pH Indicating Dyes for Biodiesel

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An Indicator Dye is a substance that changes color with pH. We use them in titration tests to find the equivalence point where we stop the titration.

Examples of Indicators:

 

pH  Phenolphthalein  Phenol
Red 

Universal
Indicator 

Red
Cabbage 
 Bromophenol
Blue
Range
  
8.3-10.0   6.8-8.4   1-11 2-12 3.0-4.6
1 colorless yellow red red yellow
2 colorless yellow orange-red red yellow
3 colorless yellow orange crimson yellow
4 colorless yellow pale orange puple blue
5 colorless yellow orange-yellow blue blue
6 colorless yellow pale-yellow blue; blue
7 colorless yellow green-yellow blue blue
8 pink red green aqua-marine blue
9 pink red dark-green emerald-green blue
10 pink red blue lime blue
11 pink red blue lime blue
12 pink red blue yellow blue
13 pink red blue yellow blue
14 pink red blue yellow blue


Phenolphthalein - Changes color from clear to pink at a pH of 8.3 to 10.0 It is used in the WVO titration used to determine your biodiesel recipe. Most consider this indicator dye to be the easiest to use. It is available online from several sources or from laboratory supply stores in larger cities. When using phenolphthalein, you look for any color change that stays visible for at least 15 seconds.

 

 

Phenol Red - Changes color from yellow to red at a pH of 6.8 to 8.4. It is used in the WVO titration used to determine your biodiesel recipe. It is the easiest to obtain and available at any pool supply store or any hardware store with pool supplies. Even though it changes colors earlier than phenolphthalein, it still works just fine on oils low in FFA.

Turmeric - Changes color from yellow to red at a pH of 7.4 to 8.6. It is used in the WVO titration used to determine your biodiesel recipe. It is a common spice used in Asian foods and found in most food markets. It can be used dry by adding a small pinch to the titration. A tincture of turmeric can be made by soaking turmeric in isopropyl alcohol. Curcumin is the name of the color pigment in turmeric.

Red Cabbage - Changes color from red to blue to green over a pH range of 2 to 12. It can be used for titrations. It can be made by boiling red cabbage leaves in water. It can also be a tincture of red cabbage in isopropyl alcohol for increased life. red cabbage spoils quickly if not stored in the refrigerator. Since it has a wide range of colors, you will need to make color chart showing the colors at various ph values using chemicals with known ph values.

Bromophenol Blue - Changes color from yellow to blue over a pH range of 3.0 to 4.6. It is primarily used for the soap titration test. Bromophenol blue is available from Internet sources and from laboratory supply stores in large cities. One thing to note is that it takes several milliliters to perform a test, unlike most of the other indicators that take several drops.

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