On the Absorption of Oxides of Nitrogen by Nitric Acid

United Kingdom...... United States......... Wine lees............. Thorium, cerium, and zir- conium salts.......... Zinc oxide................ United ...
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June, 1920

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

EXPORTS OF CHEMICALS AND ALLIED PRODUCTS (Continued) CHEMICALS (Concluded) : T a r t a r (Concluded) : Crude. United Kingdom.. United States.. Wine lees.. Thorium, cerium, and zirconium salts.. Zinc oxide.. United Kingdom.. COAL-TAR PRODUCTS: Naubthalene OTHER PRODUCTS: Blacking and shoe polishes. Candles: Paraftin, and paraffin mixed with stearin. Argentina.. Germany. Stearin. Dyeing and tanning materials : Sumac, ground.. France. United Kingdom, United States.. Sumac, not ground.. Fertilizers. chemical. Austria-'Hungary Switzerland. Glue, common.. Ink: Printing. Other.. Manna. Brazil. . . . . . . . . . . . . . . .

1915

1918

Lbs.

Lbs.

Lbs.

18,286,460 ................. .... 3,714,127 8,772,192 ....... ............. 9,400,289 ......... 2,89j:??O .............. ...... 2,004,000 .............. 274,255

.......... ............ ................

........ .............. ..... ....... .... ...... ........ . ........... .......... ............. .............. ................ .. .. ..

................

Other.. Medicinal products. n.e.s. Oils, vegetable: Fixed' Castor oil.. Argentina.. Germany..

.......... ........

.........

EXPORTS OB

1913

17,409,682 15,011,715 4,583,630 10,488,270 5,270,149 2,29.?:893

.....

320 5,643,392 4,810,706

'

395,950

.....

661,387

102,074

265,657

13,448

4,807,400 11,273,777 761,697 1,700,866 ..... 6,808,535 84.657 545,424 34,184,437 5,443,433 13,799,393 8,469,718 18,120,010 43,001,163 16,215,000 12,489,185 2,436,770

13,726 164,464

119,931

..... .....

33,730

34,529,678 24,215,789 4,613,172 12,165,106 14,530,886 12,508,366 19,2ii:kiS 13,228 28,494,746 5,833,430 16,609,625 4,217,663 3,394:457

.....

.....

....

20,062 122,356 770,736 183,204 78.485 87,964

194,668 150,575 509,268 142,860 66,800 66,139

19,621 473,332 453,932

332,898 43,652 2,474,468 425,050

184,086 135,584 1,245,390 134,702

135,364 69,887 523,156 109,570

1,169,552 221,785 4,409

672,189 247,579 77,162

207,675

.. .. ...... .....

CHEMICALS A N D ALLIEDPRODUCTS (Cond u d e d )

OTHER PRODUCTS (Concluded): Oils, vegetable (Conclrtded) : Volatile or essential: Bergamot. France. United States.. Essences of citrus fruits, n.e.s Lemon.. Germany. United Kingdom.. United States.. Mandarin. Orange. United Kingdom.. United States.. Peppermint. Other. Paints, pigments, colors, varnishes : Colors, n. e. s . . Varnishes, non-alcoholic. Soap: Common. Austria-Hungary Germany. United States. Scented.

1913

1915

1918

Lbs.

Lbs.

Lbs.

........... ............ .....

139,096 62,335 22,130

232,704 85 650 57:461

1,811,778

.............. ............. ..........

2,639 1,007,303 232,169 101,175

935 1,640,263 61,848 591,568

2,747 3,592,961

426,186 2,013 106,050 23,913 26,193 49.152 31;495

562,770 2,661 155,805 28,786 47,384 50.353 38;387

1,871.724 586,650

2,666,930 428,800

2,498 940 533:Oi7

6,808,976 23,180,276 106,704 4,776,314 12,346 11,496,444 2,371,512 2,307,798 5,639,423 315,702 1913 1915

869,944

.. ..... ........... .............. .. ..... .......... ...............

.........

.............. ...... ............ ........ ...............

Metric tons Sulfur: Crude, . . . . . . . . . . . . . . . . . Refined Ground. . . . . . . . . . . . . . . . Flowers of sulfur.. Total exports.. Australia. . . . . . . . . . . . . Austria-Hungary.. British South Africa. , France. . . . . . . . . . . . . . . Germany. . . . . . . . . . . . . Greece. . . . . . . . . . . . . . . Portugal. . . . . . . . . . . . . Russia. Sweden . . . . . . . . . . . . . . United Kingdom. , , , United States.. .......

................ ......

......... .... .

..............

..... .....

531

..

212,143 49,375 85,427 4,394 35 1,339 13,447 40,981 13,761 76,036 38,979 14,190 14,777 25,352 19,541 16,465 1,028

..... .....

..... ..... .....

4,766 689,650

..... .....

21 684 51:275

.....

..... .....

54,454 19!8

Metric tons Metric tons

190,459 28,802 65 459 9,188 293,908 358 16,162 9,821 99,090 26,378 19,109 12,779 2,791 17,992 36.880 1,845 I

..... .....

.....

..... .....

..... ..... .....

.....

ORIGINAL P A P E R S ON THE ABSORPTION OF OXIDES OF NITROGEN BY NITRIC ACID By Eric I(. Rideal UNIVERSITY OF ILLINOIS,URBANA, ILLINOIS Received December 8, 1919

I A-TR 0 D U C T I 0 N

A number of researches] have been published dealing with t h e mechanism of t h e absorption of nitrogen dioxide by water or b y dilute nitric acid. Perfect unanimity of opinion based on t h e results of experiments conducted under varying conditions has, howno been arrived at, nor has it been found possible t o explain the phenomena Occurring in nitric acid condensing towers as operated under technical conditions. The following series of experimen'b was undertaken to investigate in more detail t h e mechanism of absorption under conditions approximating those which actually obtain in practice, with a view t o gaining information on the Optimum conditions Of absorption by water and nitric acid of various concentrations, SO as t o explain 1 In a recent communication on the theory of absorption towers for nitric acid manufacture, Partington and Parker ( J . SOC.Chem. Itad., Sa (1919), 75), with the permission of the British Munitions and Inventions Board, have translated into technical figures certain d a t a which were derived by t h e writer from experimental a n d , theoretical considerations W. R. Bousfield (J. Chem. Soc , 116 (1919), 45) has likewise made use of the hypothesis then privately advanced t o explain certain peculiarities in this series of the reactions. It consequently appears desirable t o record t h e experimental work on which the above-mentioned d a t a have been founalea,

t h e mechanism of operation and render t h e scientific design of absorption towers possible. The experiments of Foerster a n d Koch' have indicated t h a t the primary reaction between nitrogen dioxide a n d water can be expressed by means of the reversible equation 2NOz H20 HNOz "03, (1) a view which was confirmed b y Briner a n d Durand.2 This reaction, however, does not Seem to explain t h e whole phenomenon of the absorption process, since it is found by experiment +,hat equivalent quantities of nitrous acid are not obtained in nitric acid produced either in t h e laboratory or in absorption towers b y t h e passage of nitrogen dioxide i n t o water. To account for this deficiency in nitrous acid two theories have been advanced, the theory of volatilization by Foerster a n d Koch, and the view advanced by SaposhnikoffS and by Lewis and that the removal is effected by secondary decomposition^ Since nitrous acid is appreciably volatile than nitric acid, Foerster a n d Koch assumed t h a t its removal by ,.he air current, Under these conditions t h e equilibrium resulting from Equation

+

+

I is disturbed and strong so'utions Of nitric acid can be obtained. T h a t a limit is set t o is t h e strength of nitric acid obtainable in this 12.

angcv. Chem., 21 (1908), 2161.

* Comfit. rend., 156 (19121,582, 1445. 8

4

J. R U S S . Chem. SOC., aa (igoi), 375;as (I~oI),506. J . A m . Chem SOL, 2 (1901). 1330;sa (1911).292.

53 2

T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y

a point which will be referred t o later. Lewis a n d Edgar, and Saposhnikoff, on t h e other hand, assume t h a t nitrous acid undergoes decomposition according t o one of the following equations: 3Hn'02 _J HNOs zNO HzO (2) 2HN02 I_ N O KO2 HzO (3) The first reaction, based on Lewis and Edgar's work, is extremely improbable, being essentially a termolecular reaction. Experiments by Saposhnikoff and these authors were made in order t o determine t h e equilibrium constant of Equation 2 . T h e values for K where

+

+

+

+

Vol.

12,

No. 6

allowed t o come t o equilibrium with t h e vapor and air. A slight vacuum was then produced in t h e flask, t h e amount being registered in t h e duplicate gauges B a n d C, B being a mercury column with t h e mercury protected b y a layer of high-boiling paraffin oil, t h e fine gauge C being filled with paraffin. All t a p s were lubricated with phosphoric acid. T h e requisite amount of nitrogen dioxide was then admitted as vapor from a small flask, D, containing purified liquid nitrogen dioxide, through t h e capillary heating t u b e EF. This operation could be performed in from 2 t o 3 sec. Readings were made with reference tc a n arbitrary mark on t h e manometer.

K = C'NO~H+ C3"02 gave exceedingly un-uniform results varying in one series of experiments from 0.0545 t o 0.0287 with changing concentrations. Equation 3 may be criticized in t h a t equal volumes of nitric oxide and nitrogen dioxide resulting from t h e dehydration of nitrous acid would react t o form t h e anhydride of nitrous acid, Nz03,a gas perfectly stable under t h e conditions of t h e experiment. This alternative suggestion gives no explanation as t o why a limit is set t o t h e nitric acid strength (from j o t o 64 per cent) when nitrogen dioxide and air are passed continuously into water. ABSORPTION O F NITROGEN DIOXIDE BY DILUTE KITRIC A C I D IK THE P R E S E N C E O F A I R

Preliminary experiments on t h e reaction velocity indicated t h a t it took place with considerable speed, and t h a t t h e methods of injecting water into a dry gaseous NOz-air mixture or of collecting a sample of t h e dry mixture over water and noting t h e decrease in volume were inapplicable, due in t h e first instance t o t h e relatively slow rate a t which water vaporizes t o exert its maximum vapor pressure a t t h e desired temperature, and in t h e second t o absorption during t h e process of filling.

The percentage absorption was calculated on t h e assumption t h a t the contraction in volume was due t o t h e removal of oxygen and nitrogen dioxide from t h e gaseous phase in t h e following proportions: 4N02 0 2 = 2N206 (4) it being assumed t h a t t h e quantity of nitrous acid formed as a permanent constituent under t h e conditions of the experiment according t o Equation 2 was negligibly small, although t h e intermediate formation of nitrous acid with its subsequent oxidation is not qegatived. Later experiments confirmed this assumption. T h e results of such determinations using I O , 5 , and I per cent NOz-air mixtures with nitric acid solutions of j o , 2 j , I O , and 8 per cent strength a t temperatures of 30°, 40°, and j o " C., are depicted in Figs. 2 t o 6. D I S C U S ~ I O N O F CURVES-From a study of these curves in t h e light of t h e following considerations some insight into t h e mechanism of absorption may be obtained. It is evident t h a t t h e absorption of nitrogen dioxide in strong nitric acid proceeds most rapidly a t low temperatures, while t h e absorption in water proceeds most rapidly at high temperatures. The point of inversion, i. e., where t h e absorption rate is practically independent of t h e temperature, is in the neigh-

+

FIG.1

IfETHOD-The following procedure led t o uniform and consistent results (Fig. I ) . A zoo-cc. flask, A (approximating t o t h e volume, of a Gutmann ball, t h e standard packing for nitric acid absorption towers), connected t o a 3-way cock, was immersed in a thermos t a t a t t h e desired temperature, j cc. of mater or dilute nitric acid were introduced, and t h e contents

T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY

June, 1920

borhood of I O per cent nitric acid. T h e dependence of t h e absorption rate on t h e partial pressure of t h e nitrogen dioxide is clearly brought out, t h u s negativing t h e statement frequently made t h a t a 5 per cent NOzair mixture exhibits t h e maximum velocity of absorption.

533

with subsequent oxidation of t h e nitmus acid formed. Assuming t h e water t o be present in large excess, t h e reaction should be mono- or bimolecular according as N 2 0 4 or NOz is t h e reacting constituent. Applying van't Hoff's partial differential method of analysis t o t h e curves giving t h e absorption of t h e oxides in water a t 50' C., we obtain, if c and c' be two dc dt

different gas concentrations and - and

dc' -

at

their

respective rates of solution, t h e order of t h e reaction 1z by means of t h e following expression: dc'

dG dt

log - - log at

= f z

log c - log c' Inserting t h e values obtained from the-curve, dc dt

-

0.1

18.5

at

G =

0.25

dc - = - 0.1 a t c = 0.65 dt 5.8

i. e . , t h e reaction is monomolecular, The absorption rate may therefore depend on t h e actual gas pressure of t h e NO2 molecules, being lim.ited b y t h e rate of diffusion, t h u s being a pseudoTime in Seconds monomolecular chemical reaction, or i t m a y depend FIG.3 on t h e N z 0 4 concentration alone, being a pure monoIt is usually assumed t h a t reaction takes place be- molecular chemical reaction. T h e figures given in tween t h e monomolecular form of nitrogen dioxide and Table I1 for t h e times of half completion of reaction water, b u t there exists t h e possibility t h a t a similar a t various concentrations of gas and solvent show t h a t hydration may t a k e place with t h e bimolecular form both take place. N2O4, with which t h e nitrogen dioxide is in equilibrium. 0

20

40

60

80

,100

/20

MO

/Go /,go

TABLEI Total Pressure NOn Mm. Mercury 10 20 30 40 50 60 70 80 90

Pressure NzOa

30' C. 0.342 1.252 2.814 4.800 7.195 9.948 12.02 16.40 20.023

50" c. 0.279 1.477 2.320 3.999 5.596 8.349 11.02 13.92 17.10

Table I calculated from t h e van't Hoff equation

where KT is t h e equilibrium constant of t h e reaction at To 2 N O z Z Nz04 (5) indicates t h a t t h e quantity of polymerized nitrogen dioxide present under t h e conditions of t h e experiment is b y no means negligible. The value of Q adopted is 2XOz = N204 12,600 cal.

U I II II I I I I I I I

/ I 1

I

I

'

'

l

'

l

I I I 1 I 111

20 /O

+

It will be noted t h a t t h e partial pressure of t h e N204 rises rapidly with increasing pressure and decreasing temperature. It is evident, therefore, t h a t t h e reaction may occur in either of two ways or simultaneously 2NO2 HzO "03 " 0 2 (6) Nz04 HzO "03 HNOz (7)

+

+

+

+

FIG.4

It is evident t h a t neither set of figures exhibits constancy for varying nitrogen dioxide concentrations under similar conditions of temperature and nitric acid concentration, b u t i t may be deduced t h a t for solution in water t h e reaction velocity is practically

T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY

534

PIC

proportional t o t h e NO2 and not t o t h e N204 partial pressure, i. e . , i t is limited practically entirely b y t h e rate of physical absorption. For absorption in nitric acid t h e conditions are not t h e same, for as t h e concentration of t h e acid increases t h e reaction becomes more chemical in character. An increase in gas pressure is also associated with a transition from a physical t o a chemical t y p e of change. From these considerations t h e reaction velocity in t h e strong acid towers may be taken as governed chiefly by t h e concentration of t h e Nz04 molecules, while in t h e weak acid towers where t h e gas is less concentrated t h e reaction velocity is governed b y t h e physical rate of absorption. TABLE I1

Per cent 0

Temp. C. 30

0

50

25

30

25

40

25

50

50

30

"08

NOz Per cent 1 5 10 1 5 10 1 5 10 1 5 10 1 5 10 5

10

hTzOa

Per cent 0.034 0.544 2.05 0.029 0.451 1.75 0.039 0.544 2.05 0.031 0.498 1.90 0 029 0.451 1.75 0 544 2.05

Time Time for Half for Half Completion Completion in Form NZO4 of NOz x ~07 51 1.73 36 3.91 28 5.75 27 0.784 24 2.16 25 4.37 96 3.27 33 3.54 30 6.16 110 3.414 44 4.38 37 7.04 220 6.4 122 11.0 71 12.4 97 10.55 42 8.01

I n t h e usual 36-in. stoneware towers the inflowing gas is maintained a t between 30" and 60" C., according t o the various authorities. The optimum is stated t o be 40" C., since under these conditions t h e heat total of all t h e reactions summarized 4N02 0 2 2H2O = 4"03 just balances t h e heat loss b y radiation, and the gas and liquid temperature is said t o be practically uniform in all t h e towers. From t h e above experiments, however, i t would appear t h a t better results could be obtained b y cooling t h e entering gases t o a still lower temperature and warming t h e weak acid towers t o above 40° C. If we assume t h a t t h e opportunity factor of liquid

+ +

Vol.

12,

No. 6

5

gas contact is t h e same in t h e spherical flask as in Gutmann ball situated in t h e packing of a tower we may make a rough approximation as t o t h e efficiency of various tower systems b y calculating the percentage removal for different acid strengths a t varying temperatures from t h e curves. I n Table I11 such calculations are made, assuming t h e entering gas t o contain I O per cent NO2 and t o vary in temperature per saltum from 30' C. t o 50' C. from t h e first t o t h e last tower. The acid strength is assumed t o vary from o per cent in t h e last tower t o 5 j per cent in t h e first. T A B ~I11 E Time of Contact Min. Sec.

Percentage Removal by Tower System 8 Towers 4 Towers 6 Towers

It will be noted t h a t these figures approximate t h e results actually obtained in practice; further, if i t be assumed t h a t t h e interior of t h e flask is completely moistened b y t h e nitric acid and t h a t reaction occurs on t h e wet surface of t h e glass (0.178 sq. f t . ) , we may calculate t h e absorption rate per sq. f t . of tower surface, obtaining with a I O per cent gas a figure of approximately 2 X IO-^ lbs. NO2 per min. per sq. f t . of surface. I n t h e operation of three towers of a total capacity of 12,000 cu. f t . and a superficial area per cu. f t . of packing of I j sq. ft. with a I O per cent KO2 gas, 3 4 lbs. of nitrogen dioxide were absorbed per minute during a long run, or 1.99 X IO-^ lbs. NO2 per min. per sq. f t . , giving a n excellent agreement with t h e calculated figure.1 The influence of nitric acid concentration on t h e rate of absorption can also be calculated from t h e curves. If we assume t h a t t h e general equation Nz04 HzO JJ HNOa HXO2 is reversible, t h e velocity of absorption of nitrogen dioxide is given by

+

1

Partington and Parker, LOC.cit.

+

June, 1920

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

535

T i m e in Seconds PIG.6

Then, provided t h a t the " 0 2 functions in solution as nitric acid, i. e . , its rate of conversion into nitric acid being comparable t o t h e above reaction velocity, we may write t h e above equation, for a constant concentration of nitrogen dioxide and with water in relatively large excess,

Dc = a -

- PC2"o3, where a! and @ are constants. Dt I n Table IV are given t h e experimental results. TABLEI V

E Dt

.

10 per cent gas a t 40' C . .

. . . . . . . . . 1.08

10 per cent gas a t 30' C..

.........

1 per cent gas a t 30' C..

..

1.0 0.65 0.09 0.96 0.95 0.83 0.52 0.79 0.80 0.51 0.20

C*HNO~ 0.0 0.01 0.062 0.25

0.0

0.01 0.062 0.25 0.0 0.01 0.062 0.25

BC~ (1.08) 1.04 0.83 (0.09) (0.96) 0.94 0.84 (0.62) (0.79) 0.77 0.64 (0.20)

It will be noticed t h a t there is a fair agreement between t h e observed and calculated figures. Even better results are obtained, however, when this equation is applied t o t h e experimental figures obtained from actual tower operation. The observed and calculated values for t h e rate of absorption of nitrogen dioxide in a set of towers are given below. nc .

D_" Dt

1.00 0.99 0.95 0.90 0.80 0.70 0.55 0.32

0.00

CHNOa Per cent 0 10 20 30 40 50 60 65 68

E

-

(Calculated from 01 BCZ where P = 1.00; B = 1.25) (1.00)

T h e calculated values between t h e C"o8 = o per cent a n d C"ol = are excellent, b u t for stronger acid t h e concentration of water becomes discrepancies, as would be expected,

0.99 0.94 0.89 0.80 0.69 (0.55) 0.48 0.32

limits 6 0 per cent strengths when smaller, bigger are noticed.

E L I M I N A T I O N O F N I T R O U S ACID

As the product of interaction of nitrogen dioxide and water, nitric and nitrous acids are formed in equivalent quantities, b u t as t h e acidity of t h e solution increases, a greater deficiency of nitrous acid is observed; for example, with 5 0 per cent nitric acid t h e following figures (Table V) for t h e nitrous acid content were obtained by two methods: ( a ) After aspiration of a NO2-air mixture through a nitric acid solution in a thermostat. ( b ) After aspiration of a NOz-air mixture through a nitric acid solution t o which nitrous acid had been added so as t o obtain a relatively supersaturated solution of t h e latter in nitric acid. TEXPERATURE

c.

TABLEV PARTSPER THOUSAND HNOz

HNOz (.b.)

(a) 1.20 0.32

18 40 50

1.26 0.50 0.18

0.10

I t will be noted t h a t equilibrium is practically reached whether nitric acid or nitric-nitrous acid mixtures are employed t o start with. From the previously recorded experiments i t appeared possible t o explain t h e mechanism of t h e absorption process in nitric acid towers b y means of two reactions which would serve t o explain t h e fundamental difficulty associated with t h e alternative theories already alluded to, viz., t h e limitation of t h e acid strength t o 68 per cent. As has already been pointed out, t h e primary reaction on solution occurs according t o t h e equation N204 HzO = "03 "02. I n t h e presence of even dilute nitric acid, nitrous acid is unstable and breaks up into its anhydride a n d water, 2HNOz = Nz03 HzO. T h e nitrogen trioxide thus produced is in solution and not immediately eliminated, b u t is removed in one of two ways:

+

+

+

T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

536

( I ) I n the gas phase or in solution b y means of dissolved oxygen according t o the equation L”z03

+

0 2

= 2N204.

( 2 ) I n solution, b y reaction with t h e nitric acid according to the equation

Nz03

+ 2HN03 = ZNz04 + Hz0.

( I ) E L I M I N A T I O N B Y oxIDATIoN-The rate Of oxidation of nitric oxide t o nitrogen dioxide by means of oxygen in the gas phase has been studied by Lunge a n d Ber1,l Foerster and Blick,2 and more recently b y Bodenstein and E. Briner.3 The reaction possesses a negative temperature coefficient and is relatively slow. The necessity of providing space a n d time for this secondary oxidation as well as of maintaining t h e gases a t low temperature has been generally recognized. No investigations appear t o have been carried out o n t h e reaction velocity when in solution in water or dilute nitric acid. Such reactions proceed generally more rapidly in solvents t h a n in the gaseous phase, and i t would appear from some of Cavendish’s experiments on t h e estimation of oxygen b y means of nitric oxide in a eudiometer t h a t this is no exception. The matter is of considerable technical importance in t h e design of Pohle air or oxygen lifts for nitric acid towers. (2) E L I M I N A T I O N B Y I N T E R A C T I O N W I T H N I T R I C A c I D - I f this hypothesis of possible interaction be correct, i t is evident t h a t less nitrogen trioxide will be eliminated t o be oxidized by atmospheric oxygen according t o t h e first reaction, owing t o the fact t h a t i t has been practically all removed b y interaction with the nitric acid.

“I

Vol.

12,

No. 6

be reduced t o nitrogen dioxide by nitrogen trioxide according t o the equation1 Nz03 Z”03 = 2N204 H2O it may be urged t h a t Fritzsche2 showed t h a t t h e addition of N204,cooled t o -20’ C., t o water was a t tended with a separation into two layers both containing t h e deep blue nitrous anhydride and nitric acid, and t h a t with increasing temperature there was an increase in t h e evolution of nitrogen trioxide, supporting the assumption t h a t t h e formation of Nz03 from t h e N 2 0 4and water is a primary reaction. The addition of liquid N204 t o nitric acid of 65 per cent a n d under a t z o o C. was attended b y a vigorous reaction a n d t h e formation of the blue liquid, nitrous anhydride. Acid over 7 0 per cent in strength when similarly treated yielded no blue liquid, indicating t h a t the equilibrium isshiftedover t o the right by the mass of t h e ’ nitric acid. Ramsay likewise prepared pure NzOc b y t h e interaction of N203 and strong nitric acid. Using I O O per cent nitric acid N ~ 0 4can actually be distilled off without t h e use of any other dehydrating agent. Fritzsche’s results were confirmed by experiment a n d attempts were made t o find t h e equilibrium constant of the reaction 2N204 HzO = 2”Oa Nz03. A definite amount of Nzo4 was added t o nitric acid of various strengths a n d after completion of t h e reaction t h e Nz03 and N204 were extracted with pentachlorethane. T h e partition coefficient of nitric acid between water a n d pentachlorethane having been previously determined, and assuming t h a t both N 2 0 a and N204 have approximately the same partition coefficient between solvent a n d nitric acid, the ratio of N 2 0 3t o N204 could be determined by titration of the pentachlorethane layer with alkali and permanganate. The following results were obtained:

+

+

+

Strength of Acid Per cent

+

Ratio Nz0a : NzOa 1 : 10.55 1 : 8.51 1 : 7.17

68.0 66.7 58.6

‘‘N20a

‘H20

C N , O ~C * H N O ~ 0.90 0.75 0.99

A series of further experiments with varying amounts of nitrogen dioxide gave t h e following values for K: Acid Strength 68.0 66.7 64.7 56.6

10

a0

30

40

SO

6b

A c i d Strength FIG.7

The mechanism of the tower reaction can be depicted graphically as in Fig. 7. It will be noted t h a t t h e reaction in t h e gas phase is practically irreversible a t normal temperatures, but in the liquid phase i t is a truly reversible reaction. I n support of the hypothesis t h a t nitric acid can Z. angew. Chem., 19 (1906), 801. Ibid., 23 (1910), 2017. 8 Arch. des Sciences, 46 (1918), 23. 1 2

K 0.78 0.75 0.80 0.93 MEAN0.815

-

The constancy of ’ K is only approximate a n d is probably due t o t h e loss of nitric oxide into the air space above the liquid, b u t the decreasing concentration of Nz04 in equilibrium with a given quantity of N 2 0 3in the presence of increasingly dilute acid is demonstrated from the above figures. T h e following example shows the method of procedure: Five cc. of 58.6 per cent acid were placed in a small separatory funnel and 0.30 cc. (0.4425 9.) of purified liquid N 2 0 4 was added and allowed t o attain equilibrium, with agitation. The solution was 1 It is interesting t o note t h a t the strongest nitric acid obtained in practice corresponds t o the formation of the dihydrate HNOa.2HzO. 2 J . prakt. Chem., 22 (1841), 29.

T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y

June, 1920

%hen shaken with 5 cc. of pentachlorethane which was subsequently r u n into I O cc. of 0.958 N caustic soda. I n i t h e present case back titration required 3.3 cc. of 1.064 N sulfuric acid. The number of equivalents of nitrous and nitric a c i d derived from t h e pentachlorethane were therefore 0.00607. From nitric acid of t h e same strength when shaken with pentachlorethane, acid equivalent t o 0.00062 was abstracted, due t o t h e partition of t h e acid between t h e water and t h e pentachlorethane. The corrected figure for t h e former experiment was therefore o.ooj4j. Titration of t h e same solution with normal permanganate neutralized 6.1 2 cc. or 0.0061 2 equivalents. T o distinguish between nitrous acid from NzO3 a n d t h a t from N204, recourse was made t o t h e use of simultaneous equations. Since Nz03 HzO = z H N O Z H20 = HNOz "03 Nz04 i t is evident t h a t if x = number of molecules of N2O3 in t h e extract and y = number of molecules of N2O4 accompanying i t , t h e total number of equivalents reacting with alkali is 2X 2y = 0.00545. But in t h e case of oxidation according t o t h e equation HNOz 0 = "03 t h e equivalent weight of HNOz in oxidation is onehalf t h a t when reacting as an acid. The total number of equivalents reacting with permanganate is therefore 4x z y = 0.00612. Hence 2 x = 0.00067, or x = 0.000335 a n d y = 0.00239,

53 7

The other values were calculated in a similar manner. Attempts t o measure t h e rate of reaction in 50 t o 60 per cent nitric acid were without success as it is practically instantaneous even a t -10' C. From this value of K we may calculate t h e equilibrium concentrations of Nz04 and Nz03 in nitric acid of various strengths, and this permits us t o obtain an approximation as t o t h e amount of absorption of NOz which would occur on passage into nitric acid when secondary oxidation does not take p1ace.l T H E P R E P A R A T I O N O F STRONG NITRIC ACID

From t h e previous considerations t h e hypothesis was advanced t h a t t h e limitation in t h e strength of acid produced by t h e interaction of nitrogen dioxide and water was due not t o t h e volatility of nitrous acid or t o its decomposition, b u t t o t h e reducing action of nitrous anhydride on strong nitric acid, ~ H h ' 0 3= 2Nz04 Hz0. It therefore appeared possible t h a t the equilibrium would be shifted over t o t h e left with t h e production of strong acid provided t h a t t h e continuous removal of N203 could be ensured without simultaneous removal of Nz04 or t h e addition of too much water. This. could be effected by strictly limiting t h e amount of water vapor in t h e condensation of a mixture of N204, air and water vapor, and converting any dissolved nitrous anhydride into nitrogen dioxide b y agitation with oxygen. To test this point a gaseous mixture of NOz and oxygen in t h e proportion of four t o one b y volume, t h e oxygen being previously charged with sufficient water vapor t o produce IOO per cent nitric acid on complete condensation, was passed directly into a cooling syswhence y = 7.13. t e m consisting of a wide glass U-tube connected t o a X The Nz03 has been derived entirely from t h e N z 0 4 spiral condenser 9 f t . long, maintained a t - j o C. since none was present a t t h e start, a n d since 2 mols. The condensate obtained was observed t o separate N204 give one mol. N203, we can substitute in t h e ihto two distinct layers on standing. Analysis of each layer gave t h e following results: place of it one-half t h e N 2 0 4converted. We have therefore UPPERLAYER LOWER LAYER Per cent Per cent N z 0 4remaining -0.4425 - Nz04 (converted) NOz . . . . . . . . . . . . 50 NOz. ......... 29.4 7.13 -HNOa. .......... 501 HNOa.. . . . . . . 4 3 . 4 l/z N z 0 4converted l / ~ s z 0 4 (converted) HzO.. . . . . . . . . . . Trace HzO . . . . . . . . . . 27.2 Concentration of HhTOa = 100 Concentration of HNOs = 61.5 whence Nz04 (converted) = 0.0972. 1 The presence of a small quantity of NzOa in the upper layer vitiated This has been converted into Nz03 and "03 ac- the accuracy of these figures. cording t o t h e equation It appears probable t h a t t h e original condensate 2Nz04 HzO = N203 2"03 is acid of 60 t o 65 per cent concentration which dis46 = 0.0401 g. KzO3 a n d has given rise t o 0.0972 X solves more nitrogen dioxide, t h e nitrogen trioxide pro184 duced being continuously re-oxidized by oxygen.

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+

a n d 0.0972 X

I 26

~

184

=

0.00666 g. H S 0 3 , and t h e water 18

= 0.0097 g. H20. 184 In the original quantity of aqueous acid there were 4.0522 g. " 0 3 and 2.0628 g. of water, so t h a t t h e weights in equilibrium are H N 0 3 = 4.0522 0.0666 = 4.1188 g. = 0.0654 mol. HzO = 2.8628 - 0.0097 = 2.8j31 g. = o.Ij8.j mol. Nz03 = 0.0401 = o.oj29 mol. Nz04 = 0.442 j 0.0972 = 0.3453 g. = 0.37 j 5 mol.

remaining

=

0.0972 X

+

-

~

The acid is raised t o such a concentration by this method t h a t t h e addition of a small excess of nitrogen dioxide causes t h e separation into two layers, owing t o t h e difference in solubility of weak and strong nitric acid in liquid N204. I t has been previously shown b y Fritzsche t h a t strong nitric acid separated into two layers on t h e addition of liquid Nz04. Meister Lucius and BriiningZ have shown t h a t separation into two layers occurs only with acid of 80 per cent concentration and over, t h e upper layer consisting of 98 t o 99 per cent nitric 1

Partington and Parker, LOG.c i t . (1915).

* British Patent 4,345

T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y

538

acid containing dissolved N204, a n d t h e lower layer of aqueous HNO3 more dilute t h a n t h a t originally taken for treatment, The above experiment was repeated and t h e condensed layers treated with a slow stream of oxygen t o remove all traces of t h e NzOs. UPPERLAYER Per cent

.......... 72.4 ......... 25.7 1.9

NO?.. "0s.. HzO Concentration of

...........

=

"08

91.9

LOWERLAYER Per cent 11.0

.......... ......

NOz "0s.. 74.3 Ht0 .......... 14.7 Concentration of "Os 84.0

Having shown t h e applicability of this process t o t h e direct preparation of strong nitric acid from nitrogen dioxide and oxygen, attempts were made t o prepare strong nitric acid from a mixture of NOz, air and oxygen. For this purpose oxygen was introduced along with t h e air supply and the concentration of NO2 in t h e gas mixture was maintained a t a low figure, approximating technical conditions of operation in denitrator towers, viz., 2 0 per cent of t h e air content. Air rate c 60 cc. per min. (at 30' C.) saturated with water vapor Oxygen rate = 60 cc. per min. saturated with water vapor a t 18' C. NOz rate = 60 cc. per mi?. dry Temperature of cooling spiral = -8" C. Analysis of the Condensate Per cent "08..

............... 85.4 .................. ..................

N0z 6.1 He0 8.5 Concentration of nitric acid = 91.2

The conditions necessary t o obtain concentrated acid from weaker gases, e . g., those containing I O t o 13 per cent of NOz b y volume, such as are obtained in ammonia oxidation processes, were next examined. From vapor pressure d a t a t h e point of commencement of liquefaction of nitrogen dioxide of various concentrations has been determined as follows: Commencement of Liquefaction of Nz04

S O 2 in Gas

c.

Per cent

z:

50 25 20 15

-12

-18 -2 3 -3 5 -60

10 5 1

It was a t once evident t h a t t h e direct condensation of dry dilute NOz required extremely low temperatures for operation. Provided, however, a more easily condensable constituent of t h e gas phase be present which is a solvent for nitrogen dioxide, t h e removal of NOz can be effected a t t h e temperature of liquefaction of t h e solvent. I n t h e present series of experiments this solvent was dilute nitric acid produced from t h e water vapor present in t h e gases. The following method of procedure was adopted: A 1 2 per cent NOz-air mixture obtained from a small platinum gauze ammonia oxidation converter was cooled quickly t o 20' C. in order t o separate o u t t h e excess of water, over a n d above t h a t necessary for t h e completion of t h e equation 2Nz04 ZHZO 0 2 = 3HNO3. ,This gas was then led into cooling vessels immersed in freezing mixtures of ice a n d salt in order t o condense o u t t h e nitric acid formed. With a flow rate of 1 . 5 liters of gas per min., a condensate of 61.2 per cent, increasing t o 78.1 per cent acid strength a t -10' C., was obtained.

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No. 6

SUMMARY

I-The rate of absorption of nitrogen dioxide by water a n d nitric acid solutions follows a monomolecular law. I n aqueous solutions t h e reaction is pseudomonomolecular, being a process of physical solution, t h e rate depending on the partial pressure of NO2 in t h e gas phase. I n strong acid solutions t h e reaction is a t r u e c h e d c a l monomolecular reaction, t h e rate depending on t h e concentration of t h e Nz04. An inversion of t h e temperature coefficient of absorption occurs a t I O per cent nitric acid strength. 2-The limitation of acid produced in nitric acid absorption towers t o concentrations of 64 per cent nitric acid is due t o the reduction of nitric acid by nitrous anhydride produced in solution, according t o t h e reversible equation 2"Oa f T\Tzo3 H2O 2Nz04. The equilibrium constant of this equation a t ordinary temperatures is approximately K = 0.81. 3-The production of nitric acid exceeding 64 per cent in strength is possible, provided t h a t t h e NzOs be continuously converted into nitrogen dioxide by agitation with oxygen. +--The production of strong nitric acid may be accomplished by a process of refrigeration of a gas mixture containing suitable proportions of nitrogen dioxide, water vapor, and oxygen.

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T H E PRODUCTION OF HYDROCHLORIC ACID FROM CHLORINE AND WATER By H. D. Gibbs COLORLABORATORY, BUREAUOF CHEMISTRY,WASHINGTON,D C. Received October 28, 1919

Since t h e utilization of t h e potentially large production of chlorine brought about through war requirements offers some difficulties, pending t h e establishment of a n equilibrium by adjustment of the chemical industries from a war t o a peace basis, t h e economic conversion of chlorine into hydrochloric acid has been investigated. Early in t h e study of elementary chemistry one learns t h a t chlorine reacts with water in the sense of t h e expression HC1 HOC1 Clz H2O a n d t h a t t h e hypochlorous acid decomposes thus, HClO + HC1 0. Owing t o t h e instability of hypochlorous acid in t h e presence of hydrochloric acid, the first reaction progresses t o a very limited extent a n d the production of t h e end-products, oxygen a n d hydrochloric acid, i s greatly accelerated by light. This accounts for t h e fact t h a t aqueous solutions of chlorine eventually become aqueous solutions of hydrochloric acid upon standing in t h e laboratory, and should thecefore be protected from t h e light. A catalyst t h a t will speed these reactions a n d afford a cheap method of conversion of chlorine and water into hydrochloric acid a n d oxygen could be made t h e basis of a valuable commercial process. This is t h e reversal of t h e Deacon process. T h e

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