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 titraarc tinic-contion. These ~~roccclures suniing 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.
couple will vary according to the relation lk:
=
TiOf2
-0.20 - 0.074 log __ Tif3
(5)
The initial reduction occurs rapidly and is a t least 10% complete almost instantaneously; therefore, the ratio ( T i 0 +2/Ti+3) is always greater than 0.01, and the potential of the titanyltitanous couple is always insufficient to reduce hydrogen ions. EXPERIMENTAL DETAILS
Apparatus. Several types of apparatus for t h e protection of titanous solutioiis are described in t h e literature a n d may be incorporated into this method. T h e simple apparatus employed in this work consists of three 500-m1. Erlenmeyer flasks which are fitted with rubber stoppers and glasstube connections and connected together with Tygon tubing. A source of oil-pumped nitrogen, gas dispersion tubes, and watch glasses to cover a 500-nil. Erlenmeyer flask arp also requi~ed. Reagents. 0.33X FERRIC AmfoNIUM SULFATE SOLUTION. Dissolve 160 grams of reagent grade Fe(?;H4) (S0J2.12H?0 in sufficient deaerated water, containing 28 ml. of reagent gratlr sulfuric acid, to make 1 liter. The sulfuric acid concentration in the solution is LV. 0.3X TITANOUS CHLORIDE SOLUTIOX. This solution is best prepared from commrwially available iron-free, reagent grade titanium hydride. Alternatively, the solution may be prepared from 20% w./w. titanous chloride solution; these solutions may contain significant amounts of iron; cognizaiice of this is neccssary when the solution is standardized with a strong oxidizing primary stantlard--e.g., potassium dichromate. PREPARATION FRO?vI TITANIUM HYDRIDE. I n a well ventilated hood, add, in small portions, 18 grams of titanium hydride (TiH2) to 150 ml. of reagent grade hydrochloric acid warmed t o about '70" C. Keep thc reaction vessel covered as much as possible and keep the solution warm until evolution of hydrogen ceases. When the reaction is complete, cool the solution, and add about 250 nil. of recently boiled, cold, distilled water. Mix the solution by passing into i t a stream of oil-pumped nitrogen for 5 minutes, filter off the unrearted titanium metal, add 150 ml. of reagciit grade hydrochloric acid t o the purple filtrate, and dilute to 1 liter with cold. recently boiled, distilled water. Again mix the solution ~ c lwith l oil-pumped nitrogen. The hydrochloric acid concmtration in thc rosulting solution is approximately 2.7N. h E P A R A T I O N FROM 'TITANOES CHLORIDE SOLUTION. Mix 150 ml. O f 2070 w./'w. reagcnt grade titanous chloride solution (Tic&,) with 100 nil. of reagent grade hydrochloric acid and dilute to 1 liter with recently boiled. cold, distilled water. DEAERATED HYDROCHLORIC ACID. I n a hood, pass oil-pumped nitrogen
through 1 liter of reagent grade hydrochloric acid (36 to 38% w./w.) for 30 minutes. POTASSIUbi DICHROMATE SOLUTION, 0.331%'. Weigh to the nearest mg. approximately 16.2 grams of dried reagent grade K2Cr2O.i; transfer to a volumetric flask, and dissolve in sufficient deaerated water to make 1 liter of solution. Calculate the normality, N , as followvs:
where TT is the w i g h t of potassium dichromate in grama. OSMIUM TETROXIDESOLUTION, 0.008M IX 0.1N SULFURTC ACID. Standardization of Solutions. Ferric solutions can be standardized directly after reduction in a Jones or silver reductor, riith a primary standard such as potassium dichromate, or indirectly by a secondary standard such as titanous chloride solution which has been standardized against potassium dichromate. The latter method was used in this work and a correction was necessary for iron impurities in the titanous solution (9). The standardization of titanous solutions has bepn discussed by Lamond ('7). TITAXOCS CHLoRIUE SOLUTION. St'andardization was performed by titrating a n aliquot with the potassium dichromate solution using 0.5% w./v. sodium p-diphenylamine sulfonate as indicator. Precautions similar to those described in the procedure were taken to aroid possihle air oxidation. Just prior to t.hc cnd point, the solution changes from purple to a deep green. The end point is a sharp change from green to deep purple. An indicator blank is run continuously. To the solution remaining a t the end point of the potassium dichromate titration, add 5 ml. of 20y0 wv./v.ammonium thiocyanate solutioii. If the color changes t'o red-purplc, iron is present; in this instance, titrate tmhcsolution dropwise with tit'anous chloride solut'ion dispensed from a 5-nil. graduated pipet until the solution regains the green appearance it had just prior to the end point in thc dichromate titration. Run a blank titration to evaluate the indicator blank. Calculate thc nornialit'j- of the titanous chloride solution as follows:
n here S t
=
N,
=
T-0
=
TV1 =
Tv2 =
normality of titanous chloride solution normality of potassium dichromate solution volume of titanous chloride solution taken originallv (50 00 ml.) volume of potassium dichromate used. corrected for indicator blank, ml. volume of titanous chloride solution required for back-titration of iron impurity, corrected for indicator blank. nil.
FERRIC AMMoNIUni SULFATESOLUSimilarly, standardize the ferric ammonium sulfate solution by titrating an aliquot of standard titanous chloride solution using 5 nil. of 20% w./v, amnioniuin thiocyanate solution as indicator. The color changes progressively from violet to yellow before the red-brown end point is observed. Run a n indicator blank concurrently. Calculate the normality of the ferric ammonium sulfate solution as follows: TION.
where normality of ferric ammonium sulfate solution -ITt = normalit,y of titanous chloride solution T i 0 = volume of titanous chloride solution (50.00 ml.) 1-1 = volume of ferric ammonium sulfate required for titration, corrected for blank, ml. =
Procedure. The equivalent weight of ammoniuni perchlorate in t h e titanous reduction is so small t h a t even when solutions on t h e order of 0.3.1are used, t h e sample weight required for assay cannot be conveniently weighed n ith commensurate accurncy (1). Therefore it is necessary to (lissolve a large sample of approximatc,ly 1 gram and to take aliquots of this solution. Transfer a 1.0-gram portion (weighed to the nearest 0.1 mg.) of the animonium perchlorate sample to a 250-ml. volumet'ric flask, dissolve in deaerated wat'er, and t'hen bring t o the mark. Transfer 50 ml. of deaerated reagent grade hydrochloric acid and a 25.00ml. aliquot of the sample to a 5OO-nil. Erlenmeyer flask. Cautiously add 1 to 2 grams of sodium hicarbonate aiid cover thc flask with a match glass. Rinse a 50-nil. pipet three times with the standard titanous chloride solution, then quickly remove the watch glass and transfer a 50.00-m1. aliquot to the Erlenmeyer flask. Quickly add 1.00 ml. of osmium tet,roxide solution aiid stopper the vcssel connectcd to a second Erlenmcyer flask cont,aining 100 Inl. of 1 2 s hydrochloric acid. This Erlenmeyer flask is connected to the third flask containing 100 ml. of a saturated solution of sodium bicarbonate (50 grams of XaHCO3 and 100 ml. of H20). Gently boil the solution in the reduction flask for 10 minutes. Remove the flask from the burner and cool its contents to room temperatsure by placing the flask in a n ice bath. As the flask cools, sodiiim bicarbonatc solution siphons into thr center flask which contains hydrochloric acid. The carlion dioxide xhich is cvolved protects the titanous solutioii from oxidation hy air, -4dd 5 ml. of ammonium thiocyanate solution and titrate immediately with standard ferric ammonium sulfate1 solution to a permanent red-brow1 end point. Run blank reductions and titmtions concurrcntly with the sample. VOL. 32, NO. 10, SEPTEMBER 1960
1317
;e 986
-
V W 0
WITH Iml. OF aOO8U OSO.
V
0.00
LT
ABSENT
CONSTANT EXCESS
96-
w
C$
T!CI,
a
-
! i
48%
9 I
94
-
L
e
-
92
-
4
b
4 i
'
! 8'
I
I2
16
20
t ' l
28
24
TIME OF BOILING, mln.
2 5 - M I H . BOILING
0
IO-MIN. BOILING
V
5-MIN. BOILING
! ml. OF 0.008M 010, ADDED AS CATALYST M ALL CASES
n
8
0
/
~
/
Figure 1. Percentage reduction of ammonium perchlorate with 48% excess of titanous chloride as function of boiling time and presence of catalyst
Calculations. Calculate the per cent ammonium perchlorate in the sample as follows:
where
T h e amount of ammonium chlorate present in t h e ammonium perchlorate used for rocketry is generally so low t h a t t h e correction term in the above equation may be ignored.
:+= ' normality of ferric ammonium sul-
B
=
S
=
JI'
=
fate solution volume of ferric ammonium sulfate solution used in titration of blank, ml. volume of ferric ammonium sulfate solution uscd in titration of sample, ml. weight of sample, grams
Table 1.
Typical Results for
Ilay 1
Run h'o.
2
1 2
Blank Titration of Titanous Chloride Solution"
FeXHa(SOJz Solution, 311.
1 2 3
14.17 44.12 44.18 44.10 44.07 44.11 44.04 44.04 44.00
3
3
RESULTS
The effects of various factors mere studied. Figure 1 relates the percentage of perchlorate reduced as a function of the boiling time in a 48 mole yo excess of titanous chloride solution. It also shows the effect of osmium tetroxide on the rate of reduction. Fnder these
1 2 3
.4verage,
111.
Standard Ileviation, * 311.
44.16
0,032
44.09
0,021
44.03 0.023 80.00 ml. of stock titanous chloride solution titrated with 0.3371A' ferric ammonium
sulfate solution in presence of 1 ml. of 0.008M osmium tetroxide solution. b Pooled standard deviation of 3-day data is 0.026 ml.
Table It. Reproducibility of Assay of Reagent Grade Ammonium Perchlorate (Lot 1 ) Sample Weight, NHaClOa Found,
Grams 0.12313 0.09081 0.09081 0.11030 0.11030 0.10624 0.10624 0.10624 Av. = 99.810/, Std. dev. = 0.14y0
1318
% 99.84 99.94 99.77 99.66 99.85 99.98 99.73 99,68
ANALYTICAL CHEMISTRY
Table 111.
experimental conditions, the catalyzed reduction is essentially quantitative in 8 minutes, while 15 minutes are required when the catalyst is absent. Figure 2 shows the pcrcentage of perchloratc reduced as a function of excess titanous chloride. The reduction is quantitative with a 15 mole % excess of titanous chloride and a 25minute boiling period, or with a 48 mole % excess and a 10-minute boiling time. A 5-minute boiling time does not yield quantitative results even Jyith an excess greater than 90 mole % of titanous solution. The results of this study permit the assignment of the following optimuni experimental parameters: 10-minute boiling time, 100 mole % excess of titanous solution, and osmium tetroxide as catalyst. Table I summarizes typical results obtained for a blank of the same titanous chloride solution on different days. The titanous solution loses strength daily; however, this loss is relatively small and if the blank is determined each day, results are good. The excellent applicability of the method to reagent grade ammonium perchlorate is demonstrated by the reproducibility study in Table 11. The results obtained on various lots of
Typical Results from Assay of Ammonium Perchlorate
Sample Ilesignatioti
Sample Weight, Grams
NHaClOa Found, yo
Lot 2
0.11732 0.11732 0,10784 0.10784 0.11668 0.11668 0.13071 0,13071 0,10847 0.10847 0,09978 0,09975
09.78 100.02 99.98 99.69 99.08 99.51 99.38 99.19
Lot 3
r,ot 4
Std. Dev.. hv., %
%
99.87
0.15
99.29
0.19
~
ainnionium perchloratc are shov n in Table 111. LITERATURE CITED
(1 j Burns, E. *4., “Analysis of Ammonium
Sitrate. Part 111, Assay,” Jet Propulsion Laboratory, Progress Rept. No. 20-365 (Sept. 2, 1958). ( 2 j Greenberg, -4.L., Walden, G. H., Jr., J. Chem. Phys. 8, 645 (1940). (3) Husken, A., Gaty, F., Chi7n. anal. 30, 12 (1948). (4)Ishihashi, Kiyoshi, Repts. Hiniejz
Tech. CoZZ. 6, 70 (1956). ( 5 ) Knecht, E., Proc. Chem. SOC.25, 229
(1909). (6) Knecht, Edmund, Hibbrrt. Eva, “New Reduction Methods in Volumetric Analysis,” 1st ed., pp. 21, 66-7, Lonymans, Green, London, 1918. ( 7 ) Laniond, J. J., Anal. Chim. Acta 8 , 217 (1953j. (8) Latimer, W. AI., “Oxidation Potentials,” 2nd ed., 11. 268, Prentice-Hall, S e n York, 1952. 19) Pierson, R. XI., Gantz, E. St C., ASAT,.CHEM.26,1809 (1SYA).
(10) Rlbaudo, Charles, “Methods of An-
alyzing Polysulfide-Perchlorate Propellants,” Technical Rept. 2334,SamueI Feltman Ammunition Laboratory, Picatinny Arsenal, Dover, N. J., September 1956. (11) Rosenherg, A., 2. anal. Chem. 90, 103 (1932). (12) Rothmund, V., 2. anorg. u . allgem. Chem. 62, 108-13 (1909). (13) Stahler, A., Chem. Ztg. 33, 759 (1909j. RECEIVED for review February 15, 1960. -4ccepted J u n e 27, 1960.
Titrimetric Determination of Combined Carbon Dioxide in Anhydrous Ammonia A. R. ADAM,’ RAYMOND SYPUTA, and W. E. STEPHENSON Western Operations Inc., Richmond Refinery, Standard Oil Co. o f California, Richmond, Calif.
b A method for determining parts per million of carbon dioxide in anhydrous ammonia has been developed. The carbon dioxide, combined as ammonium carbamate, is separated from the ammonia by weathering, and is determined by acidification and absorption in barium hydroxide. Analysis of known prepared samples in the range of 0 to 50 p.p.m. showed a precision of =t 1.5 p.p.m.
F
ox CERTAIN uses of commercial ammonia, the carbon dioxide content should tie as low as possible. Determination of this impurity is difficult, since the level iiwolved is only a few parts per million. As air contains several hundred parts per million of carbon dioxide, the usual chemical analyses are complicated by difficulty in establishing blanks and by limitations of sample size. Because of the chemical combination of carbon dioxide and ammonia, the usual methods of gas analysis. such as mass spectrometry and gas-liquid chumiatography are not applicahle. The rraction of carbon dioxide with anhydrous ammonia forms ammonium carbamate ( 1 ) . CO?
+ 29H,
/SHs +
O(”
‘OXHA The method described here is based on the observation that even very small amounts of this mat’erial are quantitatively deposited when the ammonia is weathered off at, or below room temperature. The ammonium carbamate can be deconiposed in a closed system with a 1 Present address, Technical Service Group, Oronite Chemical Co., Belle Chasse, La.
dilute strong acid to liberate the carbon dioxide (1).
“, OC\ \ONH,
barium hydroxide is approximately 0.009N oxalic acid, standardized against a dilute standard sodium hydroxide solution. Water-pumped nitrogen is iised as the flushing gas. PROCEDURE
Several procedures for determining the evolved carbon dioxide were considered. The gravimetric method of Kolthoff and Sandell (9) was tried but was abandoned because of insufficient sensitivity. Of the recent titration procedures ( 3 ) , the one based on the direct titration method of Pieters (4) appeared to be more useful as a control test. A suitable modification of this procedure was developed. APPARATUS A N D REAGENTS
The sample containers are cylinders of Type 316 stainlesh &el of approximately 500-nil. capacity, equipped a t both ends with stainless needle valves and suitable adapters. The acid scrubber is of approximately 250-ml. capacity with a medium frittedglass plate a t the bottom. Three stopcocks are provided a t the top, one to introduce the acid, one to attach an Ascarite tube, and one as the outlet to the absorber. A tower filled with glass wool is placed between the acid scrubber and the absorber to act as a spray trap. The absorber used is specially made with four coarse fritted-glass plates placed 1 inch apart. I n other respects, it is similar to the ASTM lamp qiilfur absorber. A wet-test gas meter is used to measure the throughput of flushing nitrogen. Figure 1 shows the apparatus as assembled for the determination. The solution used to flush the sample cylinder is approximately 2N sulfuric acid. The absorbing solution is approximately 0.01N barium hydroxide. This solution must be protected from carbon dioxide in the atmosphere. The acid solution used to titrate the excess
Prepare the sample cylinder by flushing and filling with a n atmosphere of carbon dioxide-free nitrogen. Attach the tared cylinder to the flushed sample line and draw it nearly full by chilling the exterior of the cylinder sufficiently to keep the sample from vaporizing. Weigh the cylinder to obtain the sample weight. Immerse the cylinder in a water bath maintained below 70” F., leaving the upper valve exposed. Completely weather off the sample a t such a rate that no liquid ammonia is discharged. Close the cylinder valve. Draw approximately 150 ml. of the 2N acid into the acid scrubber and flush the scrubber and trap for 10 minutes with carbon dioxide-free nitrogen through a length of Tygon tubing attached to the inlet side of the scrubber. Close both the inlet tube of the scrubber and the outlet tube of the trap with pinch clamps, and connect the nitrogen supply to the absorber. While flushng with nitrogen, add distilled water and neutralize the abborber to a faint phenolphthaleiii pink. Discard the neutral solution and add a measured volume of the barium hydroxide solution that provides for a t least lOOyo excess of the absorbant. Titrate with the oxalic acid to a faint pink end point. Record the titration as A . Discard the neutral solution and immediately add the same amount of barium hydroxide, discontinue the nitrogen flushing, and place the absorber Ltnd the gas meter in the train. Close the scrubber stopcock leading to the trap and open the one leading to the Ascarite tube. Place a few drops of water in one tip of the cylinder and connect the scrubber inlet to this end with tho Tygon tubing. ReV O L . 32, NO. 10, SEPTEMBER 1960
0
1319
