Ammonia sensor

Hello all,

I am searching for an Ammonia sensor for a school project, I have tried googling it ( google.com) and have failed to find one. For some reason its been hard to find one.

Can anyone help me by guiding me as to where to look or where I can find one? Thank you.

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These indicate Ammonia sensing is not an easy task, there are color-changing cards available, these might be the cheaper option.