MIXTURES OF NITROGEN PEROXIDE AND NITRIC ACID. 45 I1 I.-Mixtures of hTitYogen Peroxide and Nitric Acid. By WILLIAM ROBERT BOUSFIELD K.C. NITRIC acid and nitrogen peroxide' are mutually soluble in certain proportions and in other proportions give rise to a double layer. When t o the simple binary mixture1 water i s added a more com-plex mixture arises. A systematic study of these mixtures was projected beginning with the simple binary lrixture and passing on t o consider bhe modifications which are introduced by the addi-tion of water. The present' communication deals with the firs 46 BOUSFIELD MIXTURES OF part only of this research. The nitric acid and nitrogen peroxide for the research have been specially prepared by Nobel’s Explosives Co. LM. with whase chief research chemist, Mr.Rintoal I have been in consultation from time to time. Nitrogen Peroxide .-Nitrogen peroxide absorbs moisture readily, and in the absence of excess of nitric acid appears to decompose according to the equation N,O + H20 = HNO + HNO,. The presence of a very small quantity of water will therefore cbange the colour of liquid nitrogen peroxide to a dirty green but traces of moisture may involve the presence of nitrogen trioxide without noticeable change of colour. Two samples of liquid nitrogen peroxide were supplied by Messrs. Nobel. Nearly all the work was donel with sample No. 1 which was subsequently found to have contained traces of nitrogen trioxide. Sample No. 2 had been purified by distilling i t with phosphoric oxide. The prob-able reactions which result in this purification appear t o be 2HN0 + P205 = 2HP0 + N205, N,O,+ N20,=2N,O,.This sample No. 2 may be regarded as pure’ liquid nitrogen peroxide. I n the meantime before receiving this pure sample I had used up sample Not. 1 in nitric acid solutions and recovered it by dis-tillation and rectification with a pear still-head. This sample to which I may refer as No. 3 appears by the density given below to be nearly as pure as No. 2. This is probably due to the oxida-tion of the nitrogen trioxide by nitric acid according to the equation ZHNO + N,O = 2N,O + H,O. Thus with excess of nitric acid and very little water the reaction N20 + H,O = HNO + HNO, appears to be reversed and in the presence of excess of nitric acid the water appears to have no decomposing effect on t.he nitrogen peroxide.As further evidence of t.his the addition of a few drops of water to the orangecoloured liquid nitrogen peroxide turns itt a dirty dark green presumably due to the mixture of the blue nitrogen trioxide or nitrous acid with the1 orange nitrogen peroxide but t3he addition of a few drops of nitric acid destroys the green and restores the orange colour. Thee green d o u r cannot be eliminated by simple rectification, as the nitrogen trioxide and peroxide appear to pass off together, the resulting gas being of a somewhat deeper red colour than that of pure nitrogen peroxide. Nor does the addition of syrupy phm NITROGEN PEROXIDE AND NITRIC ACID. 47 phoric acid help matters. Solid phosphoric oxide appears to be necessary to get rid of the nitrogen trioxide unless excess of nitric acid is added.Another sample (No. 4) of a bulk of about half a litre which had been accidentally contaniinated with sufficient water to turn i t to the dark green colour was mixed with sufficient nitric acid to restore hhe orange colour and then distilled with phosphoric oxide. The resulting sample No. 5 was of the same colour and density as the pure sample No. 2. I n t.he rectification of sample No. 4 the first few C.C. passed over green a t a temperature at4 the top of the still-head of 19-20'. The bulk which constitutes sample No. 5 distilled a t 21-9-t0*lo which may be taken as the boiling point of pure liquid nitrogen peroxide. TABLE I. Specific Volu~i7 es o f Samples o,f YitrogetL Peroxide.NO. 1 ............ 0.67390 0.68110 0-68864 NO. 2 ............ 0.67435 0.68172 0.68938 NO. 3 ............ 0.67432 0.68168 0.68935 No. 4 ............ - - 0.68946 No. 5 -4". 11". 18". - 0.68170 ............ A set of density observations on sample No. 1 was taken a t the temperatures given in table 11 which gives the observed specific volumes and those calculated from the formula 2r = 0.66994 + 0.0009767t + 0.00000344t2. TABLE 11. Specific Volumes of Sanzple N o . 1 at Various Tenbperatures. t. D. v observed. v calculated. Difference. 0.08" 1.49250 0.67002 0.67002 i 11 1.46822 0.68110 0.681 10 rt 15 1.45909 0.68536 0.68536 rrt 20 1.44750 0.69085 0.69085 A 7 1.47704 0.66703 0.67695 - 8 18 1.45214 0.68864 0.68863 - 1 For the pure sample No.2 the specific vo81ume-tempesature curve derived from the three duplicated observations a t 4O 1l0, and 1 8 O is which may be taken as giving the correct specific volume of pure nitrogen peroxide a t temperatures from Oo to 20° within + 2 in the fifth place of decimals = 0.67027 + 0.00100'75t 4- 0.000003t2 48 BOUSFIELD MIXTURES OF iVitm'c Acid.-The nitric acid used was specially purified and sent to me by Messrs. Nobel with the' following analysis: Nitric acid ........................ 99.68 per cent. Hydrochloric acid ............... 0-007 , ,, Sulphuric acid .................. 0.068 .... Nitrous acid ........................ 0.012 , .. Mineral matter .................. nil 99.767 , ,, I have taken the percentage of water by difference as 0.233 per The densities of this acid a t the temperatures indicated are: cent.4O. 1 1 O . 1 so. 1.5381 1.5254 1.5126 I n calculatdng the strength of the various mixtures of nitric acid and nitrogen peroxide, to which reference1 is made later the nitrous acid given in the above analysis has been reckoned as nitrogen peroxide since for the reasons above given it is assumed that with very concentrated nitric acid the nitrous acid present is oxidised to give nitrogen peroxide and water. In tab10 I11 are given the resulting values of the specific volumes derived from the density determinations.* P is the per-centage by weight of nitrogen peroxide in the mixture. The temperature coefficients a from 4O to 1l0 and from 1l0 to 1 8 O TABLE 111. Specific Volumes o:f Mixtures uf ATitric A ck? an,d Nitrogen Peroxide.P. 0 1,2168 8.021 16.88 26-09 34.925 37-60 42.01 43.71 46.70 48.66 49.96 51.37 53-10 93-86 96-93 98.49 100~00 v4. 0.65015 0.64839 0.63719 0.62372 0.6121 4 0.60445 0.60296 0.601 12 0.60082 0.60088 0.60113 0.60145 0.60205 0-60341 0-67305 0-67371 0.67435 I 2'11. 0.65557 0.65374 0.64211 0.62821 0-6 1655 0.60885 0-60743 0.60568 0.60543 0.60561 0-60599 0.60639 0.607 15 0.60873 0.68030 0;68098 0.68172 -V18* 0.661 13 0.65920 0.64715 0.63291 0.82113 0-6 1346 0.6 1204 0.6 1044 0.61026 0-61059 0.61110 0.61161 0.61248 0-61436 0.68610 0.68786 0.68855 0.68938 Temperature-coefficients. A r -.4-11' 11-18O x 105. u x 106. 77 79 76 78 70 72 64 67 63 65 63 66 64 66 68 68 66 69 68 71 69 73 71 75 73 76 76 80 104 108 104 108 105 109 - -* The actual density determinations have been omitted at the request of the Publication Committee to save space NITROGEN PEROXIDE AND NITRIC ACID. 49 TABLE IV. Coiztraction of Mixtures of Xitric Acid and Nitrogen Yerolxide. Contraction per C.C. of solution. Contraction per gram of solution. P. 1.22 8.02 16-88 26.09 34-92 37-60 42-01 43-7 1 46.70 48.66 49.96 51-37 53-10 93.86 96-93 98.49 4". 0.0032 0.0234 0.0489 0.0724 0.0896 0.0934 0.0985 0.0997 0.1008 0.1011 0.1011 0.1005 0.0988 0.0008 0.0004 -11". 0.0033 0.0242 0.0506 0.0743 0.0917 0.0954 0.1005 0-1017 0.1027 0.102s 0.1026 0.1019 0.0998 0.0009 0.0005 -IS&.0.0034 0.025 1 0-052 1 0.0763 0.0938 0.0976 0.1025 0.1036 0.1044 0.1044 0.1040 0.1031 0-1005 0.0023 0.0009 0.0006 4 O . 0.002 1 0.0149 0.0305 0.0443 0.0542 0.0563 0.0592 0.0599 0.0606 0.0608 0.0608 0-0605 0.0596 ---1 1 O . 0.0022 0.0156 0.0318 0.0458 0-0559 0.0580 0.0609 0.0616 0.0622 0.0623 0.0622 0.0619 0.0607 ---18" 0.0023 0-0163 0.0330 0.0474 0.0575 0-0597 0-0626 0-0632 0.0637 0.0638 0.0636 0.0632 0.0618 ---are also set out in table 111 as they give an important clue to the nature of the combination which is taking place1 in the mixture.Another important matter bearing on this is the contraction which takes place a t various constitutions of the mixture. The euthetic point that is t,he point of closest packing (see Bousfield, T. 1915 107 1412) may be obtained by calculating ths ratio of the volume of the constituents before mixture t'o the volume a t the same temperature after mixture which is where v,- = specific volume of nitrogen peroxide w = specific volume of nitric acid v =specific volume of the mixture. It should be noted that R-1 is the contraction per C.C. of solu-tion formed tho values of which are set out in table IV. I n the same table are set out the values of the difference between the volume of a gram of the constituents before mixing and the volume after mixing which is Pv,+(lOO -P)w ~- - v.A = 100 Consideration. of t t e Results.-The results given in the tables are exhibited in Figs. 1 2; 3 and 4 where they are set out on the values of P the percentage by weight of nitrogen peroxide as abscissae. There are shown i n 50 BOUYEIELD MIXTURES OF Fig. 1 the specific volumes of the mixtures. Fig. 2 the values of 22 - 1 near the maximum. Fig. 3 the valueis of h near the maximum. Fig. 4 the temperature coefficients for the intervals 4-11° aiid 11-18°. FIG. 1. 90 100 0.66 0.65 0-64 0.63 0.63 0.61 0.60 0.63 0.67 0 10 20 40 50 60 Specific volume of mixtures of nitric mid and nitrogen :c =Percentage of nitrogen peroxide. y = Speci$c volume. 4 O 11° and 18O. peroxide ut The notab16 heat of evolution on mixing approximately equal weights of nitric acid and nitrogen peroxide (which it.is proposed to determine accurately a t a later st'age) indicates a powerful com-bination. The minimum values of the specific volume curves give the same indication showing a notable contraction of about 1 NiTROGEN PEROXIDE AND NITRIC ACID. 51 The minima do not however locate the exact. composi- per cent. tion. They occur : for for 1 8 O , 43.8 ,, 4O a t about 44.6 per cent. for 11° , 44.2 ) ) F I G . 2. 0.104 0.103 0.102 0.101 0.100 0.099 0.098 40 50 Contraction per C.C. of solution. x = P . y = R - 1 52 BOUSFIELD MIXTURES OF The position of the minimum is so largely determined by the mere density differences of the two components that it cannot' locate precisely the percentage of the combination.The euthetic point (see Bousfield Zoc. c i t . ) is generally very close to the neighbourhood of the point of definite composition. The FIG. 3. I 55 Contraction per grain of solution. x=P. y = A. values of the contraction per C.C. of solution which determine the euthet,ic point are given in table IV and set out in Fig. 2. The maxima correspond with the euthetic point and occur: for for 1l0 ,? 48.4 >, for 1 8 O ? 47.6 ,, 4O a t about 49-2 per cent NITROGEN PEROXIDE AND NITRIC ACID. 53 The actual position of the euthetic point is determined not only by contraction due t o combination but also by contraction due to changes in the polymerisation of the constituents. The high temperature coefficients for both constituents indicate that these changes are notable.The percentage 49.2 a t 4 O corresponds very nearly with the composition 3HN0,,2N2O4. If we! take the curves in Fig. 3 in which the values of the con-traction per gram of solution are set out the effect of polymerisa-tion is ts some extent excluded and the maxima f o r 4O and 1.l0 appear to occur a t 49.3 per celnt. whilst that for 1 8 O is again shifted slightly to the left. On the' whole then itq may be said FIG. 4. 80 70 60 0 25 50 Temn~~crature-coe~cients of specific volums of mixtures of nitric acid and nitrogen peroxide. x = P . y = Temperature-coe$icient a. that the indications point to the composition of the definite com-pound 3HN03,2N,04 which co'rresponds with 49.33 per cent.An inspection of Fig. 4 in which the temperatxre coefficients are set out shows a definite minimum at. 26.7 per cent. which appears to be the same for each range of temperature; this corre-sponds with the definite composition 4HN0,,N20,. The first part of the specific volume curve shown in Fig. P is approximately straight' which indicates that the wholel of the nitrogen peroxide added up tto about 15 per cent'. enters into this combination with nitric acid. Furthermore the proportions of nitric acid and the compound 4HNO3,N,O4 derived from the mass-act& relation give a calculated specific volume which corresponds closely wit 54 MIXTURES OF NITROGEN PEROXIDE AND NITRIC ACID. the actual specific volume curve for a considerable distance that is until the e'ffect of the increasing proportion of the still denser cmbination becomes sensible.On the whole we may conclude that a t least two definite compounds exist in the solutions namely, 4HN03,N,04 and 3HN0,,2N,04. The Composition of t h e Double Layers.-The specific volume curves have a gap bet'ween about 54 and 92 per cent. If at 4-18O a mixture between these limits is made the solution separates into t8wo layers which are mutually saturated. Nitrogen peroxide is soluble in nitric acid up to about a 54 per cent. solution but the solubility of nitric acid in nitrogen peroxide is very much less. I n order to determine the maximum solubili-ties a t different' temperatares the two components were shaken together from Cime to time a t the required temperatures forming a cloudy liquid and kept in a thermostat until the two clear layers were completely separated.The different layers were then drawn off into a pyknmeter a t the required temperatures and again kept in the thermostat a t these temperatures during the adjustment of the pyknometer. The resulting deasity determinations enabled the compositions to be determined. I n table V are given the density observations for saturated solutions of nitric acid in nitrogen peroxide together with the specific volumes a t the temperatures and the resulting percentage, p of nitric acid in the saturated solutions. TABLE V. Saturated Solutions of Aritric d cid in Nitrogen Peroxide. Percentage of D. V. nitric acid. 4 O 1-48742 0.67231 4.90 11 1.47351 0.67865 6.67 18 1 - 459 40 0.68521 8.05 Since at this end of the specific volume curves they are practic-ally straight lines the percentages are easily calculated from the f armula where vo =specific volume of nitrogen peroxide vt =specific volume of solution containing p per cent.of nitric acid the values of the constants being : uo - vt = Bp, 4O. 1 1 O . 18". 0 0 0.67435 0.68172 0.68938 B 0*000424 0.000476 0.00052 6 It will be observed that the solubility of nitric acid in nitrogen peroxide rises rapidly with temperature being about doubled i EFFECT OF DILUTION IX ETJECTRO-TITRIMETRIC ANALYSES. 55 the range from Oo t o 20°. The values indicate that the solubility would vanish a t about -loo. In table VI are given the density observations for saturated solutions of nitrogen peroxide in nitric acid together with the specific volume a t the temperatures and the approximate per-centages of nitrogen peroxide' in the saturated solutions. TABLE VI. Sktiwuted Solutions of Nitrogen Peroxide iit A'itric Acid. 4 O 1.65432 0.60448 54.4 11 1.63942 0-60997 54-3 18 1.62501 0.61 538 54.0 D. V . P. I n t.his case the approximate compositions are determined diagrammatically from a large-scale specific volumel curve. The temperature of my laboratory in the end of May when these last observations were taken made them very difficult. It is however, clear that the change of solubility with te,mperat,ure is in this case very small and i t appears t o diminish with rising temperature. ST. SWITHIN'S, HENDON N.W. [Receiwed August 29th 1918.