Application of' Conductivity Measurement of Nitric Acid Concentration to Plant Control 13,
1.. 5. Kava1 l'owcler Factory, Iridian Head, hld.
The eiectricul ~~oriduclicily of riilric ucid hus brrn delrrniirird and lhe < f e d of oxides .S ndrogeri or1 coridu,clirilymeuswed. A syslemfor rri~asiiriri,g/he coricentruliorr, qf riilric acid in the process tlse(f hus (pen dei:eloped. With the aid .Jspeciul conductirily ~.cll,s mourilrd iu lhe acid line and a conduclii~ily bridge, lhe ~~oncrntrution churig~.soceurrirr,g llrrough
/he whole cycle of operatiori curl be followed, step by step, by a n occasiorlul press of uri electric button. The conduclioity curzw and the system have resulkdfrom the condnctivity meusnrerrienlsundanalysis of about fiue hundred samples, in, the process, drawn from the process, und prepared in the laborutory. free of oxides arid u p tu a roiiecutration of 99.92 per cent in t,liislaboratory. The cells used in these tests veie home const.ructed, but the conductivity bridge was of a standard Leeds & Xorthrup type. The toristant for the cell was determined in tlie usual matiner with 0.02 N potassium chloride solution. C o n d u c t i v i t y values plotted are b e l i e v e d to be correct, hut no esteridecl tests Iiarc besri m a d e to prove tliern. Resistance values were rlc-
NCOUiiAOEU by t l w R U C C ~ S SIterr in det,erm i n i n g sulfuric a c i d coticentration in t.lic process Iiy ronductivity rricasuremeiit of its solutions and by the excellent, resilks ihtainetl on applying t Iie,sc rneasiminent~s to plant control (g), at,t e n t i 11ri ,vas directed to a siniilar dcvelopment arid applicat,ion bo nit,ric acid operatimi.
E
OLD I\IETROD
In the process for the iiiaiiiiineture of smokeless powder
practicc was to c u t at irraiiy iiiiiiutes after prinre (the time at which large volumes of acid vapors are rapidly delivered to t,he condenser-) but tliis pnived very unsatisfactory as acid strength by tliis r n e t i d varied more than 5 per cent from day to dav, resulting in a loss of strong acid and, therefore, a gnin in weak which tiad to be redistilled at an added and uiinecessary \(I
Cspelise.
The reciprocal of the measured resistance is the emduetarice The \;aliics for nitric aeid shown in Figure 2 wwc taken froiri tbc lit,crature (5'); tliose showii in Figures :ia i d 4 wcre deteriiiined on eiirmically pure acid, prepared irf t.he solution.
Tlien: results, wkren plotted, using ohms resistance as ordinates and acid strengths a.s abscissas, gave a curve (Figure 5 ) wliicii indicated values as a method by which cuts could be made at definite acid concentrations. 980
I N D U S T R I A L A N D E N G I N E E R I ?: G C H E M I S T R Y
September, 1933
981
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2
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IS
t
>- low 5 2 2500
?9
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=
8 " awo u
i*
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3500
10
lo
E
t
f lr
os
'=OD I000 5
O
500
0 0
0
o
10
FIGURE3. TIES OF
ao
XI
40
so
PER CENT
60 70 "03
ao
90
50
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40
70
60
10
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FIGURE5. NITRIC ACID CONCESTR.4-
FIGURE4. NITRICACID COXCE\VS. OHMS RESISTANCE AT 28" C.
TION
us.
O m s RESISTANCE
Samples drawn from t h e process of manufacturing nitric acid from Chile saltpeter and measured with laboratory cells a t 28' C.
Cell constant, 0 45
There were, however, a few values for strong nitric acid known to contain a high percentage of oxides of nitrogen which did not fall exactly on the curve, but, since the oxides distill over first and along with the strong acid, they are out of the way before the nitric acid concentration falls below 90 per cent and, therefore, do not militate against the practical value of the curve. Xitric acid containing both oxides of nitrogen and chlorine, and nitric acid containing only oxides of nitrogen, when measured gave practically tht. same difference in conductivity readings taken before and after airblowing; this indicates that the inflection of the points on the curve was caused entirely by the presence of oxides of nitrogen. Interest of a general nature led to the investigation of this influence of oxides of nitrogen on the conductivity of nitric acid. In the investigation, samples from the proces were drawn into 8-gallon stoneware crocks which were p1act.d in a warm room and blown with air until the oxide content had been reduced to zero. ilnalysis and conductivity checks were made daily. It was interesting to observe the building up in
so
80
P € R C€NT " 0 1
TRATION
Author's curve; cell constant, 0.45
m
CO
SO
PER CENT " 0 ,
-1
ELECTRICAL CONDUCTIVINITRIC. ~ C I DAT 28" C.
m
to
acid concentration of the samples so long as there were oxides of nitrogen present, and the more or less rapid falling off in concentration as soon as ail the oxides of nitrogen had been dispelled. Oxides of nitrogen increase the conductivity of nitric acid. Much time was required and a large number of samples investigated before sufficient data could be collected to measure properly this conductivity increase. These values plotted in terms of both nitric a c i d a n d oxides of nitrogen concentration (Figure 6) supply all the corrections necessary for oxides of nitrogen up to 3.5 per cent. Although the data as plotted list only tests on nitiic acid concentration between 90 and 100 per cent, there were some tests made on acids of lower concentration containing oxides of nitrogen, but in these the oxides of nitrogen did not appear appreciably to affect the conductivity. The influence of oxides of nitrogen is, therefore, concentrated in a small area and thus has a minimum effect on the utility of analysis by this method since a conductance minimum between 96 and 97 per cent already precludes the use of any value3 above this concentration.
C P RClO FREE P L R H T K I D BLO
0
PLRWT RClD CON
6000 5500
I
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Y
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1500
PER
CENT H N O j
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FIGURE 6. COSDUCTIVITY CURVESFOR NITRIC ACID COSTAININC OXIDESOF NITROGEN
LL-I
I N D U S T R I A L A N D E N G I N-EE R I N (3 C A E M I S T R Y
982
APPLICATION OF
MEASURGMENT CONTROL
CONDUCTlVlTY
Vol. 25, No. 9
TO PLANT
I n order to apply the conductivity system to plant control, special Bishop conductivity cells ( 1 ) with adjustable lava head and platinum electrodes were furnished by the Leeds & NorthruptCompany. A special trap to collect the samples
FIGURE7. Flaw SneeT
OF
SYSTEM
and also act as a reservoir for the cells was developed with the aid of the Duriron Company. The standard cells as installed were charged with approximately 70 per cent nitric acid because of the high boiling point of this concentration. A water jacket around the trap and a k e d s & Northrup conductivity bridge completed the system (Figure 7). Control of temperature and pressure was also difficult, and a bypass line had t o be added before satisfactory results were finally obtained. Plotted data collected from a number of test runs furnished the work curve (Figure 8).
CELLS WITH P L A T M U M ELECAND A W ~ T A B LLAVA E HEADS PREPARED FOR Nmnrc Acio MEASURE TIVIW
TROD-
MENT
in tlie process itself and, therefore, enables cuts to be made at exactly the nitric acid concentration desired. Although these investigations did not include nitric noid produced from the oxidation of ammonia, being limited to nitric acid from Chile saltpeter, both the determined values and the system are applicable to the concentration measurement of nitric acid in any process. CONCLUSION
FIGURE 8. Nrrnic Acin CONCENTIITION os. OHMSRESISTANCE Work E U E T ~
This conductivity system is both interesting and effective: interesting because each wap-e of the bridge's galvanometer indicates concentrat,ion changes occurring too rapidly to be observed by any other method and heretofom unknown; effective because it measures tlie concentration of nitric acid
By conductivity measurements the concentration of nitric acid between certain limits of concentration can be measured with speed, ease, and accuracy unequaled by any other method known here. By applying conductivity measurements to plant control cuts can be made at exactly the concentration desired. hTERATURE CITED
(1) Bishop. U. S. Patent 933,015 (Aue. 31. 1909). (2) Coster, IND.ENS Cnwd., 23, $63-66 (1931). (3) Cottmll, "Manulecture of Nitric Acid and Nitretea," p. 888. Van Nostrand. 1923. R a o e r v ~ oMarch 13, 1933. Permission t o publieh granted by the Bureau of Ordnanoe. U. 9.Navy Department.
