POLYMERIZATION OF
smcrc ACID:
RETARDISG EFFECT OF CHROMATE ION BY R. K. ILER
Grasselli Chemicals Deparlmenf, E.cperincenta1 Slalion, E. I . dii Por~tdc :\rciriour.s and Company, Inc., Wilniington, Delaware Recezued November I d . I961
The chromate ion is unique among the common inorganic anions, in that it retards the gelling of aqueous silicic acid in the pH range about 1 to 2. I t is postulated that the Concentration OF polymerizable silicic acid may be reduced through formation of an unstable silico-chromic acid which exists in equilibrium with the free acids. However, the complex does not Heem to be sufficiently stable to prevent polymerization, since the silicic acid eventually gels,
It has been found that chromic acid is unique among inorganic acids in retarding the polymerization of silicic acid. Since chromic acid is effective in retarding gel formation over the range where either hydroxyl ions or fluoride ions' function as catalysts, it probably does not inactivate these catalysts, but instead may reduce the concentration of polysilicic acid by forming a loose complex, possibly analogous to a heteropoly acid. While no further investigatioh is planned, the identification of the postulated silicochromic acid would be of interest. Experimental Reagent Solutions. Sodium Dichromate .-One hundred grams of Mallinckrodt Analytical Grade was dissolved in 100 ml. of water, and the pH adjusted by adding 2.0 .ml. ot 4 molar CrOasolution and 1.50 ml. of 6 N iYaOH solution. The final volume was 132 ml., and the pH was 1.9. Chromic Acid.-Four moles per liter of reagent grade CrOa. Sodium Silicate.-6453 g. of Grasselli J M Grade sodium silicate solution, containing 29.7% by weight of Si02 and !1.2% NagO, was diluted to 17.7 liters, giving a solution I .8molar in Si02 and 1.08 normal in sodium ion. Sulfuric Acid.-Grasselli C.P. HSSOl was diluted to a strength of 4.65 N . Procedures. (a) Preparing Sols from Acids and Sodium Silicate.-In all cases, silicic acid sols were prepared by placing the solution of acid and required additional wate? in ti beaker fitted with a high speed stirrer operating upwards
I
2
3
PH
Fig. 1.-Gelling of silicic acid sols made with various sti,ong acids. (I)
R. I
