The cyanide ions obviously. However, it is not a real hazard as it requires reactions with strong acid to produce hydrogen cyanide gas. This is only theoretically possible in the sink disposal scenario (i.e. you dispose ferricyanide into the sink at the same time someone uses a strong acid to clean the clogged toilet, thus producing hydrogen cyanide. This however never happened in real life and only a theoretical possibility). Technically it is totally safe to dispose of it into the sink. However, some local regulation may penalize you for that. If you want to be on the safe side neutralize the solution with hydrogen peroxide. There was a topic on research gate regarding it: https://www.researchgate.net/post/Does_anyone_know_how_to_neutralize_potassium_ferricyanide

Honestly added (also for the convenience of other interested readers of this thread):

Also read about the components usually used in the GUS-Stain (RG-Q/A- thread posted May 15, 2020):

"What is the function for all of the components in the GUS staining solution?"

(==>

• https://www.researchgate.net/post/What_is_the_function_for_all_of_the_components_in_the_GUS_staining_solution )

In:

• http://aspendb.uga.edu/downloads/GUSstainingSOP.pdf one can find an example of an 'SOP for Hazardous Chemicals' including a Staining Protocol too (established 2009 at http://aspendb.uga.edu = University of Georgia, USA).

Unfortunately don't know whether protocol and components are the same as Orlando de Lange does use for his staining or some component have been changed or replaced by other components.

IMHO Disposal down the sink [besides legal commandment, be it national, governmental, city, water authority which should followed, naturally!] might be always a "question of quantity": certainly (only) 1 ml reacted staining solution vs. 1 L of staining solution to dispose might be worth considering to ignore rules.....

So, on the other hand, what is the problem collecting the small volumes after staining into a plastic or amber glass bottle (should have tight scre-top) and [with correct labeling, especially of the "most hazardous components methanol*), K-ferrocyanide, K-ferricyanide, and DMF (Dimethylformamide) ] to handle over to the Safety officer or the "disposing facility/institution" usually nowadays present at a University.

Acetone, and/or Triton X - which also is /are used with the stain - might not pose a real problem regarding correct and safe disposal.

*) Depending on the method used, methanol in other protocols substituted by ethanol; other component used: EDTA, Chloramphenicol

Another GUS staining protocol (2013) can be assessed at:

• https://s3-us-west-2.amazonaws.com/oww-files-public/6/64/GUS_staining_OWW.pdf

or from Rice University (2003)

• http://www.owlnet.rice.edu/~bios311/bios311/bios413/413day7_Sp12.html

Fernández-Piñán et al, 2019:

Agrobacterium tumefaciens and Agrobacterium rhizogenes-Mediated Transformation of Potato and the Promoter Activity of a Suberin Gene by GUS Staining, doi: 10.3791/59119 state: ".... The GUS substrate and the disposal material should be disposed safely. Remove GUS staining solution and discard it in appropriate containers. ..." to be found at:

• https://www.jove.com/t/59119/agrobacterium-tumefaciens-agrobacterium-rhizogenes-mediated

If you have to consider a 'deactivation' of the "most" hazardous components ( K-ferrocyanide, K-ferricyanide) H2O2 treatment might be a solution...as Anton Lennikov has anticipated/suggested (also cf. e.g.:

• http://www.h2o2.com/industrial/applications.aspx?pid=106&name=Cyanide-Treatment)

To see the context why correct disposal of cyanide containing waste might be important: cf.:

• https://sensorex.com/blog/2016/04/26/cyanide-wastewater-treatment/
• https://www.nmfrc.org/bluebook/sec623.htm

Nevertheless - in your special case - I'd suggest to use sodium hypochlorite ( 'industrial strength bleach', or e.g. even 'fresh'(!) household bleaching solution) adding some NaOH (to increase the pH to 9-11/12), providing a 'neutralization' of cyanides by oxidizing those to ==>cyanates [alkaline chlorination]. ADD THE BLEACH SLOWLY (after increasing the pH of the waste solution by admixing NaoH, e.g. caustic, or 10N NaOH) under constant stirring and USE litmus paper for checking the achieved pH (Use of a ph-meter not recommended!)!

Not to forget: Naturally NaCN-containing waste usually is not allowed to be flushed down the sink...but perhaps you may get information proper about the waste-water-management policy at your Institution/University/City....

IMPORTANT: Never admix 'acid' to your cyanide-waste without wearing a breathing / oxygen respirator mask - even if you work / treat your waste solution(s) under a fume hood / fume cupboard.

Wish you good luck!

Caveat emptor! No guarantee /warranty adopted!

No liability assumed - check information given yourself again for correctness and completeness! Seek professional guidance, eventually a competent consultant or an artful laboratory person to help you eventually in organising a scheme and check whether you have the right and valid operation permit to do deactivation by yourself.

[Admitting to have deactivated - for years of my profession - a lot of chemical waste solutions generated - in moderate volumes - in a Lab for Electron Microscopy as well as from a Histology lab prior to )

For more information on GUS-Staining issues also you may search the RG-URL :

• https://www.researchgate.net/topic/GUS-Staining

Thank you so much for these responses. They are really helpful!!

Orlando de Lange , you're welcome.