My father* wants to synthesize Sodium orthovanadate, and has found out it is prepared from Vanadium pentoxide and a strongly alkaline solution of NaOH, or by fusion with NaOH but cannot find a detailed method, e.g. about quantities, separation from excess alkali, drying, etc.

I** haven't been able to find out much by googling or searching Pubmed: most methods I've found deal with stabilizing the solution, not with the complete synthesis. Could you point us to a complete protocol?

*chemist, but his English is not so great.

**not a chemist anymore

From Supniewski, J. "Inorganic Preparative Chemistry", 1958 (in Polish):

Melt $\pu{36.4 g}$ of vanadium pentoxide $\ce{(V2O5)}$ and $\pu{63.6 g}$ of sodium carbonate $\ce{(Na2CO3)}$ a nickel crucible until you get a uniform, clear melt with no bubbles of $\ce{CO2}$ coming out. Pour the melt on a cold plate and wait until it solidifies to a colorless mass (during the cooling process it should turn dark green, then yellow, then colorless). Pulverize the melt and dissolve it in a minimum amount of cold water, filter, and layer with several volumes of ethyl alcohol. After several hours, colorless crystals of $\ce{Na3VO4 * 16 H2O}$ should separate. Wash the product with ethanol and dry in a desiccator.

Some clarifications as requested in the comments:

"Minimum amount" means that you need to add only so much water to dissolve the salt completely, no more. Just add water in small portions and stir well - if the salt is still visible add some more water, etc. Suppose it took you about $\pu{50 mL}$ of water to dissolve your substance. Now you need to add twice or thrice (i.e., $\pu{100 .. 150 mL}$) the amount of ethanol ("several volumes"). Add the alcohol slowly so the solutions do not mix immediately but form two separate layers, and leave it undisturbed for several hours (I think it may take a day or two).

• Thanks! If someone could clarify what does "a minimum amount" and "several volumes" meant... – Adriano Varoli Piazza Nov 14 '16 at 17:53
• Could you edit this into the answer? Also if you have any idea of the expected yield for those quantities, that'd be grrrreeat. – Adriano Varoli Piazza Nov 15 '16 at 13:58
• @Adriano Varoli Piazza Unfortunately the book does not give the yield for this particular procedure, but I estimate it to be over $90 \%$. I did not perform this synthesis myself. Feel free to report your yield. – vapid Nov 15 '16 at 15:06

According to Wikipedia, sodium orthovanadate is prepared by mixing vanadium(V) oxide in a solution of sodium hydroxide:

$$\ce{V2O5 + 6 NaOH → 2 Na3VO4 + 3 H2O}$$

But, there is a different reaction scheme as mentioned in this site:

Sodium Orthovanadate, $\ce{Na3VO4.12H2O}$, is the orthovanadate most frequently met with. It is readily obtained by adding excess of caustic soda to a solution of sodium pyrovanadate:

$$\ce{Na4V2O7 + 2NaOH <=> 2Na3VO4 + H2O}$$

It can be conveniently crystallised from caustic soda solutions, as it is less soluble than in water. It forms hexagonal prisms isomorphous with the corresponding phosphate and arsenate [...] The anhydrous salt, $\ce{Na3VO4}$, melts at 850°-866° C. It can be prepared by fusing sodium carbonate and vanadium pentoxide in the required molecular proportions. Extraction of the product with water and precipitation with alcohol gives colourless needle-shaped crystals of a hydrate containing sixteen molecules of water, $\ce{Na3VO4.16H2O}$. The hepta- and octa-hydrates, $\ce{Na3VO4.7H2O}$ and $\ce{Na3VO4.8H2O}$, have also been prepared.

By dissolving vanadium pentoxide in a large excess of caustic soda, Ditte obtained two crystalline vanadates which contained a larger proportion of the basic oxide than is present in the orthovanadate, and to which he gave the formulae $\ce{4Na2O.V2O5.30H2O}$ and $\ce{4Na2O.V2O5.26H2O}$.