THE ACTION OF SIUCA ON ELE-LYTES. PABT n. 2813 CCCLXXXVII1.-The Action of silica on Electrolytes. Part 11. By ALFRED FRANCIS JOSEPH and HENRY BOWEN OAKLEY. THE action of silica on two classes of electrolytes-acids * and salts-has already been studied (Joseph and Hancock J. 1923, 123,2022). Its effect on bases is now described. It must again be The experiments with acids have been repeated with purified ‘‘silica gel” with the same result. There has been some correspondence in Natecn (Jan. 31& March 28th and April 4th) on this wbject but the preaent authors have not yet obfained experimentd evidence req- 8 change of opinion. 5 c 3814 JOSEPH AND OAKLEY : emphasised that a high degree of purity of the silica is essential for work of this kind as the presence of a small quantity of basic impurity has an important effect on its properties.For example, a sample of silica gel which retained 6% of wafer aiter drying a t 180” retained only 302% if it had been washed with hydrochloric acid (followed by water) before being dried. Silicates of the AlMi and the AlMine-earth M&.-Mtky studies on these substances a t high temperatures have been carried out in recent years but data as to reactions taking place in presence of much water at the ordinary temperature are scanty. The following facts bear on what follows Potassium seems to form silicates of greater stability in water and lower solubility than sodium the properties of potash glass bear this out and also the fact (Morey J . Amer. Chem. Soc. 1914 36 230) that the compound KHSGO may be left for hours with water a t 100” without undergoing appreciable decomposition.Calcium silicate is less soluble in water than the barium salt. The sodium silicates, even when containing a high proportion of silica are all acted on by water to a noticeable degree. The equilibrium betwem silicic acid and sodium hydroxide was studied by constructing a titration curve for a “water glass ” solution containing 2 mols. of silica to 1 mol. of sodium oxide. The first points on the curve were obtained by the addition of hydrochloric acid and the later ones by the addition of sodium hydroxide. In each case the hydroxyl-ion concentration was determined by p measurement and the amount of sodium silicate present was found by subtracting the quantities of hydrochloric acid and hydroxyl from the total sodium.The results are in Table I. If the sodium oxide neutralised is plotted against p H , the curve shows a definite inflexion when the solution contains 2 mob. of neutralised sodium oxide to 1 mol. of silica. The pH a t which the ratio is half this is very nearly 10 and this corresponds to the value for the fist dissociation constant of silicic acid. The combination between the second molecule of sodium oxide takes place between ~ I I values of 11-6 and 12-6 indicating that the second dissociation constant of the acid is about 10-l2. The curve is typical of the combination of a strong brtse with a weak dibasic acid. TABLE I. Solution contains 0.0186 mol. of silica per litre. pa ..............................3.2 9.40 9-90 10.0 * 10.72 Mols. Na,O neutrdised.. . 0.0 0.0017 0-0043 0-0046* 0.0086 .............................. 11-77’ 12.28 12.31 12.53 12.63 Mols. Na,O neutralised ... 0.0112 0-0180 0-0278 0.0745 0.1570 The point marked * was obtained by interpolation THE ACTION OF SILICA ON ELECTROLYTES. PART II. 2815 The Action of AZMine Hydroxkk8 on SiZica.-When pure silica is acted on by an alkaline hydroxide neutralisation of the base commencw h e d i a t e l y and the extent to which the reaction proceeds is dependent on the nature and the concentration of the base the quantity of silica (at the surface of which the reaction takes place) as well as on the usual factors such as time of contact and temperature. The hen- of the silica will of course deter-mine the amount of reactive surface so that different specimens will not necessarily give the same quantitative results although Merent series will be comparable amongst themselves.The immediate effect is the production of a silicate at the surface of the silica. This is followed by its partial dissolution c a d by the solvent and hydrolytic action of the water. After a time, equilibrium is approached between the silicate in the solid and that in the liquid phase. Increase of concentration of either of the reagents causes an increase in the amount of silicate in each phase. For equal concentrations of different bases the amount of solid silicate formed depends on the solubility relationships of the silicate of the base used; being great where the solubility is small (as in the case of calcium) and small where the solubility is large (as in the case of sodium).These statements are illustrated by the results tabulated below. The amount of silica taken is given as g. per 100 C.C. in all other cases concentrations are expressed as g.-equiv. per litre. The silica was purified and the pE measurements were made M previously described. The reactions were carried out in wax-coated glass flasks. The amount of hydroxyl-ion remaining in solution was calculated from the pH measurements by means of the values of Michaelis for l.ilog K at the temperatures at which the experiments were made these varied between 33" and 38". From the hydroxyl-ion concentration the concentration of free base was calculated from its known degree of dissociation at the working concentration.Titration with standard hydrochloric acid gave the concentration in solution of the free base plus soluble silicate and this subtracted from the amount of base taken gave the amount of the base retained in the solid phase. Neither thermostats nor shakers were used in this work. TABLE 11. Effect of time on the progress of the reaction. Silica 0.5%. Base 0-0365N. Time ........................... 20 hours 48 hours 70 hours 12 days 96 NaOH neutralised . . . . . . 53 63 86 94 yo Ca(OH) neutralised ... 68 79 98 9 2816 JOSEPH AND OAKILEY : TABLE III. Comparison of the amounts of solid silicate formed from different bases after 1 day. Initial concentration of base 0.405N. Baae Fraction of Solid silicate Silicate Solid as Base.neut. base neutr. per litre. in soh. yo of total. NaOH 0.027 0-66 0.0014 0.026 5.2 KOH 0.029 0.71 0.0019 0.027 6.5 Ba(OH) 0.021 0.54 0.0063 0.014 30 Ca( OH) 0.034 0-84 0.034 0.000 100 The results follow the solubility relationships of the silicates of The barium experiment was made at a different time and the these four bases. initial concentration of the base was 0.0392. TABLE IV. Effect of the concentration of the base on the amounts of silicate formed in the solid and liquid phases after 2 days. Initial Final Total Solid Silicate in NaOH. NaOH. silicate. silicate. soh. 0.004 0-00016 04038 Small 0-0038 0.014 0-00058 0.0134 0.00024 0.0132 0-030 0.00159 0.0285 0.00032 0-0282 0.060 0.00828 0.0515 0-00112 0.0504 TABLE V. Effect of the amount of silica taken on that of the silicates formed in the solid and liquid phases after 8 days.Base. 0440N. KOH 0.040N. Ba(OH), 0.039N. CdfgFk NaOH, yo silics ~&CWL 0.5 1.0 0.5 1.0 2.0 0.5 1.0 0.5 1.0 Solid SiliCfbt0 silicate 04014 0-0027 0.0011 0-0021 0.0047 0.0063 0.0138 0-0349 0.0351 in SO^. 0.0366 0.0369 0,0379 0.0383 0-0358 0.0054 0.0092 0.0028 0.0027 neutr. 94 97 97 100 100 30 58 99 99 %B-(Note.-The experiments with sodium hydroxide were carried out with a different specimen of silica this does not affect the relationships which the above table is designed to show.) From the above two tables it appears that there is (a) a direct equilibrium between solid silica solid silicate and liquid phase (hydroxide and silicate in solution) which is the main factor in the case of sodium and potassium; and ( b ) an ordinary solubility relationship between solid and soluble silicates which is the main factor where as in the case of calcium and barium no easily soluble silicates exist.It follows from (a) that any reduction in th THII ACTION ox SILICA ON ELECTROLYTES. PABT n. 2817 amount of free silica should be accompanied by a reduction in the amount of solid silicate and this is shown by the following Sgures for the time effed of a stronger solution of sodium hydroxide on silica the gradual pa8sctge inta solution of the silica is accom-panied by a fall in the amount of solid eilicate. TABLE VI. Simultmeous hppearance into solution of silica and solid silicate. 1% Silica = 0.167 mol. per litre. NaOH 0*0101N.Time. PK. TOM silicate. Solid silicate per litre. 5 hours 11.94 0-0768 0.0057 1 day 11-41 0.0937 0-0046 4 $9 11.00 0.098 1 0.0039 8 9 ) 10-93 0.0986 0-0037 2 days 11-15 0-0972 04040 E8e.d of Neutrtd Salts on the Reaction.-The reactions between neutral salts and silica have been dealt with in the previous paper : acidity is always developed and some of the base goes into the solid phase. The addition of a salt to a mixture of silica and alkali should favour the formation of insoluble silicate by depressing the solvent and hydrolytic action of the water and this is found to be the case. Table VII gives the results of parallel experiments cmied out with and without the addition of the chloride of the base concerned. TABLE VII. 1% Silica and O.OQN-base.Time of standing 1 day. Conc. of salt soh. = N. Fraction of added base found in solid phase. Base. Without salt. With salt. %OH 0.030 0.078 KOH 0.035 0.149 0-70 0.85 0-161 0-210 Ca( OH) * BWH), Experiments carried out with sodium chloride and different amounts of silica showed that as before the amount of silica determined that of the solid silicate formed (which is of come, very small in the absence of free base). Titration methods could not be applied but the hydrogen ions liberated were equivalent to the solid silicate and this is seen to be roughly proportional to the silica taken. TABLE VIII Silica taken. p~ of mixture. H-ion conc. 4-01 o-Ooo10 3-74 0*00018 1% 2 2818 CRAPUN ISOMERIC CHANGE IN ILBOUTIC COMPOUNDS. summary. (a) The action of an alkaline hydroxide on silica results in the formation of a solid silicate part of which passes into solution. The amount of solid silicate formed is very small with sodium and potassium about 50% in the case of barium and nearly 100% with calcium. These results are in accordance with the solubilities of the silicates of these bases. ( b ) In the case of sodium and potassium the small amount of solid silicate produced is roughly proportional to the weight of silica taken although the amount of silicate in solution is only shghtly affected. These relationships show that the solid phases (silica and silicate) are directly concerned in the equilibrium. (c) The addition of a neutral salt increases considerably the amount of solid silicate produced. WELLCOME TBOPICAL RESEARCH LABORATORIES, KHARTOUM. [Received J d y let 1926.