CARBOHYDRATE TESTS:
1) Benedict's Reagent - ONLY tests for "reducing sugars" like glucose, galactose, and fructose. Will NOT detect non-reducing sugars like sucrose. This reagent must be mixed with the solution you are testing and then HEATED in a hot water bath for about 3 minutes--enough time to produce a visible color change IF the solution contains even a small concentration of reducing sugar(s).
Your NEGATIVE CONTROLS (-) should be: (a) DI water and/or (b) sucrose solution. DI water
can be your negative control for any of the following tests, since we
know pure water does not contain carbohydrates, lipids, protein, or
nucleic acids (DNA, RNA). However, a BETTER negative control for Benedict's Reagent would be a sucrose solution, as sucrose is NOT a reducing sugar, and thus should NOT produce a positive indication (a color change) when the Benedict's Reagent is heated.
Your POSITIVE CONTROL (+) should be a pure glucose solution (or a solution containing a different reducing sugar).
COLOR CHANGE INDICATION (when the solution tests "positive" for reducing sugars): green (low content), orange (moderate content), red (high content), brown (very high content).
2) Iodine (i.e. IKI, I2KI, or Lugol's Iodine. Gram's Iodine will also work, but it tends to be a more expensive reagent) - ONLY detects starch (strings of glucose monomers joined together). Iodine will NOT detect individual glucose monomers.
Your NEGATIVE CONTROLS (-) should be: (a) DI water (deionized water) and/or (b) a pure glucose solution.
Your POSITIVE CONTROL (+) should be a pure starch solution.
COLOR CHANGE INDICATION (when the solution tests "positive" for starch): brown.
PROTEIN TEST:
Biuret Reagent - ONLY detects protein (amino acid chains). The Biuret Reagent WILL NOT detect individual amino acids (monomers of proteins/polypeptides).
Your NEGATIVE CONTROLS (-) should be: (a) DI water and/or (b) a pure amino acid solution.
Your POSITIVE CONTROL (+) should be a pure protein solution.
COLOR CHANGE INDICATION (when the solution tests "positive" for protein): purple or black.
LIPID TEST:
Sudan III or Sudan IV - ONLY detects lipids (i.e. fats, oils, triglycerides).
Your NEGATIVE CONTROLS (-) should be: (a) DI water
Your POSITIVE CONTROLS (+) should be: (a) pure lipid/fat/oil MIXED WITH a little bit of soap/detergent (this helps to prevent separation of any water content in the Sudan III/IV Reagent from the lipid/fat/oil), (b) whole or reduced-fat milk (emulsified fat).
COLOR CHANGE INDICATION (when the solution tests "positive" for lipids/fats/oils): red.
NUCLEIC ACID TEST:
Dische diphenylamine test - ONLY detects DNA.Will NOT detect RNA. Requires heating/boiling then cooling (ice bath).
Your NEGATIVE CONTROLS (-) should be: (a) DI water and/or (b) pure RNA solution.
Your POSITIVE CONTROL (+) should be a pure DNA solution.
COLOR CHANGE INDICATION (when the solution tests "positive" for DNA): blue.
For photographic examples of these tests, go here.
Friday, August 30, 2013
Friday, August 23, 2013
How Do I Measure Liquid Volumes? Proper Lab Equipment Choice
In your basic biology lab (college-level course), it is very likely that you'll be required to know how to use various common tools used to measure volumes of liquids.
Rule 1: If the tool isn't calibrated (if it is the kind of tool that needs calibration), then your measurements are NOT RELIABLE (won't be accurate).
BAD equipment for measuring accurate and precise volumes of liquid:
These are ACTUALLY TERRIBLE FOR MEASURING ACCURATE AND PRECISE VOLUMES OF LIQUID (but are very useful in other functions!)
a) Beakers - usually made of glass or plastic. DON'T USE THIS when you need to measure an accurate and precise volume of liquid. DO use a beaker for storing or mixing things together (when appropriate) or when the measurement of volume does not have to be very exact (like how you would use a measuring cup for baking).
b) Erlenmeyer flask - same problem/use as the beaker (see above).
VERSUS
GOOD tools/glassware for measuring accurate and precise volumes of liquid:
For large-ish volumes (1 mL - 1000 mL):
a) Graduated cylinder - usually glass or plastic. Pretty good for measuring large-ish volumes (around 1 mL - 1000 mL) of liquid. BE SURE to avoid the "paralax error" by reading off the volume of liquid in the graduated cylinder at the "meniscus". A graduated cylinder is nice when you need to measure a bunch of different "something" milliliter volumes, like 5mL, 7 mL, and 9mL, for example.
b) Volumetric flask - glass. Great for measuring EXACTLY ONE volume of liquid--that is, a single volumetric flask is designed for measuring only one specific volume, such as 5 mL, 100 mL, 1000 mL (1 Liter), etc. Wonderful for mixing/swirling components of a solution together, as well. With the volumetric flask, also read the "meniscus" as it coincides as closely as possible with the single line on the neck of the flask that indicates the "one specific volume" that flask measures.
For smaller volumes (1 mL or less):
c) Pipette - there are many subtypes, but they tend to be bulb and Pasteur pipettes (in old-school chemistry labs or classes), serological and mohr pipettes (graduated pipettes that are very similar to each other and are used in modern labs and some classes), or volumetric pipettes (modern labs and some classes). Pipettes of any kind usually require some kind of pipette "aid", as in some apparatus or add-on tool that helps the user to suck up/expel liquid. In the old days, people pipetted by mouth instead of using a pipette "aid"--and just so you know, pipetting by mouth is a super bad idea because you might inhale or swallow part of your experiment, which is dangerous, expensive, and anti-scientific. Just don't do it! Use a pipette aid.
Bulb pipettes:
Pasteur pipette (with balloon bulb)
d) Micropipette - lots of varieties due to different manufacturers, but they all do the same thing: allow you to measure and/or transfer insanely small volumes of liquid. Most labs have micropipettes that measure volumes in MICROLITERS. 1 microliter is so small, it's less than 1 "drop" of liquid (think of a single rain drop). Micropipettes come in variety of subtypes based on what range of "small volume" it is intended for measuring.
Rule 1: If the tool isn't calibrated (if it is the kind of tool that needs calibration), then your measurements are NOT RELIABLE (won't be accurate).
BAD equipment for measuring accurate and precise volumes of liquid:
These are ACTUALLY TERRIBLE FOR MEASURING ACCURATE AND PRECISE VOLUMES OF LIQUID (but are very useful in other functions!)
a) Beakers - usually made of glass or plastic. DON'T USE THIS when you need to measure an accurate and precise volume of liquid. DO use a beaker for storing or mixing things together (when appropriate) or when the measurement of volume does not have to be very exact (like how you would use a measuring cup for baking).
b) Erlenmeyer flask - same problem/use as the beaker (see above).
VERSUS
GOOD tools/glassware for measuring accurate and precise volumes of liquid:
For large-ish volumes (1 mL - 1000 mL):
a) Graduated cylinder - usually glass or plastic. Pretty good for measuring large-ish volumes (around 1 mL - 1000 mL) of liquid. BE SURE to avoid the "paralax error" by reading off the volume of liquid in the graduated cylinder at the "meniscus". A graduated cylinder is nice when you need to measure a bunch of different "something" milliliter volumes, like 5mL, 7 mL, and 9mL, for example.
b) Volumetric flask - glass. Great for measuring EXACTLY ONE volume of liquid--that is, a single volumetric flask is designed for measuring only one specific volume, such as 5 mL, 100 mL, 1000 mL (1 Liter), etc. Wonderful for mixing/swirling components of a solution together, as well. With the volumetric flask, also read the "meniscus" as it coincides as closely as possible with the single line on the neck of the flask that indicates the "one specific volume" that flask measures.
For smaller volumes (1 mL or less):
c) Pipette - there are many subtypes, but they tend to be bulb and Pasteur pipettes (in old-school chemistry labs or classes), serological and mohr pipettes (graduated pipettes that are very similar to each other and are used in modern labs and some classes), or volumetric pipettes (modern labs and some classes). Pipettes of any kind usually require some kind of pipette "aid", as in some apparatus or add-on tool that helps the user to suck up/expel liquid. In the old days, people pipetted by mouth instead of using a pipette "aid"--and just so you know, pipetting by mouth is a super bad idea because you might inhale or swallow part of your experiment, which is dangerous, expensive, and anti-scientific. Just don't do it! Use a pipette aid.
Bulb pipettes:
Pasteur pipette (with balloon bulb)
d) Micropipette - lots of varieties due to different manufacturers, but they all do the same thing: allow you to measure and/or transfer insanely small volumes of liquid. Most labs have micropipettes that measure volumes in MICROLITERS. 1 microliter is so small, it's less than 1 "drop" of liquid (think of a single rain drop). Micropipettes come in variety of subtypes based on what range of "small volume" it is intended for measuring.
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