Removing sodium borate for the test proving thiocyanate ions

I've just learned that when carrying out the identification test for thiocyanate ions borax can induce a false positive and therefore needs to be removed. I couldn't find the answer on google so I'm asking a question here.


I would think you first carry out the test to see if borax is present; but if it is, how would you remove it?




*Lehrbuch der analytischen und präparativen anorganischen Chemie* by Jander et al. [1] suggests to precipitate borate with $\ce{Ba^2+}$ in neutral solution beforehand as barium(II) thiocyanate is one of the few barium(II) salts actually soluble in water, or use an excess of $\ce{Fe^3+}$ to make sure all the interfering anions are precipitated.


From [1, p. 358]:



> 
> **6 . Nachweis als $\ce{Fe(SCN)3}$**
> 
> 
> […]
> 
> 
> **Störungen:** Um den störenden Einfluss von $\ce{F-}$, $\ce{PO4^3-}$, $\ce{AsO4^3-}$, $\ce{H3BO3}$, $\ce{C4H4O6^2-}$, $\ce{C2O4^2-}$ usw., die mit $\ce{Fe3+}$-Ionen Komplexe bilden, auszuschalten, wird $\ce{Fe^3+}$ im Überschuss zuge­geben.
> 
> 
> 


From [1, p. 418–419] (emphasis mine):



> 
> **9. Nachweis von Fe(III) als $\ce{Fe(SCN)3}$**
> 
> 
> $$\ce{Fe^3+ + 3 SCN- <=> Fe(SCN)3}$$
> 
> 
> […]
> 
> 
> **Störungen:** […] Ferner beeinträchtigen
> $\ce{Hg^2+}$-Ionen durch Bildung von wenig dissoziiertem $\ce{Hg(SCN)2}$, $\ce{F-}$ durch $\ce{[FeF6]^3-}$-Komplexbildung, die Anionen organischer Säuren ebenfalls durch Komplexbildung und auch $\ce{PO4^3-}$, $\ce{AsO4^3-}$, Borat-Ionen sowie ein größerer Mineralsäureüberschuss die Reaktion. **Es ist
> daher ratsam, vor der Prüfung […] die störenden Anionen in neutraler Lösung mit $\ce{Ba^2+}$ abzutrennen**.
> 
> 
> 


### References


1. Jander, G.; Blasius, E.; Strähle, J.; Schweda, E.; Rossi, R. Lehrbuch der analytischen und präparativen anorganischen Chemie, 16. Auflage.; *Hirzel: Stuttgart*, **2006**. ISBN 978-3-7776-1388-8. (in German)


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When you ask how to remove borax, I assume it is Sodium tetraborate decahydrate ([Wikipedia](https://en.wikipedia.org/wiki/Borax)). Thus, I agree with @andselisk of using $\ce{Ba^2+}$ solution to precipitate it. Even though it is seemingly an excellent answer, the reference given and the text body are in German, and I didn't understand it much. :-) Thus, I want to give some clues to make it sense.


According to Ref.1, tetraborate-ions ($\ce{B4O7^2-}$) form a white precipitate of barium metaborate $\left(\ce{Ba(BO2)2}\right)$ with $\ce{Ba^2+}$ solution ($\ce{BaCl2}$).


However, if it is a neutral solution, only following reaction occurs:
$$\ce{B4O7^2- (aq) + Ba^2+ (aq) + 3H2O (l) -> Ba(BO2)2 (s) + 3 H3BO3 (aq)}$$


Meaning, removal of borax is only partially completed. Complete precipitation of $\ce{B4O7^2-}$ ion would be reached only in the strong alkaline medium:
$$\ce{2 H3BO3 (aq) + Ba^2+ (aq) + 2 OH- (aq) -> Ba(BO2)2 (s) + 4 H2O (l)}$$


Keep in mind that this white precipitate is dissolved in inorganic acids:
$$\ce{Ba(BO2)2 (s) + 2 H3O+ (aq) -> Ba^2+ (aq) + 2 H3BO3 (aq)}$$


Thus, white precipitate of $\ce{Ba(BO2)2}$ should be filtered off before adding $\ce{Fe^3+}$ solution.


Alternatively, I believe testing of thiocyanate ions can be done in presence of borax by doing two back to back reactions: (i) Check the solution with the blood-red color formation reaction with $\ce{Fe^3+}$ solution as usual; and then (ii) Check again the same solution with the blue color formation reaction $\left(\ce{(NH4)2[Co(SCN)4] (aq)}\right)$ with $\ce{Co^2+}$ solution:


$$\ce{Co^2+ (aq) + 4 SCN- (aq) + 2 NH4+ (aq) <=> (NH4)2[Co(SCN)4] (aq)}$$


**Reference:**


1. O. A. Ievtifieieva, V. V. Bolotov, T. A. Kostina, O. M. Svechnikova, T. I. Yuschenko, N. I. Kaminska, A. E. Kosareva, L. V. Slobodyanyuk, O. P. Yashchuk, In *Analytical chemistry (Qualitative analysis) Part I: The manual for students of higher schools*; O. A. Ievtifieieva, Ed.; Publishing house the CLL: Kharkiv, Ukraine, 2014 (168 pp.).


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