An Automatic Hydrogen Fluoride Recorder Proposed for Industrial Hygiene and Stack Monitoring DONALD F. A D A M Division o f Industrial Research, Washington State University, Pullman, Wash.
b The increasing use of fluorine and fluorides in industrial processes requires close surveillance of the fluoride content of the air in working areas and adjacent environs. ConventionaI sampling and analytical techniques are time consuming and are not suitable for an alarm system to warn personnel of the presence of obiectionable exposure levels. A method is needed for continuously and instantaneously recording changes in the fluoride content of both indoor and outdoor air. High fluoride levels in the range 0.1 to 200 mg. per cubic meter are determined by measuring the color change produced by the reaction between the green ironferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid) complex and fluoride ions using an automatic recorder.
T
of fluorine and fluorides in electrochemical and thermochemical processes, fluorocarbon synthcses, gaseous diffusion procrsses, rocket fuel oxidation. pctroleuni refining. aerosol packaging. etc., requires close surveillance of the fluoride content of the air in the working areas and the adjacent environs. Thus, many industrial air pollution specislists, rocket rngincers. and intlusti i d hygirnists h a w been called upon to monitor for airborne fluorides in the concentration range of 0.1 to 200 mg. per cubic meter. I n techniques coniniorily uscd to determine the levels of fluoritlc, air is dran-n through absorption qolutions for varying periods of time. Thcw solutions are then chemically analyzed in the laboratory. Such procedures are laborious and time consuming. I n many situations, analytical data obtained several hours or days after the sampling may have lost significance, diereas an immediate and continuous recording of the atmospheric levels may be required for most effective use of manpoirrr or for efficient procc~ss control. Conventional sampling and analytical techniques are not suited for use with an alarm system to warn personnel of the presence of objectionable exposure levels. A d a m et al. (1, 5) described a prototype of a versatile, modified dosimeter HE ISCRE.4YISG USE:
1312
ANALYTICAL CHEMISTRY
which is potentially applicable to the automatic coloriinrtric determination of any gaseous air pollutant. Recently, Adanis and Koppe ( 3 ) improved and miniaturized this prototype. This instrument is referred to as the hliniAdak. Reagents were reported for the determination of the fluoride ion in the atmospheric concentration range of 0.2 to 200 pg. per cubic meter (3) and for sulfur dioxide in the range of 0.1 to 5 p.p.ni. ( I ) . Since conipletioii of this study. Howard and Weber (8) described a portable, continuous analyzer for gaseous fluorides in industrial environments which measures the internal electrolysis occurring in an aluminumplatinum galvanic cell in the presence of fluoride. A survey of the colorimetric procedures for milligram concentrations of fluorides suggested a method based on the rraction of fluoride ions n-ith the green complex produced betn-een iron and 8-hydrosy-7-iodo-5-quinolinesulfonic acid (ferron) as first reported by Yoe (11). Further data on the ironferron color complex were reportcd later (6, 9, 1 2 ) . This paper proposes the automatic determination of high fluoride levels in the range of 0.1 to 200 mg. per cubic meter by measuring the color change produced by the reaction between the green iron-ferron complex and fluoride ions.
reagents in the ;\lmi-.ldak analyzer. Changing from one reagent to another requires a simple exchange of optical filters and cleaning and rinsing of the reagent system. X Corning 640 mp optical filter is used 11ith the iron-ferron reagent. The absorbance scan of this filter and the iron-ferron reagent with and without fluoride are shown in Figure 1. Other suitable automatic colorinietric analyzers might also be adapted for use with these reagents.
EXPERIMENTAL
Data lia:.r been presented (6, 9, l a ) concerning thc effect of diverse ions on the green complex. Of the ions reported :md possibly present in polluted air and emission streams, only Fef3. SOa-?, :tnd PO40 appeared to offer serious limibations to the application of the iron-frrron complex in the direct sampling of complex atmospheres containing high concent'rations of fluoride. Some disagreement' was noted among the (lata coiirerning the interference of Alf3 and on the iron-ferron complex as well as the importance of pH control. Examination of the experimental details indicated that' t'he ironfcrron ii-oiglit rat'io used by Fahey (6) was ~pprosimatelj- 1: 10; Sn-ank and Jlellon ( 9 ) , 1:40; and Yoe ( l a ) , a rangc of ratios from 1: 6.7 to 1:80. Interference of Sulfite Ion. Because sulfur dioxide could be present' with fluorides in a polluted atmosphere, tho sulfite resistance of the REAGENTS rcagrnt appeared to be a critical Preparation, 1 to 3 Ratio. FERRIC (,onsideration. Sulfur dioside readily reduces the ferric iron present in the CHLORIDE. Dissolve 9.734 grams ot FeC& 6H20 in 172 ml. of conceniron-forron reagent. thus bleaching trated HCI and dilute t o 1 liter. t h r glcen c d o r of the complex. The FERRON.Dissolve 1 gram of 7-iodoaddition of a number of oxidants such 8-hydroxy-5-quinoline sulfonic acid in 1 as prmulfate and hydrogen peroside liter of boiling water. Cool, filter, did not increase the resistance of the and make up to volume. iron-ferron green complex to the sulfite SODIUM TETRACHLOROMERCURATE. ion s:itisfsctorily. Dissolve 41.2 grams of mercury(I1) The sulfur dioxide sampling method chloride and 17.7 grams of sodium proposed by West and Gaeke (10) chloride in distilled water and dilute to 1 liter. involved the complexing of the sulfur RECORDER RIAGIXT. X i x 20 ml. of dioxide with the tetrachloromercurate ferric chloride, 120 ml. of ferron, and ion. The disulfite complex minimized dilute t o 1 liter with sodium tetrathe auto-oxidation of sulfur dioxide chloromrrcuratr. -idjust the final pH collectcd in an alkaline absorption to 2.0. liquid. An iron-ferron reagent conThis reagent may be used intertaining t,ctrachloromercurate might rapchangeably m5th the previously deidly form R stable disulfite complex with scribed low fluoride and sulfur dioxide
30:
IRON-FERRON Y -
RATIO 1:40 1:20 1:10
A - 1:5 !3 0
25-
0
//
1- 1:1.5
*-
q I5
4 -1
350
400
450
500
550
600
650
700
750
LENGTH IN my
/
100 ma. AI"'/
25 rrl
PH
Figure 2.
Effect of pH on aluminum interference
Figure 1 . Absorption spectrum of ferron reagent and optical filter
+~mpled sulfur tiioxitie : i d thus niinimize or prevent thr redox reaction l~ctwcenthe ferric and the sulfite ions. Sodium tetrachloroniercurate n-:is added to the reagent t'o produce a f i l i d coiiceritration of 0.13-11 incrciirate. l'fici addition of tet,racl-iioromrrciirate g i ~ ~ i t lincreases y t'hc t,olcr:mcc of the rcagrnt to sulfur diosido. dthough it not completely rlirninatc the pnstj. of SOn interferencv. Interference of Io Reagent Conditions. fcrr,t~ii reagents was \\.it11 various iion-fcmcin ratios (by waiglit) including 1 : I . 5 , 1 : 3. 1 :5, 1 : 10. 1 :20, and 1:40 ~ ~ ~ n t : i i n i ithe ig tcti~:ic~liloromc~rcuratc ion. I3ach reagriit \vas prcp:ircd so :IS to have a~~pi~osiinstcly the s:inic initkt1 ahsorbaiicv. IGacli of t,hc various ixtios \v:is p 1 ~ ~ ~ ) a rtoc d four initi:il :icitlit'ics, pH I..;. 2.0. 2.5, and 3.0. ' I ' l i ~ dcgwe of intc.i,fercnce with the i!.ciri-fcrroii-iucrcvii,:itc iwgent producc.(l 1)tlic. :iddition of 1'04-3( 1 , 2: and 4 nig.), SO3+ (3, 5 , and 10 mg.), and ..UT:' (20. 50, nn(l 100 ing.),, n - ~ dcttwiiiiicd s hy iiicasw i n g t1-w changc in ahsorh:i!ic~cof 25 ml. of * ~ c ratio h reagent :tt tlic four pII va1iic.s. Sevrrd of tlicw c,oiirlitioiis ~vcrv ~ni:ittnin:ible with tho 1 : 1 ,,j :tiid 1 : 3 ratii) rcxgmts becaurc. of j)i,t!cipit%t,i(in of citiiw ferric hj-drosit-iv or pho In :tdtlition, the fluorid(, scwiti e:icIi r t q e n t a t c w l i 1)H 1wr1 W:IS siniiI:~rly detcmiinetl b!. tlw :itltiitioii of 1 , 2, and 3 mg. of fluoi,itl(x. Aluminum Ion. T h i 3 ti:it:i slioiving t l i v iiitcrfercnce of .41A3 :m plottcd in Figurcl 2. The adtlitioii of 100 nig. of to 25 nil. of rc:igent l i d nn incrcwingly greatc,r effect upon thr color of tlw iron-fclrron c~mplr:, :IS t h r iroiif r i v i i i ra tiri \vas c!iangc~l irici~c~inri it:\ 1I>f i . i i i n 1 : I . 5 to 1 : 10. -4t ratios of 1 : 20 t h i s
a i d 1:40, the influcmcc of AI + 3 bccaiiie less. Hoivevcr, the intcrferencc nith the reagent \vas still greater than was found with ratios betj\vc,en 1 : 1.5 and 1: 3 . These data also sliow that the d l + 3 interference increases :is the pH of the reagcmt is changed from 1.5 to 3.0. Parallel families of curves \yere obtained from t'he addition of 20 :md 50 mg. of Ai-3. Sulfite Ion. T h c ~ (lata showing tlie change in interfci,cnce produced by the addition of 10 mg. of SOS-2 to 25 nil. of reagent with chmging pH and iron-fcrron ratio arc plott,ed in Figure 3. Similar f:tniilic~~of curves were obtained by t,hcL addit'ion of 2 and 5 mg. of SOa-2. These rl:it,a stioiv a generally iiwmsing interference with the iron-fcwon color as tlic iron-ferron ratio \vas ch:mgcd from 1 : 1.5 to 1 :40. The SO3-* intcrfercncc, tlccrcmed as the pH of the reagent w:is climiged from 1,s
to 3.0. Phosphate Ion. Pliosphatv int,erferchncr was reduwd similarly as tlie iron-ferron ratio \vas ch:ingcd from 1 : 1.5 to 1:40. Figure 4 sho\vs the dtlgrec of interference resulting from the sdtlition of 4 nig. of P04-3 to 25 ml. of reagent. Similar f;tmilii,s of ciirws \~.crcobtained from thc :irldit,ion of 1 and 2 1ng. of 1'04-8. Effect of pH. The effciot of pH ul)on tlic color of t h r iroii-feri,on reagent a t thc a i s ratios stndicd is shon-n i n Figurc 5 . Thcw data s h o n that i.onsid(si,:il)l('tliffrrriic*c.sin change of alisorlianc*c (>xist :IS t h e pH of the various iron-feriwi ixtio rcagrnts is clinngrti from 1.5 to 3. Masimum color stability \vit,li change in pH \v:is found in the iron-fcrrori mt'io rang(' of 1 : 1 , 5 t o 1:5. r 7 I hc. varying rtxlationships bct\veen t!ie iron-ferron ratio and pH with tlie
degrc,cl of interf(1reiirc produced b j .41-3, YO3->, and PO4+ undoubtcdly explain the contradictory reports in the litrratul r concerning interfeiing ions and the ~c~lativc significance of pH control. thorough cv:aluation i q precludd because of the unavaihbilitj of complete information concerning all of the pH and iron-ferron 1 atios used in reaching tliesc varying conclusions. Thc addition of a potassium acid phthalate buffer wili aid in maintaining a constant pH during the air sampling, ho\{erer, this buffer is not compatible with the 1 : 1 . 5 ratio reagent. Fluoride Sensitivity. pnrallcl study v a s made t o detcrminc, t h e rclatirc. fluoridc smsitivity of the sei-era1 lion-fnron ratio icagcnts a t til? four c h o ~ c l l pII T~allle,. on(. milligiani of fluoiitic. ion p r r 23 inl. of icagcwt 1\23 used. l ' h i w (lata arc portiaycd i n I'igiir(x* 6 and 7 . Incieawig fluoiidc sonsitivit! \\ as obtained as t h e iron-feiron r a t 1' 0 \ \ a s incrcascd from 1 1 .!I to I : I O tind then decreased again at ratios of 1 . 2 0 and 1:40. The preparation uf the most appropriate iron-ferron reagent to bc used for the determination of the conccntration of fluoride in the air under all possible conditions cannot be defined in terms of a single iron-ferron ratio and pH. Alany of the problems involved haw been shown in Figures 2 to 7 . Othei similar problems may evist upon examination of other interfering ion. which might be present in a complex industrial atmosphere. The best rc,igcnt must bc srlectcd after a thorough tsainination of the optimum pH and iron-ferron ratio for any given set of conditions. If no interfering ions arp present, then selection of the optimum reagcnt would be bawd upon fluorido sensitivity and pH stability. Rcfercncc A\
VOL. 32, NO. 10, SEPTEMBER 1960
1313
)RON-FERRON RATIO 1:40 0120 a- i:in
*-
.-
A-
r
i:S i:3
2 .o
1.5
3.0
2.5
PH
Figure 4.
Effect of pH on phosphate interference
werv niade 16 minutes following :&ition of the stated xeight to the reagmt. The :ipprosimate limiting concentrations of the aiiions studied is wnimmized in Table I.
I , . . . . 2.0 , . . pH . . 2.5 , . . . . 3 .O , . .
1.4 1.5
Figure
3.
Effect
of
pH
on
sulfite
Table I. Effect of Common Ions on Iron-Ferron-Mercurate Reagent
interference
Appro\. Limiting Corlcn.,
:iluminum with somc s:icrific.c of fluoride sensitivit~r. Limiting Concentration of Selected Common Ions. Tlicir influrncc~upon thc iron-ferron color \vas sturlicd next using the 1: 3 iron-fctrron-xiin,c,urate YCagent. Sclect,iori of ioix studied was based on a consideration of compounds ivhirh might hr present \r-ith fluoride in pollutecl air or 21 process tmission strcain. A11 ahsorhaiicc~ iiirwiircmcnts
to Fxgurrs 6 and 7 will ronfirni that inafimuni pH stability a i d fluoride sensitivity were found in the ironfcrroii range of 1 : 5 to 1:20 and a pH range of 2.0 to 3.0. Thc 1 : 3 iron-ferron ratio reagent at pH 2 was sdected for further study as thr Mini-Adak high fluoride reagent on the basis of the data presented in Figures 2 to 7 . This choice combined minimum intrrferericr from sulfur diovide and
IT&
Added As
I011
Reagent Tests on Fluoride Atmospheres. Atmospheres containing hydrogen fluoride in t h e concentration range of 0.9 t o approximately 20 nig. of fluoride per cubic meter w r e
IRON-FERRON RATIO
- 1:40 - 1:20 1:10 A1:5
20 -
x
0-
:3 - 1K1.5
m-
15-
L I -
IRON-FERRON RATIO XK40 01:20 ai:in . .-
'E
0-
E-
1.3
1.5
Figure 5.
1314
2.0
25
PH Effect of pH on absorbance
ANALYTICAL CHEMISTRY
3.0
5%
'
'
. 2.0 '
'
'
.
. 2.5 '
'
1:3
t1.5
'
3.0
PH Figure 6.
Effect of pH on fluoride sensitivity
"
-
r
20 -
50
-
15 -
40
-
c
r r a
-
30
to-
I-
#
-
a 20 1:40
IRON-FERRON
-
/
-
L
I
1:ro Figure 7.
-
RATIO
Effect of iron-ferron ratio on fluoride sensitivity
gciieiated by tht mt~thod dcycribed by Hill et d.(7) l'he chamber air as iimiiltancously
miiplcd by two methods
~IAYUAL RIIXHOD. i i i \i:is snnipled :it 0.0 r.f.m. through t n o scries-coniic'ctc~l fritted glsss wruhhing columns (>,ivli containing 400 nil of 0.01211'
iodium hydro\ide (5) The L oluine of t h air sampled was 1nr:iiurctl nit11 a d i ~tcist meter and corrected foi prmwre drop through thc system. No tcnipcrature correction\ J( rrc' made as tciiilwatures v ere equivalciit for each siinultaneous run. In most instances, tlic two absorption solutions were cornl i i i i i d and the total fluoride drtermined b j :L photometric thorium nitratt. t1ti:ition using a modified high salt 1)' occdure (2). ,ihsorption solutions from each of t h c s two toners uere analgzcd separately a t the start of this btutly to establish t h r need for two columns in series. 'I'hc+e analyses showed t h t a n average of 93.57, of thc fluoridc itas collected in tlie first tower. The concentration of hydrofluoric w i d in the sampled XI \vas calculated tq dividing the total inillgrains of fluoride found by the corr w t r d volumr of air c y m w e d in cubic inetcrs. IASTBUMENTAL X~ETHOD. d i r Was t1i:in-n into the automatic analyzer at 'i ~ , i t e of approximutc,lv 0.7 c.f.m. Tlic ratr of air sampling \\ah iiieasurcd nit11 the rotameter on tho face of the analyzer. The anall zer was opeiated for either 1 hour or until approximately 3 nig. of fluoride had brcn accumulated as indicated by the strip (-hart recorder. 'Tlic elapsed sampling tiinc for each iuii was recorded to 15 scconds. The total volume of air sampled uas then obtained by multipll ing thc sampling rate by the total minutes sampled. The weight of fluoridc collected nas determined from a standard curvr relating milligrams of fluoiide and recorder chart divisions, 47,T, Figure 8.
The s t a n d a d curve \\asprepared by adding 0.5 to 4 mg. of fluoride in 0.5ml. incremrnts of a 1.0 mg. of fluoride per ml. solution t o a 15-ml. volume of circulating reagent in the hlini-Adak. T h r reagent volume was evttporated back to the original 1 5 d . volume aftel addition of each increment by d r a ing ~ filter air through the sampling system Plastic snmpling tubes, inch in outer diameter and 2 feet long, were used to convey the air from the chambei to the two types of sampling equipment. The inlet ends of the two tubes were held togethvr by a piecc of Scotch tape so that the two samples might lie obtained froin :is nearly the same point as possible. The sample tubes were interchanged during the scrips of comparisons and no bias appeared to reiult from the switching of the tubes betncrn the sampling equipment. Prior to the brginning of the sampling period, fluoride-containing air was drawn from the tcit chamber through each of the sampling tubes for a period of approximately 15 minutes to attain cquilibrium b e t w e n the atmospheric concentration and the adsorbed layer of fluoride on the inner surface of the tubes. A prriod of 2 minutes elapsed prior t o the start of each run during M hich the suction was disconnrcted from the tubes and rach of the two sampling systems was charged with a new volume of its respective absorption solution. The sampling periods for both \y+ms mere begun and dis-
Table 11.
3
2
I
mg. F-/ 15 mi.
Figure
8.
Standard
fluoride
curve
continued simultaneously SO that should any variations in atmospheric choncentration exist, they would not d e r into the comparison of methods. Statistical comparisons of the air sampling results obtained by the t n o methods are given in Table 11. The standard deviation of the average difference between each paired sampling for 19 pairs of samples was then related t o the standard deviation of the average difference between 14 pairs of aaniples obtained nith duplicate scrubbing towers. The t and F values obtained indicate that tlie differences found in the comparisons made between the scrubbing tower method and the MiniAdak automatic inethod are not statistically significant. Assuming that duplicate tower? have identical precision then the s t m d s r d deviatioiis are: -(
tower
=
(11 i ) 2 - = & 8 27'; 2
Therefore: y
automatic. = (10 8 ) * - (8 27)' =
* 694%
Statistical Comparison of Manual and Instrumental Methods of Air Analysis
qtd. Dev., 'ourre of Yariation
(-
NO. F
Duplicate fritted glass towers i l l 7 14 Fritted glass scrubbing tower vs. automatic 1Iini-Adak f10 8 19 l>ifiercmce+iirp not statistically significant.
Ratio 1 178a
Fl % 3
201
t
0 43"
t*?&
2 745
~~
VOL. 32, NO. 10, SEPTEMBER 1960
1315
DISCUSSION
The air piebsure to the hydrogen fluoride generator n :ts increased by approximately 5 iniii. of water betn-een each comparative sampling lvithin each series. This resulted in :t step-wise increase in fluoride concentration in the ch:tmber atmosphere before sampling. Comparison of the series of data obtained by each mcthod n-it11 the respective air pressure rcadings indicated that the :iutoni:itically obtained data followcd thc incrc~mental increase in hytlrogeii fluoride concentration between samples much inore closely than did the scrubbcr t o w x clctta. It is likcly that this improvement in (lata n a y achieved through minimizing the hniiian elernent in sampling and analysis. 13y substitution of an automatic analyzer, the personnel conducting the sampling can read pertinent infvrni:Ltion off the strip chart recorder ininiedintely and coniplete the necessary calculations to determine the fluoride concentration of the prc.vious sam-
pic I\ hilc the subsequcnt saniple is being t:ikeii. This would result in an e h n a t e d sal ing in manpower of 50% over that rcquircd to study the fluoride concentrations in industrial hygiene and stack monitoring situations by convcntional techniques. Prior to the use of the pioposed ztutomatic twhnique in an uncyilored situation, it n.ill be nccessary to determine the concciitration ranges of other chemicals n Iiicli may co-nist with tht. fluorides in the air t o he sampled. T h c possibility that one or more intcricring ions might b(1 found in an air or cniisaioii qtreain s:iniple and that thcii c3ffccts iiiight be additive should bts cu-c’fully considcrctl when applying thii nicthod in a n c n or coniplex situation.
( 2 I Xt!;inia, I ) . F., Kioppe, R. K., . i x . i ~ . C H E M . 28, 116 (1956). (3) Ibid., 31, 1219 (1959). ( 4 ) Adzam, 1). F., Koppe, R. K., I ~ I I ~ > H. J., J . .lir Pollution Control .isaoc. 9, 160 (1950). ( 5 ) .2dame, I ) . F., Mayhew, 1). J., Gilagy, I < . AI., Richey, E. P., K O I I ~ P , R . I.I) for review .lugust 25, 1059. dccepted June 6, 1960. Division of IVater, Sen age, ant! Sanitation, 136th Meeting, ACS, ltlantic City, T. J , Scpttmher 1950.
Volumetric Assay of Ammonium Perchlorate EUGENE A. BURNS’ and R. F. MURACA’ Jet Propulsion laboratory, California lnsfitute o f Technology, Pasadena, Calif.
b The volumetric determination of perchlorate by reduction with titanous chloride has been studied as a function of boiling time, excess reagent, and the presence or absence of the osmium tetroxide catalyst. This study permits the assignment of the following optimum experimental parameters: 10 minutes’ boiling, 100 mole excess of titanous chloride solution, and osmium tetroxide as catalyst. The stability of stock titanous chloride solution is such that the procedure is recommended for the assay of rocket grade ammonium perchlorate.
yo
A
P1:RCHLURkTE Of the purity used in pyrotetahnic compositions for rocketry can be assayed by the dcterniinatioii of perchloratc ion. The procedures set forth in Military Specification .J*iS-A-192 and by the Joint Army-Savy-Air Forw P:incl on the Lhalytical (’heniistrj of Solid Propellanth spec if!^ the reduction of perchlorate to chloride by fusion in a Parr bomb and the iuhscquent tlchmination of rhloride by an nrgmtinietric titration. These ~~roccclures arc tinic-consuniing and subject to errorb :wiiing from incomplete reduction and th(, inevitable spattering of the sample. The method describcd in this paper specifies the reduction of perchlorate ion
M&iONICM
1316
ANALYTICAL CHEMISTRY
by an excess of titanous chloride and a subsequent hack-titration of the excess titanous ion with ferric ammonium sulfat’?. This procedure gives accurate results in a short time and is submitted as an alternative or a substitute for the official military method. The advantages of a titanous reduction method over a Parr bonib fusion and precipitation with tctraphenylphosphonium ion have been demonstrated in the analysis of polysulfid(.-perchlorate propellants (10). ‘The determination of perchlorates by reduction with titanous salt5 was first proposed in 1909 (5, 6, 12, 13) and has not been widely used bccause prolonged boiling in the absmce of air is rpquired for quantitativc reduction
(2-4, 11). The reaction occuriiig TI-hen perchlorate ion is reduced with an excess of titanous ion in strongly acidic nietlia is rcprrscntcd by the equation: C10;-
+ 8 Ti+3 + 4 H 2 0 C1- + 8 T i O f 2 + 8 H + -+
(1)
I n :i quantitative. detcwniiiation, t h e e x c ~ ~titanous s ion is tit’rated with standard ferric aninioniuin sulfatr solution. Ti-3
+ Fei3 + H20 TiO+2 + F e + * + 2H+ -+
(2)
using :I t,hiocyanatcl solution a b im intlicator. Thch end point is tlisc*cwctl 1):-
the :i1)pe:iraiiw of the rcd-1)rowi ferric. thiocyanntc cmiplex ion, F(3*3
+ C‘SS -
-
FeC9S +‘ i:i j
The reduction and titration niurt be pc~rfornietlin the absence of oxygen I)ctxuse acidic solutions of titanous and fwrous ions oxidize easily. Other oxidizing impurities in the ~aiiiple will also bc reduced by titanous ions and give, high results for perchlorate j suit:il)lr corrections can bc made for t,hesc suliatances if their identity is knon-11. Sinrc the potcantial of the titaiioustitaiiyl c~oupleis strongly tlependcnt on thct :iiaiclity of tlw solution, 0.074 log
Ti0 + 2
- T ~ K+ 0.118 pH (4)
then at liydrogi~nion concentration of loss t,li:in 0 2 M , partial reduction of liytlrogc~n niay occur. [Latimer ( S ) lists E” for t h k couple as -0.1 volt.] ‘Thus for n u nnalq-tical procedure to be quantitntivc. the conditions must be sucli that this wliuction does not take p l n w . T h t ~rcduction of perchlorntr :IS tlrscribctl untler Experimental Lktnils is pc~rfornii~ti in SM hydrochloric avid; thercforr. at 100” C.the potential of the Present address, Propulsion Depirtinrnt, Poulter Laboratories, St,anforcl I?(,w t r c h Institrite, LItdo Park, Calif.
