phenolphthalein test

allmaple

allmaple

Judge
this one always confused me, if theres something red and sticky at a crime scene they swab it, spray it, the swab turns pink and they say 'phenolphthalein test positive for blood'. but phenolphthalein turns pink for any acidic compound, so for all they know it could be tomatoe juice. i know they have to do other tests in the lab, i just found it odd that at the scene they so confidently say 'its blood'
 
But what could resemble blood so much? I mean its not like theyre swabbing metal, then its different, but to swab a red substance I guess with experience they learn to see the difference. I do remember reading that once. That over time, a professional can tell the difference between blood and any other compound. Sort of like how blood experts can tell if its animal or human too! (sounds absurd , I know but hey anything's possible these days)
 
Actually, human blood and animal blood cannot be determined by a visual inspection. A species origin test in a lab needs to be run on the blood sample. This is a simple ring precipitation test where antigens and antibodies diffuse and result in a precipitate.

The Kastle-Meyers test is a presumptive test that can be used in the field. When a swab containing blood is subjected to phenolphthalein, it turns bright pink. This indicates that the sample is blood, but it does not indicate whether the sample is human or animal.
 
Okay you're right, I just realized what I had written and it makes no sense whatsoever :lol: an experienced inspector can decide if its blood or a reaction with the metals upon visual inspection, but can not find out the origin of the blood, it being an animal or human sample. My apologies for that :lol:
 
Well I guess if it was tomatoe juice they could determine it by smell.....;)
 
No need to apologize for anything Roka4csi. This forum is all about learning and sharing ideas. As a Forensic Science student, (Yeah! Griffin Tech in Griffin, Georgia) I have a lifetime of learning ahead of me, so I am always happy when I can help someone out with something technical, and I look forward to learning new things as well which I have since visiting this thread. :)
 
:eek: You're a Forensic Science student! That's great, I'm so happy for you! We need more CSI's in this world, and definetely in this thread :D
Does anyone have any idea if its possible to make phenolphthalein out of scratch? Perhaps replacing some of the ingredients with others - though it wouldn't be phenolphthalein, but I mean, if Hydrogen is needed, I wouldn't get Hydrogen from a lab, but replace it with another hydrogen rich mixture... :lol: I'm talking too much again. I'll let you know what I come up with when its done.
 
this may sound odd but i have noticed blood has like a very faint matalic smell coz of the iron in the blood and also blood smells diffrent from ketcup so it not hard to tell the diffrence.
 
allmaple said:
this one always confused me, if theres something red and sticky at a crime scene they swab it, spray it, the swab turns pink and they say 'phenolphthalein test positive for blood'. but phenolphthalein turns pink for any acidic compound, so for all they know it could be tomatoe juice. i know they have to do other tests in the lab, i just found it odd that at the scene they so confidently say 'its blood'
Click to expand...

Your description of the phenolphthalein test is a common misunderstanding of the chemistry behind the test. The Kastle-Meyer test is not a pH test. As a matter of fact, the solution (if prepared properly) is basic (which would normally be "pink", acidic solutions turns phenolphthalein clear. The solution is then refluxed and reduced under a reducing agent (usually zinc), and as it is reduced it returns to a colorless state. When blood comes into contact with it, and the solution is hit with the Hydrogen Peroxide, there is a reaction which occurs that oxidizes the phenolphthalein, causing the solution to return to the pink color it should be under the basic pH it is prepared at.

So for a substance to turn the KM solution pink, it isn't pH dependent, it's based on it's ability to act as an oxidizer. Which is why there are few things that look like blood, that would give a false positive.

And in the "real world", CSIs know that a pink phenolphthalein test isn't "positive" for blood, it is only a presumptive test. So blood is merely indicated. Further testing to required to confirm the substance is blood.


BTW. Don't go out and buy phenolphthalein, make a X% concentration solution, and expect to have working Kastle-Meyer solution. It doesn't work that way. I often get people asking me whey their solution doesn't work, and that's because there is more to Kastle-Meyer solution, than simply a solution of phenolphthalein.
 
I can't remember ever using a presumptive test (like phenolphthalein) for blood or anything else at a crime scene. Wet/dried blood is pretty easy to recognize and distinguish from other wet/dried 'non-blood' materials at a scene; the presumptive tests waste/destroy a portion of the evidence that might be better utilized in the lab with sophisticated instrument analysis; and besides, you have too much to do at the scene (preservation of the scene, photography, sketching, dusting non-movable and/or 'fragile' items for latent prints, etc etc) to waste time performing essentially non-informative presumptive tests.
 
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