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Title: Forensics Aqueous Solutions Notes
Description: Chapter 6 in our textbook
Description: Chapter 6 in our textbook
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Page 1 of 5
6
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A solution consists of a solvent, the dissolving medium, and the solute, the material (s, l, or
g) being dissolved
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If water is the solvent we say it is an aqueous solution
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It is also
3
2
called grain alcohol
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•
The eyeball contains the vitreous humor with mainly proteins as solutes, but it is also
checked for alcohol in autopsies because other bodily fluids can give misleading results as
the body and its contents decompose (ferment) and make alcohol
...
6
...
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Sugars and alcohols are examples of compounds that are nonelelctrolytes because they
form no ions in solution
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Strong electrolytes are compounds that are fully dissociated into ions in water solutions
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Weak acids and weak bases
are weak electrolytes
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A tiny crystal of the dissolved solid is about to be added to a supersaturated solution prepared by
heating and then slowly cooling without disturbing the solution
...
3
Fingerprint Powders
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Commercially available in a variety of colors and textures (photographing)
•
•
Lightly applied to nonabsorbent surfaces with camelhair brush will ADHERE TO
PERSPIRATION RESIDUES AND BODY OILS
...
Fluorescent Powders (Q67 on page 192)
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Fluorescent powders that fluoresce under ultraviolet light- used when color or pattern of
background obscures visibility of the print
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Prints are typically not tape lifted, but photographed and digitized or transferred to a
computer for analysis
Superglue Fuming (Q67 on page 192)
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Superglue fuming- works great on nonporous surfaces- metals, leather, plastic bags
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Heating produces vapors that polymerize on the oily residue of the print
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Produces white latent print
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CHEMICAL METHODS FOR VISUALIZING LATENT PRINTS
Iodine fuming
Iodine is a solid crystal that when heated, turns into a vapor without passing through a
•
liquid phase
– this transformation is called sublimation
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Iodine prints are not permanent and begin to fade once fuming is stopped
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Ninhydrin (Q68 on page 192)
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One of the major components of fingerprints is amino acids
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Page 3 of 5
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It is sprayed or poured onto the evidence, and a permanent purple or violet fingerprint
results
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6 ACID CHEMISTRY
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An acid is a substance that can release hydrogen ions (H+) into an aqueous solution
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A strong acid like HNO may
3
be concentrated or dilute (less dangerous)
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restoration kit uses acids to restore the S/N etchings
...
7 BASE CHEMISTRY
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A base is a compound that produces hydroxide ions (OH1-) in aqueous solution
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Acids are said to be
corrosive
...
NaOH
! Na1+ + OH1- (100%)
Weak bases, like weak acids, exist mainly as molecules and not ions
...
9 THE pH SCALE AND BUFFERS
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The pH scale tells how acidic or alkaline a water solution is
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999 for solutions and from 7
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7
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The pH is defined as the negative logarithm of the H+ concentration
...
O
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” Drano gel
*************************************
Citrus
rain
ocean
NH bleach
3
Some household substances are acidic
...
Other products have a higher pH than 7
...
pH Calculations with Calculators
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To calculate pH from [H+]:
Page 4 of 5
–
Enter [H+] (or [OH-]) value
–
Press “log” key
–
Change sign by pressing “+/-” key
6
...
010 M HCl solution? (0
...
0 x 10-2 M)
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Answer: Since for strong acids the concentration of H+ ions is the same as the strong acid,
[H+] = 0
...
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So the pH = -log(10-2) = -(-2) = 2
...
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A change of two units is a 100-fold change in [H+]
...
0
...
0
...
6
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HCl + NaOH
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acid + base
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HBr(aq) + KOH(aq) → KBr(aq) + H O(l)
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Neutralization reactions can be monitored by the addition of an indicator
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Phenolphthalein is pink in base and colorless in acid
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BUFFERED SOLUTIONS
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In chemistry, a buffer is a substance or a mixture of substances that is put into a solution to
help maintain a constant pH
...
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A typical buffer contains both a weak acid and its salt, which acts as a weak base
...
00
2
...
00
9
...
00
! NaCl + H O(l)
2
yield a salt water
2
Page 5 of 5
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Title: Forensics Aqueous Solutions Notes
Description: Chapter 6 in our textbook
Description: Chapter 6 in our textbook