963
V O L U M E 21, N O . 8, A U G U S T 1 9 4 9 Wait ( I O ) did not report any difference in the point of maximum absorption for natural vitamin A esters when dissolved in cyclohexane from that observed n-hen this form of the vitamin was dissolved in isopropyl alcohol or in ethanol. Awapara et al. ( 1 ) have reported that vitamin .1 alcohol, when dissolved in benzene, exhibits a maximum absorption a t 322 mp. Rawlings and Wait, (10) noted differences in the magnitude of the E:Fl,l,maxima of solutions of the vitamin A in ethanol and in isopropyl alcohol similar to t,hosc shoxn in Table 11. They also noted that the EtFm.-maxima of a vitamin h ester concentrate decreased in the following order of the solvents used: ethanol, isopropyl alcohol, and cyc1ohr:xanc. LITERATURE CITED
(1)
Gray, E. LeB., and Cawle), J. D., J . AVutrition,23, 301 (1942). Guerrant, N. B., Chiloote, X I . E., Ellenberger, H. ..\.. and Dutcher, R. A., ANAL.CHEM.,20,465 (1948). Kreider, H. R., 1x1.ENG.CHEM.,ABAL.ED.,17, 694 (19451. Morgareidge, K., Ibid., 14, 700 (1942). Morton, R. A., and Stubbs, A. L., Analyst, 71,.348 (1946). Morton, R. A,, and Stubbs, A . L., Biochem. J . , 42, 195 (1948). Oser, B. L., Melnick, D., and Pader, M.,ISD.EN:. CHEM., ASAL.ED., 15, 717 (1943).
Rawlings, H. W., and Wait, G. H., Oil & Soap, 23,83 (194tjI. Taylor, R. J., .Vatwe, 149, 474 (1942). V. S. Pharmacopoeia Vitamin Advisory Board, Letter 172, 11. 3
4
I1 947). ~ -.,..
Vandenbelt, J. M.,Forsyth, J . , and Garrett, A . , ISII. EX(;. CHEM., ANAL. ED.,17, 235 (1945). IVilkie. J. B., 0. S.P. Vitamin ;Idvisory Board, Li,flPr 164,p . 3% (1946)
16, 1948. dutliorized for publication . i u g u > t PO, 1948, as Paper 1463 in the Journal Series, Pennsylvania Agricultural Experiment Station. Financed in part b y a grant from The Sutrition Foundation, Inc. ~ ~ L C K I Y E DSeptember
Awapara, J., Mattson, F. H., Mehl, J. W.,and Deuel, If. J., Science, 104, 602 (1946). E.D., IND. ENG.CHEM.,d n - a ~ED., . 13, 144 (1941).
(2) Embree,
Amperometric Determination of Primary and Tertiary Mercaptans in Their Mixtures By lodometric Combined with Argentometric Titrations 1. 111. TiOLTHOFF AND .W. E. HARRIS’, L‘nivsrsity of Minnesota, llfMinneapolin,Minn. All mercaptans upon aniperometric titration in ammoniacal niedium with silver nitrate react i n a molar ratio of 1 to 1. Upon titration with iodine a primarj mercaptan reacts with 0.5 mole of iodine, whereas a tertiarj mercaptan reacts with 1 mole of iodine. The titrations can be carried out amperometricall?, using a rotating platinum electrode as indirator electrode and a saturated calomel electrode as reference electrode. When a mixture of a prim a r j and tertiar? mercaptan is titrated iodonietricall?, the amount of iodine used is much less than t h a t calculated because part of the tertiarj mercaptan R‘SH reacts with the formation of a mixed (tert.) R’SH 11 4 disulfide: (prim.) RSH RSSR ’ 2HI. Transformation of t h e mercaptans into slightly dissociated mercaptides, such as lead
+
+
+
T
H E appiicatioii ( i f t h r i,ut;iti~igplatinum electrode as indicator elrrti,otlt. i n the amperometric titration of mercaptans (thiols) \vith sil\-c.i, iiitrate has been reported (b). Vhen a pi,imary, secontlwy, or tertiary mercaptan is titrated amperoi n :immoniacal solution with silver nitrate, 1 mole in(~trira11~o f ~silvc~i~ ir used for each mole of mercaptan present. Hence, i i 1 :I mixture of primary and tertiary mercaptans no distinction t w t w r n the two can be made by amperometric titration with silver nitrate. Thc reactions can he represented by the following equations : (prim.) RSH (tert.) R’SH
+ Ag(XHO2++
+ Ag(jSHr)2+ --+
+ SH,+ + S H 3 R’SAg + S H l f + S I I s RS Ag
(1)
(2)
where RSH represents a mercaptan and (prim.) and (tert.) indicate that the -SH group is attached to a primary and tertiary carbon atom, respectively. 1
Present addres-. Department of Chemistry, Vnirersity of Alberta,
k:dmnnton, Alberta. Canada.
mercaptide, prevents this reaction from taking place. When mixtures of primary and tertiary mercaptans are titrated iodometrically in t h e presence of a n excess of lead nitrate or perchlorate, the priniarj and tertiary mercaptans react, giving the disulfide and t h e sulfenjl compound, respectivelj The composition of a mixture of a primary and tertiarj mercaptan is found bj an argentometric amperometric titration combined with a n iodometric amperometric titration in the presence of lead salt. Best results are obtained when mercaptan solutions are dilute and cold. Concentrated mercaptan solutions favor partial disulfide formation by t h e tertiary mercaptan. The method is of limited accuracy because it is indirect and some tertiarj mercaptans react incompletely with iodine.
.
Primary mercaptans are readily oxidized by iodin? (lisulfide. (prim.) 2RSH I, +RSSR 2HI
+
+
to
the
(3)
An iodonietric method of analysis, based on this reaction, has been worked out by Kimball, Kramer, and Reid ( 1 ) . In the analpis an excess of iodine is added to the mercaptan and then back-titrated with sodium thiosulfate. ;\lore recently Rheinboldt (4)has shown that tertiary mercaptan.? react with iodine to form sulfenyl iodides. (tert.) R’SH
+ Iy
--f
II’SI
+ HI
(4)
A method for the iodometric determination of tertiary mercaptans analogous to the method of Kimball, Kramer, and Reid ( 1 ) for primary mercaptans has been described by Tyler and Brown ( 5 ) . Accordi1.g to Equation 3, when a primary mercaptan is titrated with iodine the equivalence point corresponds to a consumption of 0.5 mole of iodine per mole of mercaptan. On the other hand, according to Equation 4 a tertiary mercaptari consumes I mole of iodine per molp of mercaptan.
964
ANALYTICAL CHEMISTRY
In the prestwt iiivestigation it has been found that mercaptans can be titrated directly amperometrically with standard iodine solutions, using a rotating platinum electrode as indicator electrode and a saturated calomel electrode as reference electrode. Experimentally it was found that when a primary mercaptan is titrated amperometrically with iodine the end point corresponds to a consumption of 0.5 mole of iodine, per mole of mercaptan and with dilute solutions of tertiary mercaptans 1 mole of iodine is used a t the end point. However, when a mixture of primary and tertiary mercaptans is titrated, much less than the calculated amount of iodine is consumed. Undoubtedly, this is due to the formation of a mixed disulfide by the following reaction: (prim.) RSH
+ (tert.) R’8H + 1%
---f
RSSR’
+ 2HI
(5)
Tertiary mercaptan which participates in a reaction of this nature reacts with only one instead of two atoms of iodine. Transformation of the free mercaptan groups into insoluble or slightly dissociated mercaptides impedes this undesirable reaction. Lead ion can be used for this purpose, because it forms a slightly dissociated or complex lead mercaptide. When mixtures of primary and tertiary mercaptans are titrated with iodine under the proper conditions in the presence of lead nitrate or perchlorate, the mercaptans in the mixture react according to Equations 3 and 4. In the iodometric amperometric titration the current remains zero or nearly zero until the end point is reached, then increases rapidly. Plotting the galvanometer or microammeter deflection against volume of reagent added yields two straight lines whose point of intersection near or on the abscissa gives the location of the end point. APPARATUS AND MATERIALS
The apparatus used for the iodometric amperometric titration is the same as for the argentometric amperometric titration (2). The reference cell, F , of Figure 1 in that article is changed to a saturated, calomel electrode for the iodometric titration in place of the mercury-mercuric iodide reference electrode used in the argentometric titrations. Axproximately 0.002 to 0.005 N alcoholic iodine solution was ma e by dissolving solid iodine in alcohol. This iodine solution is standardized by titrating amperometrically against 10 to 20 ml. of standard 0.01 N arsenic trioxide solution added to 100 ml. of water containing approximately 1gram of sodium bicarbonate. Because dilute alcoholic iodine solutions are not stable over long periods of time, they need to be standardized every day or two. Standard 0.005 N silver nitrate solution was made by weighing the correct amount of pure dry silver nitrate and dissolving in a known volume of water. Lead nitrate (1 M ) and perchloric acid (1.0 M ) were made up as aqueous solutions. PROCEDURE
Procedure A. Take an aliquot portion of a solution containing 1to 40 mg. of mercaptan and titrate with standard silver nitrate solution amperometrically as described in ( 2 ) . Procedure B. Dilute a second aliquot portion of the solution containing not more than about 5 mg. of mercaptan to about 100 ml. with 95% ethanol. To the solution add 1 ml. of 1 211 lead nitrate or perchlorate solution and enough 1 M perchloric acid to make the solution 0.01 M in perchloric acid. Titrate amperometrically in the cold with standard 0.005 S iodine, wing a rotating platinum electrode short-circuited with the saturated calomel electrode through the microammeter. The end point in the titration of Procedure A corresponds to a consumption of 1 mole of silver for every mole of mercaptan; no distinction is made between primary and tertiary mercaptans. The end point in the titration of Procedure B corresponds to an iodine consumption of 0.5 mole for every mole of primary mercaptan and of 1 mole for every mole of tertiary mercaptan. If the total amount of mercaptan sulfur present from the silver nitrate titration (Procedure A) is known, the amounts of primary and tertiary mercaptans present are readily calculated as follows: Milliequivalents of silver nitrate (Procedure il)
Milliequivalents of iodine used (Procedure B). V total sample B = Viodine X lviodine X (7) V aliquot Per cent of mercaptan present as tertiary mercaptan = B A - A x 100 = c (8) Per cent of mercaptan present as primary mercaptan 100
-c
=
(9)
EXPERIMENTAL
The argentometric part of the procedure has been denionstrated to be reliable (2). The iodometric titration portion of the procedure has been tested with pure n-dodecyl mercaptan (dodecanethiol) and two tertiary mercaptans, dimethyl-nnonylcarbinthiol and di-n-butyl-n-propylcarbinthiol. Results in Table I illustrate the accuracy that can be expected of the method in the titration of primary and tertiary mercaptans alone and in mixtures of various concentrations. The figures under the heading “mercaptan present” are based upon the results of amperometric titration of the mercaptan with silver nitrate in ammoniacal medium.
Table I. T i t r a t i o n of Mercaptans a n d Their Mixtures Amperometrically in Presence of Lead Ion w i t h Iodine Mercaptan Present, Mg. ~ i - , , - b ~ t ~ l - M1. of 0.00495 N Iodine Dimethyln-nonyln-propylCaln-Dodeoyla carbinthiol‘ carbinthiolO Used culatedb 8.22 ... 8.25. 8 . 2 1 8.22 ... 1.06 2.03 2.12 ... 1.77 .. 3.40 3.54 3,55 .. 6.87, 6 . 8 1 7.10 5.32 .. 10.13 10.64 ... 8.88 16.82 17.76 ... 17.75 33.1 35.50 ... 35.50 63.2 71.0 7.27 7.76 1.84 2.96 3.68 2.96 9.18 9.60 3.68 14.82 .. 29.97 33,32 18.42 7.41 29.43 33,24 ... ... 6:40 12.71 12.80 3.68 2.56 8.84 8.80 3.68 6.40 16.30 16.48 2.56 14.31 14.33 9.21 9.21 6.40 21.51,21.47 22.01 Samples prepared and supplied by C. 5. Marvel, University of Illinois. b Calculated on basis that 1 mole of primary mercaptan requires 0.5 mole of iodine, while 1 mole of tertiary mercaptan requires 1 mole of iodine.
... ...
I . . I
...
... ... I
.
.
I t is apparent from Table I that, a t least with the mercaptans tested, most reliable results are obtained when the total concentration of the mercaptan is small. At higher concentrations the amount of iodine consumed by tertiary mercaptans alone or in mixtures is less than expected. This is no doubt due to disulfide formation by the tertiary mercaptan. The data also indicate that the sample of dimethyl-n-nonylcarbinthiol used may not be completely a tertiary mercaptan, for even a t very low concentrations theoretical quantities of iodine are not consumed or it may react abnormally. On the other hand, the di-n-butyl-npropylcarbinthiol sample is probably relatively pure and has very little of other types of mercaptan as contaminants. Attempts to titrate secondary mercaptans by the amperometric iodometric procedure have shown that they react with neither 0.5 nor 1 mole of iodine per mole of mercaptan but with a quantity of iodine between these t x o values. The exact amount of iodine consumed depends upon the conditions of reaction. At high dilutions the number of moles of iodine consumed approaches 1 . Titration of concentrated solutions requires amounts of iodine approaching only 0.5 mole per mole of mercaptan. Apparently a t high concentrations the formation of disulfide is favored, while a t low concentrations formation of the sulfenyl iodide is favored. Disulfide formation is also favored if the solutions are titrated hot. The end point determined amperometrically with secondary mercaptans is somewhat indefinite. At or even before the end
V O L U M E 21, NO. 8, A U G U S T 1 9 4 9
965
point some of the RSI formed apparently decomposes slowly with the formation of disulfide and free iodine.
2RSI
+RSSR
+
Table 11. Analysis of Commercial Mercaptans for Their Primary and Tertiary Mercaptan Content
(10)
12
This formation of free iodine gives a constantly increasing current a t the microelectrode. For this reason the end point must be determined very rapidly if the amperometric method is used. Such a procedure is not practicable and :he results are uot accurate. Some evidence that all tertiary mercaptans do not react *toichiometrically to form the sulfenyl iodide has been obtained by Laitinen ( 5 ) . A qualitative measure of the tendency to disulfide formation by a tertiary mercaptan can sometimes be obtained a t the completion of the iodometric titration. If Reaction 10 is taking place to any appreciable extent, the microammeter will register the constantly increasing current due to the formation of free iodine. If the rate of increase is smallLe., less than a few tenths of a microampere per minute-the formation of disulfide by tertiary mercaptan is probably slight. By the above test it has been noted that the dimethyl-n-nonylcasbinthiol has a much higher tendency to form disulfide than does the di-n-butyl-n-propylcarbinthiol. Some commercial mercaptans have been analyzed by thP method described in this paper (Table 11). INTERFERENCES
The substances that interfere in the argentometric titration have been discussed ( 2 ) . Substances oxidizable by iodine under the conditions of the titration mill interfere in the iodometric proeedure. I n addition, substances that form insoluble precipitates with lead should be absent or enough excess lead nitrate should be added to precipitate all of the interfering substance. .%]though in many instances the iodometric procedure could be applied to pure mercaptans without the addition of lead, this is undesirable because the iodide formed by the reaction reduces the accuracy of the determination of the equivalence point. When iodide is present the current a t the rotating electrode for any given excess of iodine is greatly reduced (see Table 111). The addition of excess lead as recommended in the procedure precipitate3 the iodide ap lpad iodide, so that iodide desenpitiza-
Mercaptan (Calcd. as CnHzaSH). Mercaptan Titrated Fraction 3B, Sharples
Primary Mercaptan,
%
%
Tertiary Mercaptan,
%
Table 111. Diffusion Currents at Rotating Platinum Electrode with Varying Excesses of Iodine in Some Solutions Solutions to Which Iodine 100 ml. 95% ._ ethanol 0.01 M HClOA 0.001 AM KI
loo ml. 95% .-
ethanol 0.01 .W HClOi 0.005 N I9 sulutlor~ in Excess, SI1. 0.07 0.14
1.7 3.2 4.7 6.3
n , 30
' -
Microamperes
,~---
0.21
--
Is Added 100 ml. 95% ethanol 0.01 M HClOi 0.001 M K I 0.01 iM Pb(NOd1
0.3
0.5 0.7
1.0
1.9 3.7
5.3 7.6
tion is rliininated. In all caseb the eiid point obtained is the same whether or not iodide is present, although less precisely determined when iodide is present. The same electrode should not be used for the iodometric titration as is used for the argentometric titration unless the silver plated onto the platinum has first been removed with nitric acid. LITERATURE CITED
(1) Kimball, J. W.,Kramer, R. L., and Reid E. E., J. Am. C'hern. SOC.,43, 1199 (1921). (2) Kolthoff, I. M.,and Harris, W. E., IND.ESG.CHEM.,ANAL.ED., i8. 161 (1946). (3) Laitinen, H. A,, private communication. (4) Rheinboldt, H., Ber., 72,657 (1939). (5) Tyler, W. P.,and Brown, W. E., B. F. Goodrich Co.. Drivate ,
I
communication.
RECEIVED August 11, 1948. Investigation carrled out under sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corporation, in connectinn with the government synthetir rubber program.
Determination of Inorganic Phosphate Modijficat ion of Isobutyl A lco hol Procedure J A M E S B. MARTINLAND D. M. DOTY Purdue C'nirersity Aprirultitral Experiment Station, Lufayette, Ind.
T
HE procedure employing extraction of phosphomolybdic
acid by isobutyl alcohol described by Berenblum and Chain (2) and recently evaluated by Pons and Guthrie ( 4 ) affords a good method for determination of inorganic phosphate in colored solutions. In studies of phosphatases and of phosphorus fractions in alfalfa, the use of this method seemed desirable because colored solutions were encountered. Investigation of the isobutyl alcohol extraction procedure has led to improvements that simplify the color development, shorten the extraction procedure, and eliminate the interference from proteins. In the improved procedure the sample is in contact with an acid solution for only a short time (60 t o 90 seconds). This should enhance the value of the method for the determination of inorganic phosphate in the 1 2
Present address, Procter and Gamble Co., Cincinnati, Ohio. Present address. American Meat Institute Foundation, Chicago, I11
preseiice of easily hydrolyzable phosphate compounds. Ferric ions do not interfere in the isobutyl alcohol extraction procedure, REAGENTS
Isobutyl Alcohol-Benzene Solution. Mix equal volumes of isobutyl alcohol and thiophene-free benzene. (Some technical grades of benzene impart a cloudiness t o the solutions after color development.) Molybdate Reagent. Dissoh e 50 grams of ammonium molybdate in 400 ml. of 10 N sulfurir acid and dilute to 1 liter with water. Silicotungstate Reagent. Dissolve 5.7 grains of sodium silicate nonahydrate and 79.4 grams of sodium tungstate dihydrate in about 500 ml. of water. Add 15 ml. of concentrated sulfuric acid, boil for 5 hours, cool, and dilute to 1 liter with water. Stannous Chloride Stock Solution. Dissolve 10 grams of stannous chloride dihydrate in 25 ml. of concentrated hydrochloric acid and store in a small glass-stoppered brown bottle.
