbenzyl)-methylpiperidinium bromide (Ro 2-0911) Choline dihydrogen citrate Choline dihydrogen tartrate Anhydrous basis.
A
Table IY. Heterocyclic Nitrogen Salts Perchloric Acid Titration, %" Papaverine hydrochloride
A B C
100.0 99.9 99.9
3-Pyridyl carbinol tartrate (Roniacol tartrate)
A B C
99.9 99.8 99.9
I
Colorimetric Method ( B S ) , % 99 100 100
Codeine phosphate (as anhydrous codeine base)
U.S.P. Method, % A 73.3 72.9 B 72.9 72.6 C 73.6 72.8 .4dditional heterocyclic nitrogen salts successfully titrated by perchloric acid method: Arecoline hydrobromide dZ-3-hydroxy-h'-methyl morphinan hydrobromide (Dronioran hydrobromide) Betaine hydrochloride Codeine hydrochloride Morphine hydrochloride dl-a-1,3-diniethyl-4-phenyl-4-propionoxypiperidine hydrochloride (Xisenti1 hydrochloride) Brucine sulfate Caffeine sulfate Morphine sulfate Tropic acid-2,2-dimethyl-3-diethylaminopropanolphosphate (Syntropan phosphate) 2-Methyl-9-phenyl-2,3,4,Q-tetrahydro-l-pyridindene tartrate (Thephorin tartrate) Anhydrous basis.
The method proved to be useful for the rapid titration of several vitamins containing basic heterocyclic nitrogen groups (Table V), for some of which only tedious fluorometric or colorimetric assay methods existed. Nicot.inic acid and nicotinamide were included in this table because they also could be titrated in the manner described, although, not being salts, they do not come particularly within the scope of this paper. Typical e.m.f. curves for this group of vitamins are plotted in Figure 7. Titration values obtained on quaternary ammonium salts
are compiled in Table VI. Perchlorate titration is the only method perfected at present for the determination of (&hydroxyphenyl)-ethyldimethylammoniunichloride (Table VI), a stable and chemically rather unreactive quaternary ammonium salt. ACCURACY AND PRECISION OF METHOD
The basic part of an organic salt, is most frequently found to be its physiologically active portion. -4s the perchloric acid titration Kill determine just this part, the accuracy of the results may be considered equivalent to that obt,ained by more cumbersome conventional methods, such as the extraction of the base from alkaline solution and titration of the solvent extract (Tables 111, II-, V, and VI). The precision obt,ainable with this method on compounds giving a sharp potential break is exemplified in Table VII. A dried sample of pyridoxine hydrochloride was titrated on five different days with perchloric acid. The results indicate a refor this compound. I n a similar experiproducibility of 3 ~ 0 . 2 % ment, conducted with anhydrous t,hiamine hydrochloride, a was obtained, using the Fisher Senior Titrimprecision of =kO.lyo eter. ADVANTAGES OF METHOD
The advantages of the method described over other procedures, used heretofore for the assay of many organic salts, are manifold.
305
V O L U M E 24, NO. 2, F E B R U A R Y 1 9 5 2 T h e method is rapid and eliminates tedious extractions and other complicated procedures. I t provides stoichiometric results without having to resort to standards, such as those used for colorimetric deteniiinations. I n some cases, such as for salts of s e a k bases or for unreactive quaternary ammonium wlts, it offers the only direct assay method known.
Table VIII.
Pharmaceutical Preparations
~___-
Perchloric .4cid Method Av. Pot. Vis. Mg. Mg. .TI@.
.
Sample
U.S.P. X e t h o d
?.I
g
.
Deviation dfg.
Neostigmine Bromide Tablets, 15 hlg. 1 9 3
+
1.5 6 16 4
1.5 16 4
S ~ n t r o p a nPhosphate Ampoules, 10 RIg. per RI1. Base Extraction and Titration Jig. 1
2 3 4 3
.4sterol Dihydrocliloride Ointment, 10% 10.3 10.2 10.2
10.2 10.3 10.4 10.2 10.2
1 7
3 4 3
10.2 10.3 10.3 10.3
10.3 10.4 10.3 10.3
10.2 10.4 10.3 10.2
-0.1 +0.1 0.0 0 0 +O. 1
.isterol Dihydrochloride Alcoholic Solution, 10% 1
2 3 4
ML 0.1 N H CLC4
Figure 7. 1. 2. 3.
Titration Curves of Vitamins
5
.
10.4 10.4 10.5 10.4 10.5
..
..
..
.. .. ..
..
.. ..
..
10.6 10.5 10.4 10.6 10.6
-0.2 -0.1
+o.
1 -0.2 -0.1
Pyridoxine hydrochloride (vitamin B6) Thiamine hydrochloride (vitamin BI) Niacinamide
Application to Assay of Pharmaceutical Preparations. The advantage of using potentiometric titration for pharmaceutical control had been pointed out by Waters, Berg, and 1,achinann ( 2 2 ) . The proposed method can be easily adapted for the assay of pharmaceutical preparations, such as tablets, ampoules, ointments, and some solutions. As cations, such as sodium, iiiagnesium, and calcium, a i l 1 cause interference, their prewice should be ewluded, n-lien thiq method is applied to pharm:iecutical preparations. Table VIT. Pyridoxine hydrochloride
2. 3. 4. 5.
Perchloric Acid Deviation Titration, %“ from Mean, %” 100.1 100.0 99.9 100.0 99.8
Mean
99.9
Mean
99.7
f0.2 +0.1 0.0 +o. 1 -0.1
1 2 3
?
Empirical Formula I(CnHzn+1)4As]+X[Aryl N=N ] -X [(C6H&Cr]+X[ (CnHzn+~)aPb 1 ‘X [C,HZ~+IPH~]+X[ ( C ~ H Z ~ + I ) Z+XPHZI [(C,Hzn+ 1)3PH1 +X [ (C,Hzn+ I ) ~ P +X ][(CnH?n+1)4Sb]+X[(C~HZ~+I)~SI+.~Y[RBS]z+S-[RzSOH]+N03-
Tin salts Dialkyltin salts Trialkyltin salts
Application for Molecular Weight Determination. The molecular weight equivalents of amines and basic heterocyclic nitrogen compounds have been successfully determined by titrating their purified oxalates or tartrates by the method described. The molecular weight of a n unknown base may be calculated hy applying the following formula after titrating its oxalate v, ith perchloric arid: K____ t . of sample (grams) x 1000 -_ nil. of HC‘IO, X normality
~
I n the case of certain tablets containing a halide acid salt of air organic base and cations derived from a lubricant, such as magnesium o r calcium stearate, interference due to these cations may be corrertetl for as follows:
Onium Salts Quaternary arsonium salts Diazonium salts Polyphenylchroniium salts Trialkyllead salts Phosphine and phosphonium salts Primary phosphine salts Secondary phosphine salts Tertiary phosphine salts Quaternary phosphonium salts Quaternary stibonium salts Sulfonium salts Sulfonium sulfides Sulfoxides
8.
4
Anhydrous basis
1.
6. 7.
Reproducibility of Results
1 2 3
Thiaininr hydrochloride
and sulfonium salts. It is believed that the following onium salts would react in a similar manner:
90 X No. of equivalent. = mol. n-t. of unknoivn base
ACKNOWLEDGMENT
After the sample is dissolved in warm glacial acetic acid and cooled to room temperature, the solution is titrated I\ ith perchloric acid in dioxane (blank titration, due to the cations in the tablet mass). Mercuric acetate reagent is then added and the titration is continued to the final end point. The value of the blank titration is subtracted from the total volume required.
T h e authors wish to express their appreciation to E. G. E Snaier for his helpful suggestions, to J. T. Plati and A. K. Ingberman for their kind communication concerning the use of perchloric acid, and to Charles h a c k for his aid in carrying out iome of the work.
Results obtained o n tal)lets, anipoules, ointments, and alcoholic solutions are reported in Table VIII. Application to ‘‘Onium” Salts. The method was successfully applied t o salts of onium-type compounds, such as diazonium
(1) Blumrich. K., and Bandel. G., Angew. Chem., 54, 374 (1941). (2) Conant, J. B., and Hall, S . F.. J . A m . Chem. Soc., 49, 3062 (1927).
LITERATURE CITED
ANALYTICAL CHEMISTRY
306 (3) Conant, J. B . , and Werner, T. H.. Ibid., 52, 4436 (1930). (4) Fritz, J. S., ANAL.C H E M .22, , 578 (1950). (5) Ibzd., 22, 1028 (1950). (6) Hall, K. F., J . Am. Chem. Soc., 52, 5115 (1930). (7) Hall, N. F., and Conant, J. B., Ibid., 4 9 , 3047 (1927). (8) Hall. N. F., and Werner, T. H., Ibid., 50, 2367 (1928). (9) Hammett, L. P., and Dietz, N., Jr., Ibid., 52, 4795 (1930). (10) Higuchi, T.. and Concha. J., J . Am. Pharm. Assoc., Sei. E d . , 40, 173 (1951). (11) Higuchi, T., and Concha, J., Science, 113, 210 (1951). 112) Hochbern. &Melnick. I.. D.. and Oser. B. L.. J . Bzol. Chrm.. 155, 109 (1944). (13) Kolthoff, J. A t . , and Willman, A , , J . Am. Cheni. Soc., 56, 1007 (1934). (14) Koithoff, J. M., and Willman, A . , I b i d . , 56, 1014 (1934). (15) La Mer, V. K., and Downes, H. C., I b i d . , 53, 888 (1931).
hfarkunas, P. C . , and Riddick, J. 4.,ANAL. CHEY., 23, 337 (1951).
Kadeau, G. F., and Branchen, L. E., J . Am. Chem. SOC.,57, 1363 (1935).
Palit, S. R . , IND.ENG.C H E X , ANAL.ED.,18, 246 (1946). Pankratz, R. E., and Bandelin, F. J., J . Am. Pharm. Assoc., Sci. E d . , 39, 238 (1950).
Plati, J. T., and Ingberman, A. K., private communication. Seaman. W.. and Allen. E.. ANAL.CHEM..23. 592 11951 ). Wateis, K. L., Berg, -1.L., and Lachmann, R. G . , J . A m . Pharm. Assoc., Sci. Ed., 38, 14 (1949).
Wollish, E. G., Kuhnis, G. P., and Price, R. T., ANAL.CHEM.. 21, 1412 (1949). RECEIYED December 8, 1950. Presented at the Meeting-in-Miniature of the Nortli Jersey Section, . ~ V E R I C A X C H E M I C ASOCIETY. L Newark. N. J., January 8, 11351.
Titration of Certain Salts as Acids in Nonaqueous Solvents JAMES S. FRITZ', Wayne University, Detroit 1 , Mich. No existing method is entirely satisfactory for titration of acid bound to moderately strong hases. Salts of ammonia, aliphatic amines, and most other organic bases may be conveniently titrated as acids in ethylenediamine or dimeth>lformamidesolution. Sodium methoxide in benzene-methanol serves as the titrant and either thymol blue orp-nitrobenzeneazoresorcinol (azo violet) is used as the visual indicator, depending on the solvent chosen. Titration of aqueous samples may be effected by first adding 15 to 20 ml. of ethylenediamine to a 1-ml. sample. The method should be particularly useful for the titration of ammonium and aliphatic amine salts.
T
HE mineral acid salts of most aromatic amines and other ueak bases can be titrated in aqueous solution uith sodium hydroxide. Difficulty is experienced, however, in titrating the salts of stronger base.-, such as ammonia and aliphatic amines. Titration in the presence of neutral formaldehyde (6) or alcohol ( 2 , 7 , 1 2 ) often permits the determination of such salts, but these modifications are not universally successful. Acid-base titrations in nonaqueous media are rapid, accurate, convenient, and very broad in scope. Several papers have dealt with the titration of weak bases in nonaqueous solvents (1, 3, 6 , 10, 11); others with the titration of weak acids in ethylenediamine ( 9 ) , butylamine, and benzene-methanol (4). Afarkunas and Riddick (8) titrated certain carboxylic acid salts as bases in glacial acetic acid solution. It is now proposed to determine certain salts by titration as acids in nonaqueous solvents. METHOD
The salt is dissolved in an appropriate solvent and titrated with a 0.1 -V solution of sodium methoxide in benzene-methanol. Either thymol blue or p-nitrobenzeneazoresorcinol (azo violet) is used as the indicator, depending on the solvent chosen. Almost without exception, the end points obtained are sharp and vivid. REAGENTS AND SOLUTIONS
Benzoic acid, primary standard grade. Dimethylformamide (DRIF), technical grade (Du Pont). Ethylenediamine, 95 to 1007, as purchased commercially. Salts were mostly commercial samples (99 to 1007, purity), analyzed as received. The amine perchlorates were prepared by dissolving the amine base in ether and adding a solution of 727, perchloric acid in ether to precipitate the salt. The precipitate was then washed three times with ether and air dried. 1
Present address, Iowa State College, Ames, Iowa.
Benzene-methanol, Four volumes of benzene were mixed with 1 volume of commercial absolute methanol. p-Nitrobenzeneazoresorcinol was made by thoroughly mixing 0.2 gram with 100 ml. of benzene, warming, and filtering. Sodium methoxide in benzene-methanol. A 0.1 solution was prepared as described by Fritz and Lisicki (4). This solution is clear and essentially colorless, but sometimes separates into two layers on long standing. This may be corrected by the addition of a small amount of methanol. Thymol blue, 0.3 gram of thymol blue dissolved in 100 ml. of methanol. PROCEDURE
Most of the solvents employed contain acid impurities and should therefore be neutralized shortly before use. This is accomplished by adding enough indicator to impart a definite color to the solvent, then titrating with 0.1 S sodium methovide to a clear blue end point. The sample to be titrated is weighed accurately into a 50-ml. beaker or flask, 15 to 20 ml. of neutralized solvent are added, and the solution is titrated ~ i t 0.1 h S sodium methoxide to a clear blue color. During the titration the beaker should be covered to exclude carbon dioxide; mixing is best accomplished with a magnetic stirrer. The titrant is added from a 10-ml. buret which can be read accurately to 0.01 ml.; the sample weight is chosen so that 4 to 9 ml. of titrant will be required. The sodium methoxide is standardized against pure benzoic acid using the above procedur methoxide should be checkcad eve SCOPE
Salts of the type B.HA can be titrated if the strength of the basic constituent, B, is not too great and if the acid, HA, is not too weak. Mineral arid salts of ammonia and aliphatic amines as well as aromatic amines gave very sharp end points in every case tried. Guanidine hydrochloride behaves as an acid, but the end point is very poor because of the strong base (guanidine) liberated during the titration. Trimethylphenylammonium iodide failed to react acidic to azo violet in any of the solvents tried. The strength of the acidic constituent of the salt may be fairly weak and still permit accurate titration-for example, ammonium benzoate gives as sharp an end point as ammonium chloride or ammonium sulfate. Aqueous solutions can usually be titrated if the salt concentration is such that a sample of small volume can be used. Ethylenediamine is added in a volume ratio to water of at least 15 to 1 and the resulting solution is then titrated directly. A few salts such as carbonates, oxalates, and phosphates precipitate when the ethylenediamine is added and cannot be titrated b y this procedure. Data for the titration of representative salts are given in Table I.
