Sodium oxide-silica molar ratio

The reaction time necessary for crystallization at a given reaction temperature can be controlled in a variety of ways, but the major way of controlling reaction time is by adjusting the water to sodium oxide molar ratio of the reaction mixture. The reaction time necessary to form these zeolites is directly proportional to the water to sodium oxide molar ratio used. For example, when the synthesis conditions indicate that the water to sodium oxide molar ratio for making zeolite A is between 15.1 and 20.1 for making a combination of zeolite X and zeolite A it should be between 25 1 and 35 1. Therefore, to synthesize zeolite A in a reaction mixture having a sodium oxide to silica molar ratio and a silica to alumina molar ratio where normally zeolite X would be formed, one would decrease the water to sodium oxide ratio. 

Another way of controlling the reaction time necessary for crystallization at a given temperature is by adjusting the sodium oxide to silica molar ratio of the reaction mixture. The reaction time necessary to form these zeolites is inversely proportional to the sodium oxide to silica molar ratio used. The effect of sodium oxide to silica molar ratio is less pronounced than that of water to sodium oxide molar ratio. 

Figure 3 includes values of potential versus immersion time in 0.1 M sodium chloride solution at pH 7.0 and 25 C for primers formulated with 57.5 and 70.0% PVC values and based on 7.5/1.0 nano silica/lithium oxide molar ratio as film-forming material, fine microzinc (D 50/50 4 pm) as pigment inhibiting and graphite as reinforcement fiber in the three levels studied. In addition, this figure displays the corresponding reference primers. 

If a preformed sodium polysilicate having a molar ratio of silica to alkali metal oxide in the range of 3.5-10 is employed before it gels, the same effects as with the amorphous silica sodium silicate system will be obtained. An aqueous sodium polysilicate containing 10-30% by weight silica and sodium oxide and having a silica to sodium oxide weight ratio of 4.2 1 to 6.0 1 can be produced as described in U.S. Pat. No. 3,492,137. 

SILICATES (Soluble). The most common and commercially used soluble silicates are those of sodium and potassium. Soluble silicates are systems containing varying proportions of an alkali metal or quaternary ammonium ion and silica. The soluble silicates can be produced over a wide range of stoichiometric and nonstoicluometric composition and are distinguished by the ratio of silica to alkali. This ratio is generally expressed as the weight percent ratio of silica to alkali-metal oxide (SiOj/MjO). Particularly with lithium and quaternary ammonium silicates, the molar ratio is used. 

The binder system should have a molar ratio of silica to alkali metal oxide which ranges from 3.5 to 10, preferably 3.5 to 7. This ratio is significant because the ratios of soluble potassium, lithium or sodium silicates commercially available as solutions lie within a relatively narrow range. Most of sodium silicates are within the range of Si02/Na20 of about 2 1 to 3.75 1. Thus, overall ratios of binder compositions obtained by admixing colloidal silica, such as ratios of 4 1, 5 1, 7 1 are mainly an indication of what proportions of colloidal silica and soluble silicates were mixed since the amount of amorphous silica in the soluble silicate at ratios of 2 1 to 3.75 1 are small. 

The common way to characterize the composition of sodium silicates is to use the ratio of silica to alkali by weight. The first refers to the Si02 content of the materiaL and the latter to its NajO content. This characteristic value is often called modulus. The ratio between silicon dioxide and sodium oxide can also be expressed on a molar basis. The conversion of modulus and molar ratio is possible by the respective formula weights. In case of sodium silicates, the modulus has to be multiplied by a conversion factor of 1.032 to obtain the molar ratio [1,5,6-13]. 

For this study, a commercial colloidal lithium silicate (3.5/1.0 silica/alkali molar ratio in solution at 25% w/w) was selected with the aim of increasing the silica/alkali ratio, a 30% w/w colloidal alkaline solution of nanosiUca was used (sodium oxide content, 0.32%). The aim was to develop a system consisting of an inorganic matrix (alkaline silicate) and a nanometer component (silica) evenly distributed in that matrix with the objective of determining its behaviour as binder for environment friendly, anticorrosive nano coatings. 

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