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Determination of small amounts of water in organic solvents by spectrophotometric analysis
Andyflco Cklmfco Acfn Ekvicr Publishing Company.Ams:erdnm Printed in Tbe Nerhcrlands
447 t
DETERMINATION OF SMALL AMOUNTS SOLVENTS BY SPECTROPHOTOMETRIC ,.
OF WATER
IN
ORGANIC
ANALYSIS
C. L. L&E BellTelephone
Laboratories,
Incorporuted,
Murray
Hill, N. J. 07974
(U.S.A.)
(Received 25th September 1970)
Small amounts ‘of water in organic solvents are usually determined by the Karl Fischer method. The latter is based on the reaction between water, iodine, and the anhydride, sulfur dioxide. The method has proved to be very satisfactory. Never-’ theless, the challenge of developing an alternative way of determining water was attractive and it seemed worth-while, as a starting point in the search for a new method, to try substituting’the anhydride, chromium(V1) oxide, for the sulfur dioxide, with the hope that some means could be found for measuring the amount of chromic acid produ’ced. Tests showed that, although the chromium oxide could not be used directly, a successful method for the determination of water could be had by taking advantage of the reaction K2Cr2g,+
H,O
= KZCrqa+H,Cr04
(1)
Thus, if a mixture of dichromate and wet 4-methyl-2-pentanone (MIBK) containing a little .magnesium perchlorate dihydrate, is shaken, the chromic acid produced i,n the reacti.on between dichromate and water dissolves in the solvent and the reaction proceeds to the right by the law of mass action. The dichromate and chro‘mate remain insoluble in the solvent. Some reaction of the solvent with the dichromate occurs to:prbducti,soluble RHCrO, but this side reaction is’usually small. Thus, it is .e.vident that if the &mount of dissolved chromate in ,the solvent is measured, and if correction’ for ,the .RHCr04 is made, it should .be possible to determine the water present. It was found that the dissolved chromate could be convenieritly determined ; spectrophotometrically’. ‘: The reaction bf water with dichromate is slow and long shaking is required to obtain complete reaction. As ,the reaction proceeds, the RHCr04 blank increases in proportion. ‘Eventually, the chromate concentration beco’mes. high *eriough to cause’ som&:oxidation of tlie’solvent;, In view of,this, to limit,the time of shaking and to make it possible to determine the waterspectrophotometrically, it ‘was ‘necessary .to ‘use, conditions where. the;reaction was only about. lo”/, complete., Fortunately, : .,, adequate sensitivity in,the analysis is assured. ‘_/ : Quantitative ,tests -of* the ‘proposed .method showed, as expected, that a ,plot of the weight of water added to dry MIBK uersus the measured absorbancewas linear. This confirms that the m&hod .will be suitable for the determination of water in MIBK. Attention wag next :fodused’on?he problem of adapting the method to the analysis of a variety of other solvents. Organic solvents differ widely in’their behavior and most .of them cannot be analyzed directly. However, a large number of these solvents can Anal. Chtm. Acta, 54 (1971) 447459
c..
448
L. LUKE
be,analyzed, provided that they are suitably diluted with 0.2’/, magnesium perchlorate dihydrate-MIBK solution before the reaction with dichromate. Data showed that, in such analyses, it is possible to determine as little as 0.002°/0 of water, depending on ). the solvent being analyzed. Since organic solvents vary in. their reactivity and thus produce RHCrO, blanks of different magnitude, it is necessary, in the new method, to calibrate for each solvent being analyzed. However, only two points are required, one below and one above the water content of the sample being analyzed. The amount of water-present in the sample can be obtained by simple interpolation, since the graphs are linear. The method calls for considerable attention on the part of the analyst, but as he or she becomes familiar with the techniques required, the average time required for the calibration and analysis of a sample is about 10 min, which is not much more than that required to make an electrometric Karl Fischer analysis. EXPERIMENTAL Apparatus
,. Reciprocal shaker. A reciprocal shaker (Eberbach Corp,,Ann Arbor, Mich.) was used; All solutions were shaken at a rate of 180 oscillations per min., ‘Spectrophototieter. A Beckman Model ,B unit with l-cm absorption cells was used, ,.
Reagents Activated molecular sieve powder. Heat powdered 3A molecular sieve (Union Carbide Co.) at 175’ for 24 h, Cool and protect from moist air. Dry solvents. Transfer 100 ml of.the solvents to be analyzed to 125-ml standard taper,conical ,flasks. Add 5 g of the,dry powdered, 3 A, molecular sieve, stopper. and shake on the. reciprocal shaker for 1, h; Allow the solutions to stand, overnight :to permit settling of the’sieve. If the amount of solvent available is limited, add 10 ml of the solventplus 1.g of 3A.sieve to,a glass,stoppered, 15-ml centrifuge.tube, shake horizontally.for 2 h and then centrifuge. Dispense the clear solutions by pipet, taking care not to,disturb the .powdered sieve on the ,bottom of the. flasks or tubes; Discard any ,solution that :becomes.discolored on prolonged contact with ‘the. sieve. .. Magnesium perchlorate-MIBK solution. Dissolve 1.5 g of: magnesium perchlorate hexahydrate: in 500 *ml. of.:MIBK and.reserve. Dissolve 2.g of anhydrous magnesiumperchlorate in.1 1ofMIBK in a glass-stoppered bottle. Ignore any,insoluble .material. Add 10 g ,of powdered: 3A.sieve; shake, for 1 .h, or, longer .if necessary, on-a ‘reciprocal shaker;. and-filter through :Whatman No: .40,filter paper supported ..on: a large,Millipore fritted ,glass filter,with the aid of suction. ,’ ‘:. ‘t ‘, ‘.v;>kdd 50 mg,of - 400 mesh dichromate to a lo-ml portion of the filtered solution and then analyze, as directed in the Procedure described subsequently, using water! as the,reference solutionThen; if necessary; add small portions of the reserved wet MIBK solution, with:intermittent analysis, until the: blank reading .is 35:to ,45°/0T., If. too mtich,:bfthe yet solution is ,added: by accident, add’ more’dry O.20/0xnagnesium~ptr‘
54 (1971)‘447-459
SPECTROPHOTOMETRY
OF WATER
IN ORGANIC
449
SOLVENTS
nitrogen being used is not water-free, dry it by passage through a tower of &mesh anhydrous magnesium perchlorate. Dispense the solution through a hole in the stopper unit by means of a lo-ml pipet (Fig. 1). ’ Stcmdurd water solution. Transfer 1.0 ml of water to a dry lOO-ml volumetric flask, fill to the mark with ethyl acetate dried with powdered 3A molecular. sieve, and shake vigorously to, assure complete mixing (1 ml = 10 mg H20). In the calibration, dispense this solution from a Gilmont Micrometer Syringe (Cole-Parmer Instrument Company, Chicago). The syringe has a capacity of 2 ml and can deliver as little as 0.002 ml through a stainless steel needle.
10
ml
PIPE1
*
300
ml
DRY NITROGEN
FLASK
Fig. 1. Apparatusfor dispensing prepared magnesium pcrchlorate-MIBK
solution.
Shake fme-mesh, crystalline potassium ;Fine-mesh pbtassium dichromate. dichromate on a loo-mesh screen and reserve the - 100 mesh salt that passes through the screen; Then pass a portion of the + 100 mesh salt,successively through lOO-,200and 400-mesh screens by ,rubbing the salt on the appropriate screen with the bottom of aX250-ml beaker. Reserve a portion ofeach mesh-size salt, Dry the crystalline - 100 and the,crushed..lOO-200, .200-400 and’-400 mesh salt in a 75’ vacuum oven. ~Reference solutions. Dissolve 0.1 g of potassium.dichromate in 100 ml of water and mix. Dilute,1 ‘and 5 ml of this solution to lOO,ml with water and,mix. Call these ” solutions 1 and 5% dichromate ieference solutidns: I .,’ pr&&&
,
‘.
.’
.’
“,
c
(.
.‘.
Sweep out two dozen or more dry, 50-ml, 19/38 standard taper conical,flasks Anal.Chim.
Acta, 54 (1971) 447459
c. L. LUKE
450
with dry nitrogen, stopper, and reserve for the analyses. Transfer 10.0 ml of adjusted 0.2% magnesium perchlorate-MIBK solution,to 3 of the flasks. Add 1.0 ml ,of the,dry solvent to be analyzed to flasks 1 and 2 and the same amount ofthe wet solvent,to‘be analyzed, to flask 3. Then add 0.1 ml of standard water solution (10 mg ,HbO/ml) to : flask 2 and analyze the 3 solutions consecutively in the,order-flasks 1; 2 and’3. Add.50.0 mg of - 400mesh potassium dichromate from a 30-ml beaker,,through a small~funnel .to the flask, tapping,the funnel vigorously with the inverted.beaker to make sure that’all the salt enters the.flask. Stopper, grasp the flask, while holding the stopper in place with the index finger, invert the flask, extend the arm down in the normal resting position and then shake vigorously and almost vertically, so that the solution hits the top and bottom of the flask, for exactly 1 min.
- RESERVOIR
CLAMP
UCTION
50 ml FLASK
Fig. 2. Filtration
apparatus. ,.
the solution immediately, with strong’suction, using a small Millipore ,ftiter funnel,(Fig. 2), through a dry 25Smm disk:of Whatman No. 40 ftiter paper.that has been-cut:.outof.a.number of sheetsof the paper with a,die. Catch the filtrate in. a SO-ml suction., flask. : Withoughdelay; fill. a l-cm absorption cell with. the : ftitered solution,:and measyre’ the percentage, transmittance on..a suitable’ spectrophotometer at :370 nm: Use water or the’appropriate dichromate solution as .the reference solution, in order thatareading’of not :lessthan about .35%T. for.flask ~No&is obtained.. For ready ‘accessibility;. water., and the two reference solutions-.should be -contained,% absorption, cells ,placed, in a multiple compartment, holderin’the spectrophotometer., Convert OAT to ,absorbance (A) by referring to a suitable table that reads in units of 0.2%T, and.then,;for the’sakeiof convenience, multiply A by 100 to obtain thevalues ; ,. of..IOO~*;.;:,.,,;;;, “‘,.; :,...‘,,:.:,;,.. ,,.‘,,, ;; ,’ :,, ; ‘).S’. :: .; ,’ ‘, ,. ; ‘, .. :. : :. ,,Filter
.Anal;.Chim; ‘Acta; 54.(1971) 447-459
451 . ‘~Ci&~&iti&& i ”
_’
If..l,OOA-for flzisk ,E%-,l OOAf&r flask ,l = B ‘f. l(KM for, flask.2 9 lOO,$ for flask 1 = C i,.ml ofsolvent taken for ,analysis = D ,, solvent being analyzed = E ,%‘water in the solvent. ‘,.
#Note .
It is’&venient,to use a,series’of 6 filter units which’can be loaded with.paper disks z&d be ready for insertion into the neck of,the .50-m] lilt$r flask. After the spectrdfih’otometric reading’of,the solution from flasks ;l, discard the s.&ions and shake :the.fil~er:fktsk and,absorption cell to remove as much of the’solvent as possible before p&&ding t6 : the .-k&t ahalysis. After, co+pletion of, .the third analysis, ,wash out ‘Ithe:twd‘:vessel~:with ‘a’ stream ,bf acetone, and dry ‘them with a jet of compressed air. &fter’~all ‘-‘of‘the, ,filf& .,\;nits‘have ‘been used+ demount’ ,them, loosen the‘ dichromate ::‘on.‘the ri.t&of the’reservoiis and the,n was,h. the reservoirs aud the.frit supports with ,,:acktone.,Xy’and reassemble.‘Wash the used SO-ml conical,flasks with water,‘and acei :,tone.~‘Wash;.theS.reserv’oirs and frit &pports occasionally with hydrochloric acid to’ ,: ,’ .&moire, chromate, ion. .’ ‘. ‘,
DISCUSSION
When ,a drop of, water, is added ,to a ‘small portion of fine-mesh, potassium dichidmate;‘h;l[IBK,.is’aiided arid the!mixture is shaken, ,little or no chromic acid ap: ,pear$.‘in the tiIBK.’ If tlie.experiment is reijeated but, a small ,amo,unt of an, anhydrous ,hjigrdscopic~alk8li.,,or~~lkaline earth ‘chloride.or, perchlorate is present with the di,chro~~‘~e’;‘t.~ejeaction of.kqn.:i~j.ocdui~:and.the’~range cdlor,of chfomic acid.@an be .s~~~,.in_ihe:‘MIIj,~.,,~,he,rdie”played, by. the, hygros,copic,salt is n&t un,derstocd.‘If the il~tferlexpejtiment ;iS repeated but-the wateri_is,,di&olved in’the ‘MIBK before mixing, .,wit,bthe ,,diChromate :plus’,h$grosccpic. salt:. only magn:esium perchlorate will Induce 1the.reactiii~.oi:k’cj~.‘“(i’)‘to any al5ljreciable,exte+ Apparently this strongly hygroscopic ., .’ ‘cfait,cati.break,.jhe,-bdnding between the’water’and MIBK, atid al1o.w the,reaction to ~q~~‘c~~~.:‘,:~~:.:.::~.~,~~,;;~..:~,~,,..:.~: ,‘i-.:.,,‘.‘.~~~ ;.,.,,_ ,, , : ‘:‘... .’ _‘:,:~,,:,:‘~,,_‘,;, ,) ‘., ::, ‘.’ ‘, ; ,,,‘, ‘) ‘,.,” ;. ,‘,,’ i, ‘. :ti;f the, magnesium, perchlorate ,lu,the,’ MIBK influences (’ :.Y ; ‘;I: -:. The,.concentratiou h@ extent {of’reaction:;This ,can, be’seen. in tbe”data” i.u ,Fig. 3;’ To obtain these data, O;O~,:,d,i::$nb:2,00/,‘,slilutiohs ‘of ‘ahhydrous magtiesium p&chlorate inJ,~IB,K ‘were ~drie$~fz$rly:well with! 3A’nioleculir~.‘sieve, ‘and: then ‘$l,O-ml;portions of the’ solutions ,‘~e~~~,inluied’,~~i~~, vai%us’ amqunts. of standard ,,water, ,solution ,and ,analy?ed. by the ~~p~~~~dzi~~~“.~~~~.or-..:-;,46q.mesh”crus’hed dichromate being used.;,F?rom curves 1 ‘and’:,3’:&he ,,Fig; : it 3s seen, thata 0:2°/~‘%olutio~~~isopthnum:~ for the, analysis, since ili~~~ritL..b,~~\Neen: ,water added and’ net~abstirbance isobtained .ov,er ,an,‘appreciable &&g&’af.water:.cc;;ntent’;arid’ the: &&Gtivit~ :is! high; The,’ graph,:for the, .0.2% sol,ution “;,~b~~dt’b~~i;r;le,lilnka~u;itii:a lOOA;value of&out:30 is obtained Thedata,$us other ~~gi~ialr;.da~a,‘~o~ta~neci,;.~it~i::co;itipletelgi’:dehydrated:0.2O/; &agnesium. pemhldratei~~iB’~.isol~t~~~~~~~~~;t~ fhat;until approximately.the’dihydrate has’been,formed, the’ ,j”~~~~~,Ithat”~i;is~de~n ‘freed, from the bonding of the MIBK, cannot react,nor,m!lly with ., 1,.(’ ; ,/’ ‘. _I.,:, .v, \ ,, .,, ‘,
..,‘:
Anal. ‘clli,;li,:act(l.‘54’(1671i44j-45~ ‘. ,, .. :
452
‘, ‘. .,’ : ‘.
‘, 110’ :
., ,
‘,I
I
I 0.2%
.’
;
,
,,’
., 1
No. 1 (-400 MESH 1
1
I
SPECTROPHOTOMETRY
OF WATER
IN ORGANIC
SOLVENTS
453
the more dichroinate used, the smaller the mesh size of the dichromate, the longer the time of shaking and the greater the intensity of shaking. .. In order to obtain reproducible analyses, the shaking of the dichromate-solvent mixtures .must be ‘reproducible. Attempts to use a wrist action or ‘reciprocal action mechanical shaker failed,, but it was shown that reasonably good reproducibility could be obtained.by hand shaking; In order to reduce the labor involved, it is desirable to limit the time,of shaking to 1 min. When this is done, it is necessary to use finelydivided dichromate in order to obtain a reasonably high net absorbanoe From the water. In the Procedure, - 400 mesh salt is recommended but, in most cases, the more easily handled 200-400 mesh salt can be used instead. A crushed salt has a greater surface area than a crystalline salt of similar mesh size, probably because of the presence of dust that cannot be removed in the sieving. The effect of the mesh size of the dichromate on the extent of reaction can be seen in Fig. 4 where calibration graphs, obtained with adjusted O.20/0 magnesium perchlorate-MIBK solution and crushed dichromate of different mesh sizes, are given. SO .
,.
0 ,.,,
Fig. 4. Effect of mesh
-400
MESH
4.0
:W&3AOOED3%. size of dichroin~te.
Anal. Chim. ,Acta, 54 (1971) 447-459
454
C. L. LUKE,
From the Fig., it is seen that linearity is’obtained in all cases and that the RHCr04 blank and the net absorbance from a given amount of water are proportional to the surface area of the dichromate. Because ofthenature ofthe new method for water; which involves an incomplete reaction between a solid and.a liquid, all variables must be closely controlledIf this is done, fairly good reproducibility and accuracy can be obtained. Data obtained with - 400 mesh dichromate (Table I) showed that the reproducibility is reasonably good. The data in Fig.%and in Table I are different because they were obtained with different batches of adjusted magnesium perchlorate-MIBK solution. TABLE DATA
Water
I
ON REPRODUCII3ILI’l-Y
irkfed (fno)
(100 A)
Average
0
43.7 43.2 39.1
42
0.5
61.3 61.3 60.6
61
1.0
82.4 75.9 78.5
79
Behavior of solvents In the determination of a given amount of water in non-polar (non-reactive) solventsthe total amount of chromate ion and the ratio of RHCrO, to H2C!r04 extracted by the, IVIIBK varies from solvent to solvent because their reactivity and their ability to bond with water varies. The variation is not great and the extent of reaction approximates that obtained in MIBK alone. The great majority of organic solvents can be analyzed on a l-ml sample, with - 400 mesh dichromate. The Pioc& dure’abpve has been written to provide, for these analyses. Whenever the solvent will permit, up’to 5 ‘ml of the sample should be used if,the water content is less ,than 0.01%. On the other hand, if the water content of the sample is greater than 6.1.x the,sample size must,be reduced so that no more than 1 mg of water is present, or the amount of water added to flask 2 must be increased se that it exceeds, by a ‘small margin, the amount present in flask 3. In this latter case, if the transmittance for flask 2 is smaller than lo%, dichromate of coarser mesh should be used in the analysis; In any event,.it is not advisable to take a sample for analysis that contains more than about 3 mg of water. ., Small samples of reactive solvents can’be analyzed with 200-400 or ‘- 400 mesh .dichromate but, if larger samples are required to provide .adequate sensitivity,.dichromate of correspondingly coarser mesh must be used to control the size c$ the’ blank. Thus;as ,much as 5 ml of the reactive’solvent adetonitrile ,cari be analyzed provi.ded that 1001200,mesh crystalline dichromate is used. Regardless of the reference solution,‘used; the blank transmittance should not.be lower than, 35”j/,T. .. .. Anal; C/h.
Acta;’ S4,(1971)
447459
SPECTROPHOTOMETRY
OF WATER
IN ORGANIC
SOLVENTS
455
Solvents such as methanol, methyl acetate and Z-methoxyethanol cannot be analyzed because they readily react with dichromate and also form such a strong bond with water that the magnesium perchlorate is ineffective. Hence the blanks are high and the’net’gbsorbance caused by water is low or negative. The same is true, to a lesser extent, ‘with solvents such as ally1 alcohol, valeronitrile, ethanol and a few others. To analyze these latter solvents, it is necessary’ to limit the sample size. A few solvents tend to reduce chromate and hence cause the color to fade in the spectrophotometric measurement. In most instances the fading is slow and can be compensated for in the.calibration. However, when fading is more rapid, as it is in the analysis of 2-butoxyethanol, the sample size must not exceed 0.25 ml. The use of a coarser mesh dichromate is not as effective. ‘Solvents such as’pyridine, tri-pz-butylphosphate, the higher alcohols and acetate esters, propionic anhydride and, to a lesser extent, 1,4-dioxane, tend to suppress the reaction of water with dichromate, Hence, in the analysis of some of these solvents, 0.1 g of -400 mesh dichromate should be used and the solution should be shaken for 2 min in order to obtain an adequate reaction. In the analysis of viscous solvents such as cyclohexanone or the higher alcohols, the mixed solvents in flask 3 should be allowed to stand for at l&st 10 min before adding the dichromate, to allow the solvent to become thoroughly dispersed in the MIBK.’ The above discussion may suggest that the ney method is not practica1, bul the fact remains that the number of solvents that require special handling*is not great and experience with the method will make it possible to predict rather closely the sample size and.mesh size of the dichromate that should be used for the analysis of a particular’solvent. :. It is obvious, from what has been said, that the various factors of reactivity of the solvent,,sample size, and mesh size of the dichromate are interrelated. Moreover, it is clear why it is necessary to calibrate for each solvent being analyzed. Attempts to eliminate the need for this individual calibration by scavenging or sequestering the water from the solvents in a state where it could then be analyzed by the reaction of eqn. (1) were not successful. The sequestermg agents used were molecular sieve 3A, silica gel, calcium sulfate and magnesium perchlorate. .,’
Solvents
which can be’analyzed i, The method has,been tested qualitatively dn more than 125 solvents that were chosen at random from those available in the laboratory. In most of this work, a l-ml portion of the dry,,solvent plus 0 or 1 mg of water was analyzed, with -400 crushed #chromate.’ It was found that all of the alcohols and their acetate esters from ethanol td octanol; iii&&ig their isomers, could’be’analyzed. TeitLbutanol had to be rnaint,iin+l’. at, ‘3&3,5,0’duiitig the drying and pipeting in ,order to keep it, from solidifying. Other solvents’tested that appear to’be’capable of beii@ analyzed are listed in Table 11;,: Future work’tiill u~~~~~~edl~ show thata great many. others c&n also’be analyzed. ‘The S&itivity ‘efariaiysis of&ganic solvents dependson the sample size used. The permissible maximum Ample size depends ‘& ‘the solve&’ being analyzed. It has been .shown that ,5-ml samples of many of the higher alcohols. and acetate esters and a’large number of the low or moderately reactive solvents listed in Table II can be analyzed;,This means that. these solvents cari be analyzed for as little as,0.002”/0 of ,
Anal. C&n. Acta; 54 (1971) 447-459
456 TABLE !SOLVllNTS
C. L. LUKE, II WHICH
CAN
DE ANALYZED
Acetone 4-Methyl-2-pentanone 5-Methyl-2-hexanone 2-Pentanone 3-Heptanone 4-Heptanonc 2-Octanone Methyl ethyl ketone Acetophcnonc 3,3-Dimethyl-2-butanone 2,4- Pentanedionc 4:Hydroxy-4-methyl-pentanone Ally1 acetate Methyl bromoacetate Methyl dcetoacctatc Methyl sulfite Methyl salicylate Ethyl bromdacetatc Ethyl chloroacqtate Ethyl trichloroacetate Ethyl cyanoacetatc Ethyl format&’ Ethyl,sulfidc. 2-Methotiyethanol acetate 2-Ethoxyethanol,atietate 2-Butoxycthanol acetate Tri-rr-butyl phosphate Phenethyl alcohol Ally1 alcbhol” B&zyl’alcdhol a,c&Dimeth~llienzyl alcohol 2-Ethqxyethnnol
2-Butoxycthanol” 2-Methyl-2-hexanol p-Mcnth- 1-en-S-o1 Acetic anhydride Propionic anhy,dride” Propionaldehyde” Benza!dchyde Butyraldehydc Chloroform Carbon tctrachloride 1,2-Dichloroethane Trichloroethylcne Tctrachlorocthane Isoamyl chloride Bchzyl chloride Bromobcnzene Fluorobenzene o-Chloropheriol 3-Chloropentcne Bromotridhloromethane Hexachloro-1.3-butadicne a,cr,a-Trichlorotqlucne Hexachloro-i-propanonc 2-Bromobu&e 1,4-Dioxane Ethyl ether Isopropyl ether Bis(2-methoxyethyl)ethet& 1,2-Bis(2-mcthoxyethoxy)cthane Trim&y1 orthoacetatea Pyridine
2-Ethylpyridine” 4-Ethylpyridine Tetrahydrofuran Tetrahydro-2-methylfuran Petroleum ether Kerosene Gnsolinc Hexane 3-Heptane 3-Heptene I-Hexene n-Nonane n-Octane Cyclohexgne 3-Methylpentane ’ I-Methyl-l-cyclopentene Benzene Toluene Xylene Tetrahydronaphthalene Acctonitrile : Acrylonitrile Propionitrile” Valeronitrile” “’ Benzonkrile Carbon disulfide Acetone dimcthylacetal l,l-Dimethoxycthanc Nikomethane Tuqkntine tert-Butylbcnzcnc
-“ 9.2%ml sample
tal;&
‘for analysis. .’
water. On the other hand, not much more than 0.5 ml ofethanol and nitromethane, or 0.25 ml of solvents which yield low net absorbance or which reduce chromic acid can be analyied; In the,analysis of 0.5 ml of ethanol and 1 ml,of acetonitrile or acetone, it is necessary, to’ use, 2004400 mesh dichromate to control the size ,of the blank.
Determination ‘0s water. in soltible’ and insoluble organic, and -inorganic solids by the : ,; ..-,, ; ,,/. .. ,. ‘. :. proposed: method; ;’ ., .:’ .’ ’ “It’seems.probable that itwill be possible to determine water in various~organic.. Anal. Chirn.
‘Acch,~54~(1971) 447459
SPECTROPHOTOMETRY TABLE SOLVENTS
OF WATER
IN ORGANIC
457
SOLVENTS
III WHICH
CANNOT
Methanol Methyl acetate Ethyl dichloroacetate Acetic acid Acetyl acetone 2-Pyrrolidinone I-Methyl-2-pyrrofidinone
BE ANALYZED
Piperidine Dicyclohexamine DiethanoIamine Triethanolamine Triethylamine 1,3_Propanediamine Ni trobenzenc
tt-Methylformamide Dimethylformamide Ethyleneglycol 2-Methoxyethanol Dimethylsulfoxide Paraldehydc cr-Methylstyrenc -.
or inorganic solid compounds if they can be dissolved in a suitable solvent and if they do not interfere in the analysis. Moreover, in certain instances, it should be possible to extract water from various insoluble solids by refluxing with a suitable dry solvent and then determining the water in the solvent by the proposed method. Other methods for determining the chromate in MIBK The spectrophotometric method cannot be used ‘for the analysis of colored solvents which absorb strongly at 370 nm. Fortunately, the number of such solvents is limited. Moreover, as long as such absorption is not excessive it can be compensated for in the calibration. In the present work, the only colored solvent tested that could not be analyzed was nitrobenzene. However, it was shown that this solvent could be analyzed by X-ray analysis. To do this, proceed as directed in the spectrophotometric method but use an adjusted O.OSO/;lmagnesium perchlorate-MIBK solution. Then add 0.5 ml of a loi0 solution of anhydrous lead acetate in methanol to the solution after filtration to remove dichromate, allow the solution to stand for 1 min, filter through a dry 2%mm, 7-m nylon Millipore disk, allow the disk to dry, using suction, -and determine the,lead in the precipitate by X-ray analysis?. The X-ray method is not as widely applicable as the ,spectrophotometric method ‘because a few solvents (c.g.*ethyl formate, methyl sulfite) precipitate lead and. a few others cause the precipitate of lead chromate to: be colloidal. Moreover, in the X-ray method, the total amount of chromate ion, that can be handled is limited by the X-ray counter. The counts per second for the precipitate on a disk must not exceed about 20,000. In meeting this limit, it is not uncommon to find that the net count from water is low when very reactive solvents are analyzed. On the other hand, in the spectrophotometric method ,it.is possible to compensate for large blanks by using a suitable reference solution. This makesSit possible deliberately to use conditions which produce a large blank,.in order, to obtain an adequate net absorbance due to water. ,, . . Another approach to the analysis of colored soivents that was tested, consisted of aspirating the filtered I$IBK,solution.directly into the flame of an atomic absorption spectrometer for the analysis ofchromium. This method has the advantage that there’ is no need to work quickly, since the reduction of chromate is of no eonsequence. On the other hand, it was found that most solvents, other than MIBK, tend to suppress: the chromium radiation in, the flame. For this reason ,the method has limited value.: .. Q~~‘~titative~,analysis: of: commercial solvents In order to evaluate the new method
for the quanti~tive
determination
of
Anuf,Claim, Acta, 54 (197 1) 447459
458
c. L. LUKE
TABLE
‘IV
DETEHMINATION
OF WATER
IN COMMERCIAL
Sulve,rt
SOLVENTS
“/* Wuter found
1,2-Dichlorocthane Trichlorocthylcnc Ethyl cthcr Ethyl acetate Tctrahydrofuran Acetonitrile 4-Methyl-2-pcntanonc Benzene Tolue~~e Xylenc Isopropanol Octanol 1,4-Dioxane Pyridine Acct’one Cyclohcxnnone
Kurl Fischer
. Spcctrophototnetric
0.007 0.006 0.003 0.019 0.02 0.06 0.012 0.014 0.030 0.02 0.022 0.065 0.09 0.07 0.30 0.23
0.007 0.004 0.002 0.018 0.02 0.06 0.01 1 0.013 0.03 1 0.02 0.02 1 0.07 0.09 0.06 0.30 0.21
water, several.J.,T; Baker Chemical Company solvents were analyzed. Samples of 5 ml were used irrthe analysis of the first three solvents, and 0.3-ml samples ,wer&us&d fdr the analysis of.acetone and cyclohexanone. The,data obtained are shown in Table IV. Included in this Table are Karl Fischer analyses supplied by J. T: Baker Chemical Company. It is’seen that the’agreement ,between the.methods is very good. Tlie:author ‘wishes to thank Mr:Jdhn Killar of the J1 T. Baker Chemical Co. for supplying the,Karl Fischer analyses showri in Table IV, and Mr. L.‘Blitier of these laboratories for performing the atomic, absorption analyses. ‘I
,,. ,‘..
SLJh4klAFk
A ‘new ,method. has .been.developed for the determination of small’ amounts of water in a ‘widd variety .df ,organic solvents, The water in ,the solvent’ is made jto react with potassium dichromate. to produce potassium chromate and ‘tihromic,‘acid. The latter is soluble in the’solvent.,By measuring its amount spectrophotometrically, the,contieritratidn. of water’ present in the,solv&t can be’determined.“:“’ .. .’
,,;: &SUM& ;,/ ,’
,,
.‘,.
.’ ’
I’
,, ‘,
i
‘,’
. ,,
‘..‘;
,.‘:.’ .Une,nduvelle,mCthode est propos& pour le d&age de faibl& qtiantit& ,d’eau dank divers sol&its organiques. L’eau dans le solvant reagit avec le dicliromate;de .potassium pour dormer chromate de potassium et .acide chromique. Ce dernier’est soluble dans le solvant. On peut ainsi d&ermine? la concentration &n,.eau du solvdtit 0, ,: .,, ” % par -spectrophotom&ie. ‘.,
Anal. C&m. ‘A&l, 54 (1971) 447459
SPECTiOPHOTOMETRY
OF WATER
IN ORGANIC
459
SOLVENTS
ZUSAMMENFASSUNG
Es wurde eine neue Methode fiir die Bestimmung kleiner Mengen Wasser in verschiedenen organischen LGsungsmitteln entwickelt. Das Wasser im Liisungsmittel reagiert mit Kaliumdichromat unter Bildung von Kaliumchromat und Chromsgure. Diese ist im LGsungsmittel lijslich ; durch spektrophotometrische Bestimmung ihrer Menge kann die Konzentration von Wasser im Lbungsmittel ermittelt werden. REFERENCES 1 E. B. 1950, 2 G. F. 3 C. L.
SANDELL, Calorimetric Determinutiorr of Trmcs of Metals. 2nd Edn., Intersciencc. p. 390. SMITH, 0. W. Rem AND V. R. HARDY, J. Amer.. Chent. Sot., 54 (1932) 3513. LUKE, And. Chim. Acrcr, 41 (1968) 237.
New
York.
And. Chim. Actrr, 54 (I 97 1) 447459
447 t
DETERMINATION OF SMALL AMOUNTS SOLVENTS BY SPECTROPHOTOMETRIC ,.
OF WATER
IN
ORGANIC
ANALYSIS
C. L. L&E BellTelephone
Laboratories,
Incorporuted,
Murray
Hill, N. J. 07974
(U.S.A.)
(Received 25th September 1970)
Small amounts ‘of water in organic solvents are usually determined by the Karl Fischer method. The latter is based on the reaction between water, iodine, and the anhydride, sulfur dioxide. The method has proved to be very satisfactory. Never-’ theless, the challenge of developing an alternative way of determining water was attractive and it seemed worth-while, as a starting point in the search for a new method, to try substituting’the anhydride, chromium(V1) oxide, for the sulfur dioxide, with the hope that some means could be found for measuring the amount of chromic acid produ’ced. Tests showed that, although the chromium oxide could not be used directly, a successful method for the determination of water could be had by taking advantage of the reaction K2Cr2g,+
H,O
= KZCrqa+H,Cr04
(1)
Thus, if a mixture of dichromate and wet 4-methyl-2-pentanone (MIBK) containing a little .magnesium perchlorate dihydrate, is shaken, the chromic acid produced i,n the reacti.on between dichromate and water dissolves in the solvent and the reaction proceeds to the right by the law of mass action. The dichromate and chro‘mate remain insoluble in the solvent. Some reaction of the solvent with the dichromate occurs to:prbducti,soluble RHCrO, but this side reaction is’usually small. Thus, it is .e.vident that if the &mount of dissolved chromate in ,the solvent is measured, and if correction’ for ,the .RHCr04 is made, it should .be possible to determine the water present. It was found that the dissolved chromate could be convenieritly determined ; spectrophotometrically’. ‘: The reaction bf water with dichromate is slow and long shaking is required to obtain complete reaction. As ,the reaction proceeds, the RHCr04 blank increases in proportion. ‘Eventually, the chromate concentration beco’mes. high *eriough to cause’ som&:oxidation of tlie’solvent;, In view of,this, to limit,the time of shaking and to make it possible to determine the waterspectrophotometrically, it ‘was ‘necessary .to ‘use, conditions where. the;reaction was only about. lo”/, complete., Fortunately, : .,, adequate sensitivity in,the analysis is assured. ‘_/ : Quantitative ,tests -of* the ‘proposed .method showed, as expected, that a ,plot of the weight of water added to dry MIBK uersus the measured absorbancewas linear. This confirms that the m&hod .will be suitable for the determination of water in MIBK. Attention wag next :fodused’on?he problem of adapting the method to the analysis of a variety of other solvents. Organic solvents differ widely in’their behavior and most .of them cannot be analyzed directly. However, a large number of these solvents can Anal. Chtm. Acta, 54 (1971) 447459
c..
448
L. LUKE
be,analyzed, provided that they are suitably diluted with 0.2’/, magnesium perchlorate dihydrate-MIBK solution before the reaction with dichromate. Data showed that, in such analyses, it is possible to determine as little as 0.002°/0 of water, depending on ). the solvent being analyzed. Since organic solvents vary in. their reactivity and thus produce RHCrO, blanks of different magnitude, it is necessary, in the new method, to calibrate for each solvent being analyzed. However, only two points are required, one below and one above the water content of the sample being analyzed. The amount of water-present in the sample can be obtained by simple interpolation, since the graphs are linear. The method calls for considerable attention on the part of the analyst, but as he or she becomes familiar with the techniques required, the average time required for the calibration and analysis of a sample is about 10 min, which is not much more than that required to make an electrometric Karl Fischer analysis. EXPERIMENTAL Apparatus
,. Reciprocal shaker. A reciprocal shaker (Eberbach Corp,,Ann Arbor, Mich.) was used; All solutions were shaken at a rate of 180 oscillations per min., ‘Spectrophototieter. A Beckman Model ,B unit with l-cm absorption cells was used, ,.
Reagents Activated molecular sieve powder. Heat powdered 3A molecular sieve (Union Carbide Co.) at 175’ for 24 h, Cool and protect from moist air. Dry solvents. Transfer 100 ml of.the solvents to be analyzed to 125-ml standard taper,conical ,flasks. Add 5 g of the,dry powdered, 3 A, molecular sieve, stopper. and shake on the. reciprocal shaker for 1, h; Allow the solutions to stand, overnight :to permit settling of the’sieve. If the amount of solvent available is limited, add 10 ml of the solventplus 1.g of 3A.sieve to,a glass,stoppered, 15-ml centrifuge.tube, shake horizontally.for 2 h and then centrifuge. Dispense the clear solutions by pipet, taking care not to,disturb the .powdered sieve on the ,bottom of the. flasks or tubes; Discard any ,solution that :becomes.discolored on prolonged contact with ‘the. sieve. .. Magnesium perchlorate-MIBK solution. Dissolve 1.5 g of: magnesium perchlorate hexahydrate: in 500 *ml. of.:MIBK and.reserve. Dissolve 2.g of anhydrous magnesiumperchlorate in.1 1ofMIBK in a glass-stoppered bottle. Ignore any,insoluble .material. Add 10 g ,of powdered: 3A.sieve; shake, for 1 .h, or, longer .if necessary, on-a ‘reciprocal shaker;. and-filter through :Whatman No: .40,filter paper supported ..on: a large,Millipore fritted ,glass filter,with the aid of suction. ,’ ‘:. ‘t ‘, ‘.v;>kdd 50 mg,of - 400 mesh dichromate to a lo-ml portion of the filtered solution and then analyze, as directed in the Procedure described subsequently, using water! as the,reference solutionThen; if necessary; add small portions of the reserved wet MIBK solution, with:intermittent analysis, until the: blank reading .is 35:to ,45°/0T., If. too mtich,:bfthe yet solution is ,added: by accident, add’ more’dry O.20/0xnagnesium~ptr‘
54 (1971)‘447-459
SPECTROPHOTOMETRY
OF WATER
IN ORGANIC
449
SOLVENTS
nitrogen being used is not water-free, dry it by passage through a tower of &mesh anhydrous magnesium perchlorate. Dispense the solution through a hole in the stopper unit by means of a lo-ml pipet (Fig. 1). ’ Stcmdurd water solution. Transfer 1.0 ml of water to a dry lOO-ml volumetric flask, fill to the mark with ethyl acetate dried with powdered 3A molecular. sieve, and shake vigorously to, assure complete mixing (1 ml = 10 mg H20). In the calibration, dispense this solution from a Gilmont Micrometer Syringe (Cole-Parmer Instrument Company, Chicago). The syringe has a capacity of 2 ml and can deliver as little as 0.002 ml through a stainless steel needle.
10
ml
PIPE1
*
300
ml
DRY NITROGEN
FLASK
Fig. 1. Apparatusfor dispensing prepared magnesium pcrchlorate-MIBK
solution.
Shake fme-mesh, crystalline potassium ;Fine-mesh pbtassium dichromate. dichromate on a loo-mesh screen and reserve the - 100 mesh salt that passes through the screen; Then pass a portion of the + 100 mesh salt,successively through lOO-,200and 400-mesh screens by ,rubbing the salt on the appropriate screen with the bottom of aX250-ml beaker. Reserve a portion ofeach mesh-size salt, Dry the crystalline - 100 and the,crushed..lOO-200, .200-400 and’-400 mesh salt in a 75’ vacuum oven. ~Reference solutions. Dissolve 0.1 g of potassium.dichromate in 100 ml of water and mix. Dilute,1 ‘and 5 ml of this solution to lOO,ml with water and,mix. Call these ” solutions 1 and 5% dichromate ieference solutidns: I .,’ pr&&&
,
‘.
.’
.’
“,
c
(.
.‘.
Sweep out two dozen or more dry, 50-ml, 19/38 standard taper conical,flasks Anal.Chim.
Acta, 54 (1971) 447459
c. L. LUKE
450
with dry nitrogen, stopper, and reserve for the analyses. Transfer 10.0 ml of adjusted 0.2% magnesium perchlorate-MIBK solution,to 3 of the flasks. Add 1.0 ml ,of the,dry solvent to be analyzed to flasks 1 and 2 and the same amount ofthe wet solvent,to‘be analyzed, to flask 3. Then add 0.1 ml of standard water solution (10 mg ,HbO/ml) to : flask 2 and analyze the 3 solutions consecutively in the,order-flasks 1; 2 and’3. Add.50.0 mg of - 400mesh potassium dichromate from a 30-ml beaker,,through a small~funnel .to the flask, tapping,the funnel vigorously with the inverted.beaker to make sure that’all the salt enters the.flask. Stopper, grasp the flask, while holding the stopper in place with the index finger, invert the flask, extend the arm down in the normal resting position and then shake vigorously and almost vertically, so that the solution hits the top and bottom of the flask, for exactly 1 min.
- RESERVOIR
CLAMP
UCTION
50 ml FLASK
Fig. 2. Filtration
apparatus. ,.
the solution immediately, with strong’suction, using a small Millipore ,ftiter funnel,(Fig. 2), through a dry 25Smm disk:of Whatman No. 40 ftiter paper.that has been-cut:.outof.a.number of sheetsof the paper with a,die. Catch the filtrate in. a SO-ml suction., flask. : Withoughdelay; fill. a l-cm absorption cell with. the : ftitered solution,:and measyre’ the percentage, transmittance on..a suitable’ spectrophotometer at :370 nm: Use water or the’appropriate dichromate solution as .the reference solution, in order thatareading’of not :lessthan about .35%T. for.flask ~No&is obtained.. For ready ‘accessibility;. water., and the two reference solutions-.should be -contained,% absorption, cells ,placed, in a multiple compartment, holderin’the spectrophotometer., Convert OAT to ,absorbance (A) by referring to a suitable table that reads in units of 0.2%T, and.then,;for the’sakeiof convenience, multiply A by 100 to obtain thevalues ; ,. of..IOO~*;.;:,.,,;;;, “‘,.; :,...‘,,:.:,;,.. ,,.‘,,, ;; ,’ :,, ; ‘).S’. :: .; ,’ ‘, ,. ; ‘, .. :. : :. ,,Filter
.Anal;.Chim; ‘Acta; 54.(1971) 447-459
451 . ‘~Ci&~&iti&& i ”
_’
If..l,OOA-for flzisk ,E%-,l OOAf&r flask ,l = B ‘f. l(KM for, flask.2 9 lOO,$ for flask 1 = C i,.ml ofsolvent taken for ,analysis = D ,, solvent being analyzed = E ,%‘water in the solvent. ‘,.
#Note .
It is’&venient,to use a,series’of 6 filter units which’can be loaded with.paper disks z&d be ready for insertion into the neck of,the .50-m] lilt$r flask. After the spectrdfih’otometric reading’of,the solution from flasks ;l, discard the s.&ions and shake :the.fil~er:fktsk and,absorption cell to remove as much of the’solvent as possible before p&&ding t6 : the .-k&t ahalysis. After, co+pletion of, .the third analysis, ,wash out ‘Ithe:twd‘:vessel~:with ‘a’ stream ,bf acetone, and dry ‘them with a jet of compressed air. &fter’~all ‘-‘of‘the, ,filf& .,\;nits‘have ‘been used+ demount’ ,them, loosen the‘ dichromate ::‘on.‘the ri.t&of the’reservoiis and the,n was,h. the reservoirs aud the.frit supports with ,,:acktone.,Xy’and reassemble.‘Wash the used SO-ml conical,flasks with water,‘and acei :,tone.~‘Wash;.theS.reserv’oirs and frit &pports occasionally with hydrochloric acid to’ ,: ,’ .&moire, chromate, ion. .’ ‘. ‘,
DISCUSSION
When ,a drop of, water, is added ,to a ‘small portion of fine-mesh, potassium dichidmate;‘h;l[IBK,.is’aiided arid the!mixture is shaken, ,little or no chromic acid ap: ,pear$.‘in the tiIBK.’ If tlie.experiment is reijeated but, a small ,amo,unt of an, anhydrous ,hjigrdscopic~alk8li.,,or~~lkaline earth ‘chloride.or, perchlorate is present with the di,chro~~‘~e’;‘t.~ejeaction of.kqn.:i~j.ocdui~:and.the’~range cdlor,of chfomic acid.@an be .s~~~,.in_ihe:‘MIIj,~.,,~,he,rdie”played, by. the, hygros,copic,salt is n&t un,derstocd.‘If the il~tferlexpejtiment ;iS repeated but-the wateri_is,,di&olved in’the ‘MIBK before mixing, .,wit,bthe ,,diChromate :plus’,h$grosccpic. salt:. only magn:esium perchlorate will Induce 1the.reactiii~.oi:k’cj~.‘“(i’)‘to any al5ljreciable,exte+ Apparently this strongly hygroscopic ., .’ ‘cfait,cati.break,.jhe,-bdnding between the’water’and MIBK, atid al1o.w the,reaction to ~q~~‘c~~~.:‘,:~~:.:.::~.~,~~,;;~..:~,~,,..:.~: ,‘i-.:.,,‘.‘.~~~ ;.,.,,_ ,, , : ‘:‘... .’ _‘:,:~,,:,:‘~,,_‘,;, ,) ‘., ::, ‘.’ ‘, ; ,,,‘, ‘) ‘,.,” ;. ,‘,,’ i, ‘. :ti;f the, magnesium, perchlorate ,lu,the,’ MIBK influences (’ :.Y ; ‘;I: -:. The,.concentratiou h@ extent {of’reaction:;This ,can, be’seen. in tbe”data” i.u ,Fig. 3;’ To obtain these data, O;O~,:,d,i::$nb:2,00/,‘,slilutiohs ‘of ‘ahhydrous magtiesium p&chlorate inJ,~IB,K ‘were ~drie$~fz$rly:well with! 3A’nioleculir~.‘sieve, ‘and: then ‘$l,O-ml;portions of the’ solutions ,‘~e~~~,inluied’,~~i~~, vai%us’ amqunts. of standard ,,water, ,solution ,and ,analy?ed. by the ~~p~~~~dzi~~~“.~~~~.or-..:-;,46q.mesh”crus’hed dichromate being used.;,F?rom curves 1 ‘and’:,3’:&he ,,Fig; : it 3s seen, thata 0:2°/~‘%olutio~~~isopthnum:~ for the, analysis, since ili~~~ritL..b,~~\Neen: ,water added and’ net~abstirbance isobtained .ov,er ,an,‘appreciable &&g&’af.water:.cc;;ntent’;arid’ the: &&Gtivit~ :is! high; The,’ graph,:for the, .0.2% sol,ution “;,~b~~dt’b~~i;r;le,lilnka~u;itii:a lOOA;value of&out:30 is obtained Thedata,$us other ~~gi~ialr;.da~a,‘~o~ta~neci,;.~it~i::co;itipletelgi’:dehydrated:0.2O/; &agnesium. pemhldratei~~iB’~.isol~t~~~~~~~~~;t~ fhat;until approximately.the’dihydrate has’been,formed, the’ ,j”~~~~~,Ithat”~i;is~de~n ‘freed, from the bonding of the MIBK, cannot react,nor,m!lly with ., 1,.(’ ; ,/’ ‘. _I.,:, .v, \ ,, .,, ‘,
..,‘:
Anal. ‘clli,;li,:act(l.‘54’(1671i44j-45~ ‘. ,, .. :
452
‘, ‘. .,’ : ‘.
‘, 110’ :
., ,
‘,I
I
I 0.2%
.’
;
,
,,’
., 1
No. 1 (-400 MESH 1
1
I
SPECTROPHOTOMETRY
OF WATER
IN ORGANIC
SOLVENTS
453
the more dichroinate used, the smaller the mesh size of the dichromate, the longer the time of shaking and the greater the intensity of shaking. .. In order to obtain reproducible analyses, the shaking of the dichromate-solvent mixtures .must be ‘reproducible. Attempts to use a wrist action or ‘reciprocal action mechanical shaker failed,, but it was shown that reasonably good reproducibility could be obtained.by hand shaking; In order to reduce the labor involved, it is desirable to limit the time,of shaking to 1 min. When this is done, it is necessary to use finelydivided dichromate in order to obtain a reasonably high net absorbanoe From the water. In the Procedure, - 400 mesh salt is recommended but, in most cases, the more easily handled 200-400 mesh salt can be used instead. A crushed salt has a greater surface area than a crystalline salt of similar mesh size, probably because of the presence of dust that cannot be removed in the sieving. The effect of the mesh size of the dichromate on the extent of reaction can be seen in Fig. 4 where calibration graphs, obtained with adjusted O.20/0 magnesium perchlorate-MIBK solution and crushed dichromate of different mesh sizes, are given. SO .
,.
0 ,.,,
Fig. 4. Effect of mesh
-400
MESH
4.0
:W&3AOOED3%. size of dichroin~te.
Anal. Chim. ,Acta, 54 (1971) 447-459
454
C. L. LUKE,
From the Fig., it is seen that linearity is’obtained in all cases and that the RHCr04 blank and the net absorbance from a given amount of water are proportional to the surface area of the dichromate. Because ofthenature ofthe new method for water; which involves an incomplete reaction between a solid and.a liquid, all variables must be closely controlledIf this is done, fairly good reproducibility and accuracy can be obtained. Data obtained with - 400 mesh dichromate (Table I) showed that the reproducibility is reasonably good. The data in Fig.%and in Table I are different because they were obtained with different batches of adjusted magnesium perchlorate-MIBK solution. TABLE DATA
Water
I
ON REPRODUCII3ILI’l-Y
irkfed (fno)
(100 A)
Average
0
43.7 43.2 39.1
42
0.5
61.3 61.3 60.6
61
1.0
82.4 75.9 78.5
79
Behavior of solvents In the determination of a given amount of water in non-polar (non-reactive) solventsthe total amount of chromate ion and the ratio of RHCrO, to H2C!r04 extracted by the, IVIIBK varies from solvent to solvent because their reactivity and their ability to bond with water varies. The variation is not great and the extent of reaction approximates that obtained in MIBK alone. The great majority of organic solvents can be analyzed on a l-ml sample, with - 400 mesh dichromate. The Pioc& dure’abpve has been written to provide, for these analyses. Whenever the solvent will permit, up’to 5 ‘ml of the sample should be used if,the water content is less ,than 0.01%. On the other hand, if the water content of the sample is greater than 6.1.x the,sample size must,be reduced so that no more than 1 mg of water is present, or the amount of water added to flask 2 must be increased se that it exceeds, by a ‘small margin, the amount present in flask 3. In this latter case, if the transmittance for flask 2 is smaller than lo%, dichromate of coarser mesh should be used in the analysis; In any event,.it is not advisable to take a sample for analysis that contains more than about 3 mg of water. ., Small samples of reactive solvents can’be analyzed with 200-400 or ‘- 400 mesh .dichromate but, if larger samples are required to provide .adequate sensitivity,.dichromate of correspondingly coarser mesh must be used to control the size c$ the’ blank. Thus;as ,much as 5 ml of the reactive’solvent adetonitrile ,cari be analyzed provi.ded that 1001200,mesh crystalline dichromate is used. Regardless of the reference solution,‘used; the blank transmittance should not.be lower than, 35”j/,T. .. .. Anal; C/h.
Acta;’ S4,(1971)
447459
SPECTROPHOTOMETRY
OF WATER
IN ORGANIC
SOLVENTS
455
Solvents such as methanol, methyl acetate and Z-methoxyethanol cannot be analyzed because they readily react with dichromate and also form such a strong bond with water that the magnesium perchlorate is ineffective. Hence the blanks are high and the’net’gbsorbance caused by water is low or negative. The same is true, to a lesser extent, ‘with solvents such as ally1 alcohol, valeronitrile, ethanol and a few others. To analyze these latter solvents, it is necessary’ to limit the sample size. A few solvents tend to reduce chromate and hence cause the color to fade in the spectrophotometric measurement. In most instances the fading is slow and can be compensated for in the.calibration. However, when fading is more rapid, as it is in the analysis of 2-butoxyethanol, the sample size must not exceed 0.25 ml. The use of a coarser mesh dichromate is not as effective. ‘Solvents such as’pyridine, tri-pz-butylphosphate, the higher alcohols and acetate esters, propionic anhydride and, to a lesser extent, 1,4-dioxane, tend to suppress the reaction of water with dichromate, Hence, in the analysis of some of these solvents, 0.1 g of -400 mesh dichromate should be used and the solution should be shaken for 2 min in order to obtain an adequate reaction. In the analysis of viscous solvents such as cyclohexanone or the higher alcohols, the mixed solvents in flask 3 should be allowed to stand for at l&st 10 min before adding the dichromate, to allow the solvent to become thoroughly dispersed in the MIBK.’ The above discussion may suggest that the ney method is not practica1, bul the fact remains that the number of solvents that require special handling*is not great and experience with the method will make it possible to predict rather closely the sample size and.mesh size of the dichromate that should be used for the analysis of a particular’solvent. :. It is obvious, from what has been said, that the various factors of reactivity of the solvent,,sample size, and mesh size of the dichromate are interrelated. Moreover, it is clear why it is necessary to calibrate for each solvent being analyzed. Attempts to eliminate the need for this individual calibration by scavenging or sequestering the water from the solvents in a state where it could then be analyzed by the reaction of eqn. (1) were not successful. The sequestermg agents used were molecular sieve 3A, silica gel, calcium sulfate and magnesium perchlorate. .,’
Solvents
which can be’analyzed i, The method has,been tested qualitatively dn more than 125 solvents that were chosen at random from those available in the laboratory. In most of this work, a l-ml portion of the dry,,solvent plus 0 or 1 mg of water was analyzed, with -400 crushed #chromate.’ It was found that all of the alcohols and their acetate esters from ethanol td octanol; iii&&ig their isomers, could’be’analyzed. TeitLbutanol had to be rnaint,iin+l’. at, ‘3&3,5,0’duiitig the drying and pipeting in ,order to keep it, from solidifying. Other solvents’tested that appear to’be’capable of beii@ analyzed are listed in Table 11;,: Future work’tiill u~~~~~~edl~ show thata great many. others c&n also’be analyzed. ‘The S&itivity ‘efariaiysis of&ganic solvents dependson the sample size used. The permissible maximum Ample size depends ‘& ‘the solve&’ being analyzed. It has been .shown that ,5-ml samples of many of the higher alcohols. and acetate esters and a’large number of the low or moderately reactive solvents listed in Table II can be analyzed;,This means that. these solvents cari be analyzed for as little as,0.002”/0 of ,
Anal. C&n. Acta; 54 (1971) 447-459
456 TABLE !SOLVllNTS
C. L. LUKE, II WHICH
CAN
DE ANALYZED
Acetone 4-Methyl-2-pentanone 5-Methyl-2-hexanone 2-Pentanone 3-Heptanone 4-Heptanonc 2-Octanone Methyl ethyl ketone Acetophcnonc 3,3-Dimethyl-2-butanone 2,4- Pentanedionc 4:Hydroxy-4-methyl-pentanone Ally1 acetate Methyl bromoacetate Methyl dcetoacctatc Methyl sulfite Methyl salicylate Ethyl bromdacetatc Ethyl chloroacqtate Ethyl trichloroacetate Ethyl cyanoacetatc Ethyl format&’ Ethyl,sulfidc. 2-Methotiyethanol acetate 2-Ethoxyethanol,atietate 2-Butoxycthanol acetate Tri-rr-butyl phosphate Phenethyl alcohol Ally1 alcbhol” B&zyl’alcdhol a,c&Dimeth~llienzyl alcohol 2-Ethqxyethnnol
2-Butoxycthanol” 2-Methyl-2-hexanol p-Mcnth- 1-en-S-o1 Acetic anhydride Propionic anhy,dride” Propionaldehyde” Benza!dchyde Butyraldehydc Chloroform Carbon tctrachloride 1,2-Dichloroethane Trichloroethylcne Tctrachlorocthane Isoamyl chloride Bchzyl chloride Bromobcnzene Fluorobenzene o-Chloropheriol 3-Chloropentcne Bromotridhloromethane Hexachloro-1.3-butadicne a,cr,a-Trichlorotqlucne Hexachloro-i-propanonc 2-Bromobu&e 1,4-Dioxane Ethyl ether Isopropyl ether Bis(2-methoxyethyl)ethet& 1,2-Bis(2-mcthoxyethoxy)cthane Trim&y1 orthoacetatea Pyridine
2-Ethylpyridine” 4-Ethylpyridine Tetrahydrofuran Tetrahydro-2-methylfuran Petroleum ether Kerosene Gnsolinc Hexane 3-Heptane 3-Heptene I-Hexene n-Nonane n-Octane Cyclohexgne 3-Methylpentane ’ I-Methyl-l-cyclopentene Benzene Toluene Xylene Tetrahydronaphthalene Acctonitrile : Acrylonitrile Propionitrile” Valeronitrile” “’ Benzonkrile Carbon disulfide Acetone dimcthylacetal l,l-Dimethoxycthanc Nikomethane Tuqkntine tert-Butylbcnzcnc
-“ 9.2%ml sample
tal;&
‘for analysis. .’
water. On the other hand, not much more than 0.5 ml ofethanol and nitromethane, or 0.25 ml of solvents which yield low net absorbance or which reduce chromic acid can be analyied; In the,analysis of 0.5 ml of ethanol and 1 ml,of acetonitrile or acetone, it is necessary, to’ use, 2004400 mesh dichromate to control the size ,of the blank.
Determination ‘0s water. in soltible’ and insoluble organic, and -inorganic solids by the : ,; ..-,, ; ,,/. .. ,. ‘. :. proposed: method; ;’ ., .:’ .’ ’ “It’seems.probable that itwill be possible to determine water in various~organic.. Anal. Chirn.
‘Acch,~54~(1971) 447459
SPECTROPHOTOMETRY TABLE SOLVENTS
OF WATER
IN ORGANIC
457
SOLVENTS
III WHICH
CANNOT
Methanol Methyl acetate Ethyl dichloroacetate Acetic acid Acetyl acetone 2-Pyrrolidinone I-Methyl-2-pyrrofidinone
BE ANALYZED
Piperidine Dicyclohexamine DiethanoIamine Triethanolamine Triethylamine 1,3_Propanediamine Ni trobenzenc
tt-Methylformamide Dimethylformamide Ethyleneglycol 2-Methoxyethanol Dimethylsulfoxide Paraldehydc cr-Methylstyrenc -.
or inorganic solid compounds if they can be dissolved in a suitable solvent and if they do not interfere in the analysis. Moreover, in certain instances, it should be possible to extract water from various insoluble solids by refluxing with a suitable dry solvent and then determining the water in the solvent by the proposed method. Other methods for determining the chromate in MIBK The spectrophotometric method cannot be used ‘for the analysis of colored solvents which absorb strongly at 370 nm. Fortunately, the number of such solvents is limited. Moreover, as long as such absorption is not excessive it can be compensated for in the calibration. In the present work, the only colored solvent tested that could not be analyzed was nitrobenzene. However, it was shown that this solvent could be analyzed by X-ray analysis. To do this, proceed as directed in the spectrophotometric method but use an adjusted O.OSO/;lmagnesium perchlorate-MIBK solution. Then add 0.5 ml of a loi0 solution of anhydrous lead acetate in methanol to the solution after filtration to remove dichromate, allow the solution to stand for 1 min, filter through a dry 2%mm, 7-m nylon Millipore disk, allow the disk to dry, using suction, -and determine the,lead in the precipitate by X-ray analysis?. The X-ray method is not as widely applicable as the ,spectrophotometric method ‘because a few solvents (c.g.*ethyl formate, methyl sulfite) precipitate lead and. a few others cause the precipitate of lead chromate to: be colloidal. Moreover, in the X-ray method, the total amount of chromate ion, that can be handled is limited by the X-ray counter. The counts per second for the precipitate on a disk must not exceed about 20,000. In meeting this limit, it is not uncommon to find that the net count from water is low when very reactive solvents are analyzed. On the other hand, in the spectrophotometric method ,it.is possible to compensate for large blanks by using a suitable reference solution. This makesSit possible deliberately to use conditions which produce a large blank,.in order, to obtain an adequate net absorbance due to water. ,, . . Another approach to the analysis of colored soivents that was tested, consisted of aspirating the filtered I$IBK,solution.directly into the flame of an atomic absorption spectrometer for the analysis ofchromium. This method has the advantage that there’ is no need to work quickly, since the reduction of chromate is of no eonsequence. On the other hand, it was found that most solvents, other than MIBK, tend to suppress: the chromium radiation in, the flame. For this reason ,the method has limited value.: .. Q~~‘~titative~,analysis: of: commercial solvents In order to evaluate the new method
for the quanti~tive
determination
of
Anuf,Claim, Acta, 54 (197 1) 447459
458
c. L. LUKE
TABLE
‘IV
DETEHMINATION
OF WATER
IN COMMERCIAL
Sulve,rt
SOLVENTS
“/* Wuter found
1,2-Dichlorocthane Trichlorocthylcnc Ethyl cthcr Ethyl acetate Tctrahydrofuran Acetonitrile 4-Methyl-2-pcntanonc Benzene Tolue~~e Xylenc Isopropanol Octanol 1,4-Dioxane Pyridine Acct’one Cyclohcxnnone
Kurl Fischer
. Spcctrophototnetric
0.007 0.006 0.003 0.019 0.02 0.06 0.012 0.014 0.030 0.02 0.022 0.065 0.09 0.07 0.30 0.23
0.007 0.004 0.002 0.018 0.02 0.06 0.01 1 0.013 0.03 1 0.02 0.02 1 0.07 0.09 0.06 0.30 0.21
water, several.J.,T; Baker Chemical Company solvents were analyzed. Samples of 5 ml were used irrthe analysis of the first three solvents, and 0.3-ml samples ,wer&us&d fdr the analysis of.acetone and cyclohexanone. The,data obtained are shown in Table IV. Included in this Table are Karl Fischer analyses supplied by J. T: Baker Chemical Company. It is’seen that the’agreement ,between the.methods is very good. Tlie:author ‘wishes to thank Mr:Jdhn Killar of the J1 T. Baker Chemical Co. for supplying the,Karl Fischer analyses showri in Table IV, and Mr. L.‘Blitier of these laboratories for performing the atomic, absorption analyses. ‘I
,,. ,‘..
SLJh4klAFk
A ‘new ,method. has .been.developed for the determination of small’ amounts of water in a ‘widd variety .df ,organic solvents, The water in ,the solvent’ is made jto react with potassium dichromate. to produce potassium chromate and ‘tihromic,‘acid. The latter is soluble in the’solvent.,By measuring its amount spectrophotometrically, the,contieritratidn. of water’ present in the,solv&t can be’determined.“:“’ .. .’
,,;: &SUM& ;,/ ,’
,,
.‘,.
.’ ’
I’
,, ‘,
i
‘,’
. ,,
‘..‘;
,.‘:.’ .Une,nduvelle,mCthode est propos& pour le d&age de faibl& qtiantit& ,d’eau dank divers sol&its organiques. L’eau dans le solvant reagit avec le dicliromate;de .potassium pour dormer chromate de potassium et .acide chromique. Ce dernier’est soluble dans le solvant. On peut ainsi d&ermine? la concentration &n,.eau du solvdtit 0, ,: .,, ” % par -spectrophotom&ie. ‘.,
Anal. C&m. ‘A&l, 54 (1971) 447459
SPECTiOPHOTOMETRY
OF WATER
IN ORGANIC
459
SOLVENTS
ZUSAMMENFASSUNG
Es wurde eine neue Methode fiir die Bestimmung kleiner Mengen Wasser in verschiedenen organischen LGsungsmitteln entwickelt. Das Wasser im Liisungsmittel reagiert mit Kaliumdichromat unter Bildung von Kaliumchromat und Chromsgure. Diese ist im LGsungsmittel lijslich ; durch spektrophotometrische Bestimmung ihrer Menge kann die Konzentration von Wasser im Lbungsmittel ermittelt werden. REFERENCES 1 E. B. 1950, 2 G. F. 3 C. L.
SANDELL, Calorimetric Determinutiorr of Trmcs of Metals. 2nd Edn., Intersciencc. p. 390. SMITH, 0. W. Rem AND V. R. HARDY, J. Amer.. Chent. Sot., 54 (1932) 3513. LUKE, And. Chim. Acrcr, 41 (1968) 237.
New
York.
And. Chim. Actrr, 54 (I 97 1) 447459