Determination of Acetylenic Hydrogen by Means of Concentrated Silver Solutions LUCIEN BARNES,
JR., and
L. J. MOLlNlNl
Research Laboratories, A i r Reduction Co., Inc., Morray
In an extensive program for the production of derivatives of acetylene, a simplified method for the determination of acetylenic hydrogen was necessary. The observation that concentrated solutions of aqueous silver nitrate and silver perchlorate form soluble complexes with acetylene, accompanied by a simultaneous quantitative liberation of hydrogen ions, was the basis of a rapid and accurate method for the acidimetric determination of acetylenic hydrogen, employing 2.0 to 3.541 aqueous silver nitrate or silver perchlorate as reagents. The procedure has been successfully applied to acetylenic alcohols, hydrocarbons, carboxylic acids, amines, and miscellaneous compounds with an accuThe principal advantages are racj within io.;%. speed of analysis, a minimum of reagents, and the absence of a precipitate of silver acetylide, which greatly facilitate detection of the visual end point. Halogens and aldehldes do not interfere.
A
CETTLENIC hydrogen has been determined b y reaction with silver ( 1 , 2, 9, 11, It?), cuprous ( 4 , 8, 11,IS), and mercuric ( 3 ) ions. I n all instances the reaction between acetylenic hydrogen and the metal ion results in the formation or precipitation of a metallic carbide or acetylide and the liberation of hydrogen ions. This may be illustrated by the reaction of a monosubstituted acetylene with dilute aqueous silver nitrate.
R-C=C-H
+ 2.4gYO3
; iR-CEC-Ag
,
AgKOa
+ HNOj (1)
Since most acetylides are explosive when dry, gravimetric determinations ( I O ) have not been favored. Therefore, methods of analysis have generally determined the excess reagent, or measured the liberated acidity. Cuprous procedures have generally been restricted to the colorimetric determination of trace amounts of acetylene, although Siggia (11) has described an acidimetric procedure applicable to macrodeterminations. Mercuric ion, as potassium mercuric iodide, has been utilized by Hanna and Siggia (3) for the determination of certain monosubstituted acetylenes. Silver ion, as silver nitrate, has been used in ammoniacal (11) and neutral mixed aqueous-alcoholic (1, 1 1 ) media. Marszak and Iioulkbs ( 5 )have described the use of silver benzoate in alcoholic medium for the determination of acetylenic hydrogen. This method appears to be one of general utility. Most of these methods, however, are characterized by certain disadvantages which, a t times, severely limit their usefulness. The acidimetric cuprous procedure (11 ) employs unstable reagents and the end point must be determined with the aid of a pH meter. Mercuric ion in alkaline medium is reduced by aldehydes in excess of 0.5% (5). Furthermore, consistently low results were obtained in these laboratories for acetylene (ethyne) under the conditions specified, and quantitative recoveries could be attained only by increasing the concentration of standard alkali to 2.ON. Ammoniacal silver methods (11)yield high results in the presence of halogens, cyanide, aldehydes, and other substances oxidized or precipitated by silver ion in ammoniacal media. Acidimetric procedurev utilizing dilute silver nitrate in alcoholic media (1, 1 1 ) often produce copious acetylide precipitates, which make detection of the visual end point difficult. The acidimetrir silver
Hill, N. 1. benzoate procedure ( 5 ) ,while apparently an excellent method and free from most. of the interferences cited, requires a 12-minute shaking period and a filtration prior to titration. I n an attempt to develop a rapid and accurate procedure for the determination of total acetylenic hydrogen in gaseous mixtures of acetylene and methylacetylene (I-propyne), the authors have used the observations of Fisher and Shaw (9, 10) that concentrated aqueous solutions of silver nitrate react with acetylene to form a soluble acetylide according to H-C=C---H
+ 8AgN03
+
Ag-C=C--iig.f1AgNO3
+ 2HX03
(2)
The liberated nitric acid does not decompose the complex acetylide and may be readily titrated. Application of this principle to other acetylenic compounds, utilizing concentrated solutions of silver nitrate and silver perchlorate, has demonstrated an analogous behavior. These observations were found to be in accordance with the findings of V e d n and Ralf ( 1 2 ) . The main advantages of the procedure are the ahsence of precipitated aceb j-lides, a minimum of reagents, and speed of analysis. Additional attribut,es are apparent freedom from interference due to aldehydes and, to a lesser est.ent, halogens. REAGENTS
Silver nitrate, 2.0 to 3 . 5 M aqueous solution, Silver perchlorate, 2.0 to 3.5M aqueous solution. .4nhgdrous or hydrated silver perchlorate is obtainable from G. Frederick Smith Chemical Co., Columbus, Ohio. Sodium hydroxide, 0.1N, carbonate-free. Methyl purple indicator solution. Obtainable from Fleishpr Chemical Co., Benjamin Franklin Station, Washington 4,D. C . PROCEDURE
Liquid and Solid Samples. Place 40 ml. of silver nitrate or silver perchlorate reagent solution in a 250-ml. Erlenmeyer flask, and add 3 to 4 drops (amount critical) of methyl purple indicator. Neutralize with 0.1N base or acid. Add a weighed amount of sample equivalent to 2.0 to 3.5 meq. of acetylenic hydrogen and titrate the liberated acid with 0.1N base to t h e green color of the indicator as viewed by transmitted light. Acidic or basic impurities and/or functional groups, if present, may be determined separately and appropriate corrections applied. Gaseous Samples. Bring a known volume of the gas into intimate contact with a suitable quantity of the concentrated silver reagent and titrate the liberated acid with standard base. Equipment and manipulative technique for handling gas samples may take any one of many forms. However, the following apparatus and procedure have proved eminently satisfactory in these laboratories. The manifold illustrated in Figure I provides means for the transfer of gas from a cylinder under pressure directly into calibrated sample tubes convenient for analysis. With the main cylinder valve closed but the needle valve open, and the gas sample tubes open t o the manifold, evacuate t h e system. Close the large stopcock, D,thus isolating the system from the vacuum pump. If the system is free from leaks, as indicated by a steady manometer reading, close the needle valve, and open the main cylinder valve. Open the needle valve slowly, and fill the sample tubes to slightly above atmospheric pressure. Close the stopcock on both gas sample tubes and remove the tubes from the manifold. Mount the tubes vertically on an apparatus support stand, allow 5 minutes for temperature equilibration, and open the stopcocks momentarily to adjust sample to atmospheric pressure. Record temperature and barometric pressure. By means of rubber tubing, connect a leveling bulb to the lower (three-way) stopcock of the sample tube. Place a suitable quan-
1025
ANALYTICAL CHEMISTRY
1026 tity of the concentrated silver reagent solution in the leveling bulb and displace the air from the rubber tubing. Open the threeway stopcock and allow the reagent solution to enter the sample tube. Shake gently, drain, and wash contents into a suitable vessel, and titrate with 0 . 1 s sodium hydroside. The volume of distilled water used for washing the gas sample tube should br held a t a minimum to prevent dilution and hence preclude acetylide precipitation. B y modification of sampling technique, the apparatus (Figure 1)may be used for the sampling of gases under pressures less t,hari atmospheric. RESULTS
The applicability of the method to various acetylenic ('oriipounds is indicated in Table I. All compounds listed \vew analyzed as received without purification. Some of the conipounds tested had very limited water solubility, but, with the esreption of 3-methyl-l-non,vn-.7-01, when added directly to t>he concentrated silver solution, all reacted rapidly and quantitntively to produce a clear, single-phase system. When the latter compound was added direct1.v to the concentrated silver solution a n acetylide precipitated n-hich ivould not dissolve completel:., and analytical results !!-ere erratir. Upon weighing the compound into 5 t o 10 ml. of 95% ethyl alcohol, and then adding the silver solution t o the alcoholic one, no precipitation of acaetylide+ was observed and analytical results were reproduvitile :ind quantitative.
Table I.
Tr
A
U
Application of Jlethod to Various Conipouids
Figure 1. Compound"
reagent
leagent
To v a c u u m p u m p Trap (4 X 31 em.) Stopcock, straight bore. 4 mm. Stopcock, straight bore. 10 nini. Manifold (3 X 40 om.) Joint, standard taper 12'38 Stopcock, 3-waj, 2 m m . M a i n c j l i n d e r valve Veedle valve .I. Open-end mercury m a n o m e t e r h. G a s ramplc tube, approyimatel, (3.3 X 18 c m . ) I . FIexitrIc prer-ure t u b i n g
U
,I.
B.
ALcoiioI 9
l-Plienyl-2-propyn-l-oi '2-Plopyn-I-olb 2-Methyl-3-butyn-2-01 3-Xethyl-l-pentyn-3-uI 3,4-Dimethyl-I-pentyn-3-0 3-Ethyl-1 -pentyn-3-ol 1-Ethynylcyclopentanol 3-Methyl- 1-hexyn-3-01 1-Ethynylcyclohexanol 2.6-Dimethyl-4-ethynyl-l1eptan-~-~l 3-llethyl-l-nonyn-3-ol
Gas sampling manifold
99.4 98.5 99.0. 9 9 . 2 99.3, 9 9 . 5 99.1 98.5, 9 8 . 3 97.4,97.7 98.2,98.7 97 5 , 9 7 9 98.0, 98.0 9 0 . 3 , 96 3
C. D. E. F. 6. H. I.
95 2 !I8 8 95 .i 99.5 98.5 97 9 98 8 97.8 W . 6 . 96
100-ml. raparit.
fi
HYDROCARBOSS
8 2 & 0 32 3, 99 4 9.98 1 8. 99 0 1.96 3
Acetylened 1-Propyne 1-Butynee Z-nIethylbiiten-3-yne 1-Hexynel
99 99 98 99 96
CARROSYLIC ACID Propiolic acid0
98.0,98.1
9q 85 = 0 . 1 5
.bfISES
3-AminopropyneJ~ Di(2-propynyl)amine1
99. .i 99.4
~IISCELLAXEOCJ Sodium acetylide i
97.4,97.?
Technical made samules of reasonable u u r i t r : no attemr>ts made a t further purification. b Bniline ranne 113' to 115' C C-Rptoh Drment bv livdroxvlamine hvdrochloride analvzib. a
0.38 & 0.08% nonacetylenics-i,e., 99.62% 1.3% nonacetylenios present b y Orsat analysis. I95.8% assay by potassium mercuric iodide (SI. o 98.5% assay by titration of carboxylic acid gfoup. h 98.6% assay by nonaqueous titration of amino group ( 6 ) i 99,3y0 assay by nonaqueous titration of amino grour,. j 2.1% nonacetylenics present. IS
e
The acetylenic amines listed in Table I, which formed insoluble acetylides, were titrated (without preneutralization of the amino group) with the aid of a p H meter equipped with standard glasP and external calomel electrodes connected by a sodium nitrate salt bridge. Their inclusion in Table I demonstrnteq the vermtility of the method. The effects of halogens and aldehydes are shown in Table 11. Halogens, in the amounts preqent, did not interfere. The visual
end point was readily observed after the precipitated d v e r halide had settled. Samples containing halogens have also been analyzed by filtering off the precipitated silver halide through a coarse paper, followed by titration of the liberated acid in the filtrate. Although the metathesis of silver acetylides by halidehas heen reported (?'), no such interference would likely he involved here, since the relatively high concentration of silver ion precludes the possibility of excess (free) halide ions. The noninterference of aldehydes is presumably due to the fact that the pH of the qolution is never maintained a t a high value sufficiently long to reduce silver ion. Apparently local excess concentrations of base are nithout effect in this respect. DISCUSSIOS
Certain distinct advantages :ire associated with the use of concentrated silver solutions for t,he determination of acetylenic hydrogen. The general absence of precipitated acetylides facilitates detection of the end point, and is particularly desirable in the analysis of gaseous samples where precipitates are likely to obstruct volumetric apparatus, clog stopcocks, etc. The limited number of reagents required, coupled v-ith the fact that filtration is unnecessary, makes t,he method extremely rapid. I n addition, certain compounds of very limited u-ater solubility, such as 1hutyne and 1-hexyne, ma>- he analyzed directly without prior addition of a solubilizing agent. The principal ohjection to the method lies in the high cost of the silver reagent solutions. Horvever, careful selection of compounds to be analyzed and reclamation of waste silver solutions help to reduce the cost. The concentration of silver ion necessary
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 -
-
Table 11.
~
-
___
1027 ~
~~
~~~~
liberated acidity is not extremelj. critical, Normalities between 0.05 and 0.2 have proved satisfactory in these Error, laboratories, although O.lA\r seems to L& he ideal for most compounds. 31ore 0 0 concentrated solutions-i.e., 0.5.V, set up 0.9 +O. 1 local escesses of base which are so large f0.1 0 0 that the locally precipitated silver oxide -0 .i iedissolves with considerable difficult!.. -0 2 -0 4 This lengthens the time required for the -0 .i analysis, and also produces a condition which may be more fiivorable for interference by aldehydes. Too dilute hase do not produce a sharp color change a t the end point. cause some acetylide precipitntion.
Effect of Halogens and Aldehydes upon Determination of Acet:lenic H j drogen
3.-11ethyl- 1-pen tyn-3-01
3 180 3 390 2 60; 1 542 2 358 2 6.54 2 730 3 313 3 430
3 180 3 390 2 671 1 543 2 358 2.640 2 724 3 300 3 114
Iodide Chloride Bromide Butyraldehyde Butyraldehyde Formaldehyde Formaldehyde Salicylaldehyde Salicylaldehyde
Reagent AgNO1
ii . 0
AgNOl
3.0 3.0
4gNOu AgNOa .4gC101
0,544 0,833
AgN0.I AgClO, AgKO, Apso,
0.698
0.718 0.088
0.713 ~~
to prevent acetylide precipitation is much less for some conipounds than for others. I n these laboratories, 2-methyl-3-hutyn2-01 and 3-methyl-1-pentyn-3-01 are analyzed rout,inely by t,he addition of 1.5 meq. of saniple to 100 ml. of 0.351 aqueous silver nitrate and subsequent titration of the liberated acidity with O.U.5.V alkali. Under the described conditions no nretylide precipitation is observed. On the other hand, most, hj-drocarhons. suoli as acetylene, 1-hesyne etc., whose acetylides would precipitate from dilute silver solutions, are analyzed utilizing concentrated (2.0 t o 3.5-11) solutions of silver nitrate or silver perchlornte as reagents. The amount of methyl purple indicator added to the solut.ion to i>etitrated is critical. Three or 4 drops from the conventional dropping-type indicator bottle are sufficient when silver nitrate is uied: larger amounts of indicator tend t o obscure the end point. E'u1,ir or 5 drops of indicator may be used when silver perchlorate ii imployed. For many of the compounds tested, a more distinct e d point was obtained lvith silver perchlorate than with silver nitr:ite. The effects of the high salt concentration, in the propr1.d concentrated silver solutions on the indicator, appear to haye ii negligible influence on the accuracy of the method. The wiwtion of appropriate indicators is limited to those which change color in the p H range of 4 t o 6. .4t higher p H values hydrated silver oxide starts t o precipitate. Bromocresol purple, hroii1ocresol green, methyl red, and methyl red-methylene blue iruiicators have been utilized in these laboratories; however, methyl purple is preferred by most of the analysts. Reputedly, c.iilorophenol red ( 1 2 ) has heen employed successfully for this purpose. The concentration of sodium hyclroside used to titrate the
.
solution. and ni:i!.
ACKNOW LEDG31 ENT
The authors are indebted to C. .4.\l*nmser tor valuable aid :inti :issistance during the (Boui'w of this investigation and preparation of this manuscript. .4cknon-ledgment is also estended to A. €1. Taylor and H. IT.Linde for their critical examination of this paper and to the .4ir Reduction Co.. In(*..for permission to puhlish. LITERiTL'RE CITED
Alteri. V. -1.. "Gas -4nalysia and Testing of Gaheous I I a t e i ~ i n l . . " American Gas ;\asoc., Inc.. Xew York, 1945. Chevastelon, It.. C o n ~ p t .r e n d . , 125, 245 ( 1 8 9 7 ) . Hanna. G. J., and Siggia, S.. ANAL.CHEM., 21, 1 4 6 9 - 7 0 (1949). Ilosvay. L.u.S., l l e r . . 3 2 , 2697-9 ( 1 8 9 9 ) .
llarszak. I., and Koulkk, 11.. .116m.services c h i w , P/u( ( I ' u 3 6 , 1-0, 4 . 421-6 ( 1 9 5 1 ) . Pifer. C. K., and Wolliah, E. G , , I s . & i . . CHLXI.,2 4 , :300-5 (1932).
Itobey. lt. F..Hudson, 13. E., .Jr., and FYie-e, 13. I
