THE CONSTTTUTfON OF THlll NITROPRUSSIDES. 1429 CXXXIV. -The Constitution of the Nitroprussides. Part I. By GEORGE JOSEPH BURROWS and EUSTACE EBENEZER TURNER. FROM time to time the nitroprussides have been made the subject of considerable speculation but little attempt has been made to verify experimentally the various constitutions assigned to them. Hofmann who is responsible for most of the experimental work in this field (Amnalen 1900 312 1) assigned to sodium nitro-prusside the co-ordination formula [Fe(CN)’),NO]Na,. Friend (T., 1916 109 721) apparently without further experimental work, proposed for potassium nitroprusside the formula (I). Conductid y and Cryoscopic Measurements. (1.) (11.) This formula corresponds with the empirical one given in works of reference (for example Moissan ‘‘ Trait6 de chimie minerale,” 1905 417) which formula however seems never to have had any evidence in its favour.Friend a t the same time rejected the formula (11) suggested by Browning (T. 1900 77 1238) for potassium nitroprusside in view of his theory as to the constitution of the ferrocyanides which theory has since been shown to be unnecessary (Bennett T. 1917, 111 490). It was suggested by one of us (T. 1916 109 1130) that a deter-mination of the molecular weights of some nitroprussides would throw light on the problem and as a result a number of nitro-prussides have been prepared and investigated cryoscopically and their conductivities measured. From the results so obtained the number of ions present in a solution of a nitroprusside has been calculated and conclusioiis have been drawn as t o the molecular weighh of the salts in question.I n these experiments the degree of dissociation of the salt at any particular dilution was found in the usual way by dividing the molecular conductivity at that dih-tion by its value a t infinite dilution. From cryoscopic measure-ments the molecular depression of the freezing point of water was found for various concentrations of the salt and by dividing this number by 18.7 (the molecular constant for water) the value fo 1430 BURROWS AND TVRNBR: i (the van’t Hoff coefficient) was obtained. The number of ions, k into which each molecule of the salt dissociates was thm obtained by aubdituting the experimental values of a and i in the equation i=l+ (k- 1)a.A similar method was used by Petersen (Zeitsch. physikal. Chem. 1902 39 249) in connexion with the cobaltamminea. This author was of the opinion that the conclusions drawn by Werner (Zeitsch. physikal. Chem. 1893 12 35 etc.) from measurements of the molecular conductivity at a dilution of 1000 litrea (not necessarily a t infinite dilution) were in most cases inaccurate. The figures given by Jones (Carnegie Znstitute of Washington, Publication No. 170) for the molecular conductivities of a large number of salts show that most salts are completely dissociated a t dilutions of about 1000 litres. In some cases however the mole-cular conductivity again increases beyond that dilution owing t o causes other than dissociation. From the figures given by Petersen (Zoc.c i t . ) for the conductivity of the cobaltammines it would appear that similar difficulties arose in his work. I n the present investigation the substances considered are salts of a strong acid. (This has been found to be the case from a pre-liminary examination of the molecular conductivity of nitroprussic acid itself.) The conductivities of the salts were determined for solutions diluted to 2048 litres and the values were plotted against the concentrations. The value for infinite dilution was found by extrapolation from the curve so obtained. I n all cases the value of pa differs only slightly from the value actually found for plOx a result which was expected from the nature of the salts in question. In the following tables are given the values of k calculated on the assumption that the nitroprussides are represented by the simple formula M,’[Fe(CN),NO] which will be referred to in future as type I.I n addition the value of k has been calcu-lated in each case for a molecule M4~[Fe2(CN),,(NO),1 (type 11). In the case of a dnivalent cation a molecule of type I will dissociate into three ions type I1 giving five ions. The value of k should therefore approximate to 3 if formula I is correct whilst if I1 is correct k,=5. In the same way a salt of a bivalent cation should give the values k = 2 or kl = 3. It is considered that the results obtained show conclusively that all the nitroprussides examined conform t o the simple formula (type I). They are salts of H,[Fe(CN),NO] and not of The possible effect due to hydration of the ions has not been over-looked (compare Jones Camegie Institute of Waahdngton Publi-H,[Fe*(CN) ,dNO) THE CONSTITUTION OF THE NITROPRUSSIDES.PART I. 1431 cation No. 180) and it is considered that the conclusions drawn from the figures obtained in the preseht work cannot be regarded as vitiated on this ground. Whereas hydrate formation may account for the differences between the experimental and absolute values of k the extremely large differences in the case of k cannot be accounted for in this way. It is hoped in a future communication to describe the alkyl nitroprussides some of which have been prepared although in an impure state only. EXPERIMENTAL. The conductivity measurements were all made a t 25-0°. The degree of dissociation of salts a t Oo differs only slightly from that a t 25O and the latter temperature allows of greater accuracy in determining the conductivity.In the following tables v i8 the number of litres containing one gram-molecule of the salt (calcu-lated for the simple formula I) p is the molecular conductivity, a is the degree of dissociation and is equal to pu/pLoo At is the observed depression of the freezing point of water MAt is the molecular depression and is equal t o v x At x 10 i is the van't Hoff coefficient and is equal to MAt/18-7 and k is the number of ions into which a molecule dissociates and is obtained from the equation i= 1 + (k - 1)a; k is the corresponding value of L calculated for a molecule of type I1 by doubling i and then substituting in the equation i = 1 + (k - 1)a.The values of a in the cryoscopic tables are taken from the curves obtained from conductivity data. Sodium Nitroprusside Na,[ Fe (CN),NO] 2H,O. The salt used was a pure specimen. Conductivity Measurements . 0. 4 8 16 32 64 228 266 512 1024 2048 00 P-168.2 169.4 181.8 194.3 204.9 214.2 222.0 229.2 244.6 246.0 286.2 a. 0.63 0.69 0.74 0.79 0.84 0.87 0.91 0.94 0.96 - 1432 BURROWS AND TURNER: Cryoscopic Measzcrements. 2 At. MA,. 47.1 0-115 54.1 24.4 0-218 53.2 14.56 0.348 50.2 10.81 0.466 49.3 8.76 0.660 49.1 7-12 0.678 48.3 Type I reqUim k=3. i. a=k/pa,. b. 2.90 0.81 3.35 2.84 0.77 3.40 2-68 0.73 3.30 2.64 0-71 3-30 2.62 0.70 3-31 2-68 0.68 3.32 Type I1 requires IC = 5.kl. 6.9 7.1 7.0 7.0 7.0 7.1 Potassium Nitroprusside Kz[Fe(CN),NO]. This salt was prepared by decomposing the barium salt with the calculated weight of pure potassium sulphate filtering off the barium sulphate and evaporating the filtrate a t a low temperature under diminished pressure. The residue so obtained was crystal-lised from aqueous alcohol containing about 95 per cent. of alcohol. It crystallises in pale pink crystals without water of crystallisation : C,ON,FeKz requires Fe = 19.0 per cent. 0'4010 gave 0.1076 Fe,O,. Fe = 18.8. v. 8 16 32 64 128 256 512 1024 2048 a0 Coductivit y Measurements. P* 199-2 206.2 215.7 227-2 236.6 244.0 249-3 257.0 2584 258- 1 a. 0-745 0-795 0.836 0.881 0.917 0.946 0-966 0-996 --v.34.4 24.1 14.66 11.65 7.02 4.78 Cryoscopic Measurements. At. MA,. i. a=pJFa,. k. kl. 0.160 51.6 2.76 0.85 3-07 6.32 0.213 61.3 2.74 0.82 3.12 6.46 0-316 46.3 2.48 0.79 2-87 6.01 0.405 47.2 2.52 0.77 3.00 6.25 0.641 45.0 2.41 0.74 2.91 6.16 0.909 43.4 2-32 0.71 2-86 6-11 Type I requires k=3. Type I1 requires k1=6. Barium Nit ropusside B a[ Fe (CN) ,NO] ,3 H20. This salt was prepared by precipitating a solution of the sodium salt with zinc sulphate and boiling the zinc salt so obtained with a suspension of precipitated barium carbonate. The filtered solu-tion of the barium salt was evaporated under diminished pressur THE CONSTITUTION OF THE NXTROPRUSSIDES. PART I. 1433 a t a low temperature and the salt crystallised from aqueous alcohol : 0.8344 gave 0'4734 BaSO,.C,0N,BaFe,3H20 requires Ba = 33.7 per cent. The anhydrous salt was found to be extremely hygroscopic. Conductivity Measurements . Ba= 33.4. v. 8 16 32 64 128 256 612 1024 2048 00 v. At. 37-7 0.093 16.6 0.180 9.87 0.285 6.67 0.413 4.73 0.575 P. 152.9 165.9 177.1 190.7 203.0 216-2 223.6 236.9 240.6 243.0 a. 0.63 0.68 0.73 0.79 0.84 0.89 0.92 0-98 -Cryoscopic Measurements. MA+. i. a = ~ l p m . b. k,. 35.1 1-88 0.74 2-19 4.73 29.7 1-59 0.68 1.87 4.21 28.1 1.50 0.64 1.78 4.13 27.6 1-47 0.61 1-77 4-18 27.2 1.45 0.59 1.76 4.22 Type I requires E= 2. Type I1 requires k,= 3. This salt is of especial interest owing to the rough equality in weights of the anion and cation.Ammonium Nitroprusside (NH,)2[Fe(CN),NO]. This salt was obtained by decomposing the barium salt with an equivalent weight of ammonium sulphate filtering evaporating under diminished pressure and crystallising from aqueoug alcohol, when reddish plates very readily soluble in water were obtained : C,ON,Fe(NH,) requires Fe= 22.2 per cent. 0-2390 gave 0.0768 Fe,O,. Fe = 22.5. v. 16 32 64 128 266 512 1024 2048 Qo Conductivity Measzcr ements. P* 206.7 218.0 228.5 236.7 246-6 251.2 261.3 266.2 268.0 a. 0.77 0.82 0.86 0-89 0.92 0-94 0.98 1434 BURROWS AND TURNER: Cryoscopic Measurements. V. At. MA,. i. a=k/pao. k. kl. 26.3 0.214 56.4 3.01 0.80 3.51 7.27 10.73 0.460 48.3 2-60 0.74 3-18 6.62 6.68 0.704 47.0 2-61 0.69 3.19 6.83 Type I requires k= 3.Type I1 requires k,= 6. Methylammonium Nitroprusside (MeNH&[Fe(CN),NO]. This salt and the diroprussidea of di- and tri-methylamine were obtained by treating a solution of the free acid (obtained from the barium salt and the calculated sulphuric acid) with a slight excess of an alcoholic solution of the amine. The solution so obtained was evaporated under diminished pressure and the solid residue crystallised from alcohol. The alkylammonium nitro-prusaidea crystallise in reddish plates which are very readily soluble in water. In appearance they resemble the barium or ammonium salts : 0*1117 gave 0.0306 Fe,O,. Fe=19'2. C,ON,Fe(MeNH& requires Fe = 20.0 21.32 64 128 258 512 1024 a 0. At-36.6 0.148 19.06 0-262 14-16 0.339 10.13 0.466 8.36 0.641 Conductivity Measurenwnts. P-185.2 197.7 208.0 218.5 226.8 233.4 240.0 Cry oscopic Measurements . MA,. i. a=pJpoo. 64.2 2.90 0.78 49.9 2.67 0.71 48.0 2-57 0.68 47.2 2.62 0.62 46.2 2.42 0.59 per cent. a. 0.77 0.82 0.87 0.91 0.94 0.97 -k. kl. 3.45 7.2 3.36 7.1 3-31 7.1 3.46 7.5 3.41 7.6 Type I requires k = 3 . Type I1 requires bl=B. Dimethylammonium Nitroprusside (Me2NH,),[Fe(CN),NO]. C,0N6Fe(Me,NH,) requires Fe = 18-1 per cent. 0.1028 gave 0.0272 Fe,O,. Fe= 18.5 THE CONSTITUTION OF THE NITROPRUSSIDES. 1435 21. 16 32 64 128 256 512 1024 2048 Conductivity Measurements. P. 156.5 172.1 185.2 196.3 205.8 212.5 220.2 227-3 230-0 a. 0-68 0.75 0.81 0.85 0.90 0.92 0.96 1 Cryoscopic Measurements. er. At. MA*. i. a=pufpm. k. k,. 35.2 0.149 52.4 2.50 0.76 3-37 7.05 16.4 0.294 48-1 2.57 0.68 3-31 7-09 8-84 0.500 44.2 2.36 0-Gl 3-23 7.10 5-92 0.714 42.3 2-26 0.57 3.2 1 7.20 Type I requires k = 3. Type I1 requires k = 5. Trimethylammonium Nitroprusside (Me3NH)2[Fe(CN),NO]. 0.2234 gave 0.0512 Fe,03. Fe = 16.0. C,ON,Fe(Me,NH) requires Fe = 16.7 per cent. v. 16 32 64 12s 256 512 1024 2048 00 Conductivity Measurements. P* 137.1 154.0 167.3 183.0 191-7 201- 1 208.0 210.5 212.0 a. 0.65 0.73 0.80 0-86 0.90 0.95 0.98 -cryoscopic Measurements. V . At. MA,. i. a=pVlpm. k. kl. 39.0 0.134 52.2 2.79 0.75 3.39 7.11 14.47 0.290 42.0 2.24 0.63 2.97 6.52 6-56 0.588 38.6 2.06 0-50 3.12 7.24 Type I requires k = 3. Type II requires k,= 5. THE UNIVERSITY CHEMICAL LABORATORIES, SYDNEY. [Received November lst 1919.1 VOL. cxv. 3 T