This is probably an effect of the relative acid/TTip concentration. A yellow solution is not rare in the system you describe. A very practical approach for the proper concentrations is given by B.E. Yoldas at this paper : Yoldas, B.E. J Mater Sci (1986) 21: 1087. 

Dear Kenneth,

To my knowledge uncomplete hydrolysis and condensation reactions of a Ti-alkoxide precursor in alcoholic solvent could result in precipitation of high organic content yellowish Ti polyoxoalcoxides.

I suppose there is also to some extend TiO2 and Ti(OH)4 particles which as we know we dont expect it to be yellow. 

I agree with you. One should not confuse TiO2  with other organo-Ti particles.  That is also my point in my previous answer. I guess i haven't explained it clearly enough. 

However, i think that even X-ray D cannot answer the original question of this post. That is the yellow precipitates, not the TiO2 phase in there. I would perform some FTIR measurements in order to find what the yellow precipitates are and understand what went wrong on the sol gel reactions.

Dear Villy Vrakatseli

Perhaps you have information about the conditions in which, using some operations one can bring this process to completion to give titanium hydroxide forms and subsequently after heat treatment to titanium dioxide. Please tell us what to do so that all the intermediates in this sol-gel process were converted to the desired final product. Under what conditions titanium polyoxoalcoxides can transform into titanium hydroxide forms?

With best regards,

Svetlana V. Chuppina

Hydrolysis is stepwise in a sol-gel process. Polymeric titanium oxoalcoholates including polynuclear are formed as intermediate forms. The final product of this process after rinsing - titanium hydroxide (usually amorphous) which, after dehydration at 550-700 degrees Celsius transforms to titanium dioxide. Lines of rutile appear on X-ray diffraction after heating for at least 1.5 at 700 degrees.

Titanium dioxide is amphoteric. The exact formula for its hydroxide is not known. One can say : in aqueous systems the amorphous hydrated oxides fall as precipitation. However hydrated dioxide TiO2 • nH2O is also called as titanium hydroxide (IV), oxo-titanium hydrate, titanium oxo-hydroxide. Depending on the conditions for obtaining the product may contain variable amounts associated with the Ti OH groups, structural water, adsorbed anions and cations. So there are no disagreements here.

Now I can not agree with you. {•nH2O} does not mean physically adsorbed water. For crystalline hydrates we write the same formula, we deal with water belonging to the structure of crystals. But at this stage no crystals were formed. There is another problem – the product precipitates as amorphous precipitate in which water is present in various forms, hydroxyl groups bonded to the titanium atom are also present. We write titanium dioxide on a piece of paper like a low molecule compound, in reality we are dealing with inorganic polymers. Regarding the structure of the precipitate - the precipitate is amorphous, its particles are nano-sized, and structure of the product can not be described by any polymorphic form.  We can really write the TiO2 • nH2O for a precipitate, since the process is complex and multipath (hydrolysis and anhydrocondensation), it is somewhat generalized formula, it does not specify the nature of water and structure of the product. After heat treatment at a temperature of about 550 degrees Celsius well crystallized anatase is formed. But the question was about the color of the TiO2 produced in the process. Sometimes when using titanium alkoxides colored products are obtained. In my opinion, this result may be related to the concentration and purity of starting materials, rate of mixing, reaction time, the viscosity of the system and the

conditions of isolation of the product. Perhaps there is an undesirable adsorption (in this case, reversible) of anions and cations present in the system.

Your story is a story about a product that is not produced from titanium alkoxides.

Сertainly not structural hydroxyl groups can cause undesirable color. The hydroxyl groups in the structure of uncalcined fresh precipitate is a consequence of the conditions of preparation. Regarding the results of IR spectroscopy, in fresh isolated and dried in the air products it is difficult to look for single isolated hydroxyl groups.

In the case of using titanium alkoxides as starting products, even in the case of the reactants of sufficiently high purity yellow product is obtained at a time; it is not due to the presence of iron compounds in the system. Titanium isopropoxide is a transparent, colorless or pale yellow liquid. Titanium isopropoxide is capable of forming the oligomeric (polymeric) products. Oligomeric titanium isopropoxide usually has a light yellow color, it is not colorless, then system becomes turbid as a result of hydrolysis and polycondensation, then the precipitate is formed. Moreover titanium isopropoxide is a substance that reacts also with strong acids and oxidants. If isolation of the product coming from the system having the color, as result of adsorption on the surface of solid particles the final product can be colored. So there are many reasons for the appearance of color in the final product in this case.

As Svetlana mentioned the sol-gel process result of Ti-alkoxide precursors depends on many factors. The hydrolysis of the Ti-alkoxide may be complete or not. 

Consider the reaction : Ti(OR)4 + 4H2O -> Ti(OH)4 + 4ROH (complete hydrolysis)

Then the product is in fact Ti(OH)n ...or not...! Because, in fact this is more complicate than a simple reaction. Ti-alkoxides are highly reactive. 

The reaction is : nTi(OR)4 + (4n + x - y)H20--> Ti, O[z,_(x+y)/z](OH)x(OR)y + (4n -- y)R(OH) . pH is a crusial factor, while excess of water results in higher oxide content of a ......OR-......-Ti-O-Ti-.......-OR networks

In general, in such systems the approach of getting Ti-hydroxide is a Ti-alkoxide precursor in acidic aquatic solutions.

It should be mentioned though, that one reason that the sol gel process is used instead of other comercial, well established processes is because under proper conditions it can give clear, transparent colloid solutions of a high oxide content Titanium network that it can be used for the deposition of transparent TiOx thin films. Annealing of the films at >350oC will eventually result in a TiO2 thin film.