The Biodiesel pHLip Test

By Rick Da Tech


The Biodiesel pHLip TestBiodiesel and Quality

Quality has been a problem for the commercial biodiesel industry. In 2005 the NREL conducted a study and found that one third of the fuel in the distribution chain did not meet ASTM specifications. In 2006 the same study found over one half of the samples tested did not meet ASTM specification. As long as quality is voluntary for the biodiesel manufactures and distributors, it will be the responsibility of the buyer to ensure quality standards are met. The pHLip test was developed to give the buyer a tool to quickly and inexpensively determine quality of the fuel being purchased.

There are two major quality issues with biodiesel. The first being quality of manufacture and is expressed as percent conversion. The second being a storage issue and results in oxidized fuel. Both of these problems can cause excessive filter clogging and even damage to the engine.

When vegetable oil is converted into biodiesel, it is impossible to convert 100%. This is because the chemical reaction is reversible. A reversible reaction is one that is simultaneously going forward and backwards at the same time. The ASTM test for conversion is ASTM 6584. In this test the mass percentage of glycerin left in the biodiesel is determined. Glycerine is a small molecule and does not weigh much in comparison to biodiesel or vegetable oil. The ASTM specification is that total glycerin not exceed 0.24% by mass. Glycerine will be in a number of forms and the ASTM spec also lists maximums for each type of glycerine found in biodiesel. These glycerides consist of free glycerine, mono-glycerides, di-glycerides, and tri-glycerides. These are a glycerine molecule with no, one, two or three Fatty Acids attached to them.

When fuel ages, it breaks down through oxidation. The oxidation causes FFA to be released into the fuel. The ASTM specification that measures oxidation is the Acid number (ASTM D664). It is a titration similar to the homebrew titration test of WVO.

The acid number can be performed easily using glassware and simple equipment but free and total glycerine testing requires a Gas Chromatograph to be accurate. Owning and operating a Gas Chromatograph in every gas station is not feasible, and the turn around time for outside testing is impractical, so the pHLip test was developed as a quick visual field test to act as a firewall for bad fuel. Bad fuel should then be sent to a lab for real quantitative testing.

History of the pHLip Test

The pHLip test has homebrew roots. Several years ago the popular homebrew quality test was to put water and biodiesel together in a jar and shake. There seemed to be two uses for the test. First to determine when to stop washing, and second, to give an indication of conversion. Different sources had you interpret the results differently. One source said if your water was clear then you had washed out all the soap. Another source said if you have an emulsion layer between the biodiesel and the water, the fuel was under converted. It was discovered that water hardness had a dramatic effect on the test results. You would perform the test with hard water and soft water with the hard water passing and the soft water failing. Ordinary tap water gives different results with different water making it a subjective test.

Shakem Up Test 1 Shakem up test 2 Shakem up test 3Shakem up test 4 shakem up test 5

Making the homebrew test work

Dr Randell Von Wedel of Cytoculture found that by adding specific salts in specific concentrations the test could be calibrated against a Gas Chromatograph (GC) to provide consistent test results very near the ASTM specification for conversion. He found that by adding a dye to the water a mirror surface could be created that would amplify the presence of any emulsion between the water and biodiesel. The emulsions that formed were primarily due to Monoglycerides and Diglycerides combining with the water.

pHLip Test

The homebrew tests all suggested that the turbidity of the biodiesel was irrelevant. However, Dr. Von Wedel found that the turbidity was caused by hygroscopic or water attracting molecules in the biodiesel. The hygroscopic molecules collected water molecules around them and made the biodiesel turbid. The choice and concentration of salts selected was also calibrated to cause turbidity in the biodiesel in a repeatable fashion.

pHLip Test

There are a number of hygroscopic molecules that can be found in biodiesel. These include monoglycerides, diglycerides, Free Fatty Acids (FFA) and oxidized biodiesel. FFA and oxidized biodiesel are the result of aging. As biodiesel ages it becomes more acidic. To differentiate the test results between aged fuel and under converted fuel a pH sensitive dye was added.

pHLip Test

If turbidity was the result of under converted biodiesel rather than aging fuel, it was found that the degree of turbidity was related to conversion.

pHLip Test

This relationship is charted as shown here:

turbidity chart

Performing the test

Like the homebrew test, the test results are influenced by how hard you shake up the test.  The harder you shake it, the longer it takes to read the results.  Dr. Von Wedel found that 10 gently flips using wrist action alone provided the mixing needed to be able to read the test in 10 minutes.

Please keep the following in mind when testing:

  • Gently flip the vial ten times end over end using only wrist action.
  • Perform the test on finished biodiesel, unwashed biodiesel will not pass.
  • Perform the test at room temperature, no hot biodiesel and no cold biodiesel
  • The test can be performed on B100 and B99 but not B20
  • The test is only valid 10 minutes after adding the biodiesel and flipping after that the dye starts to react with the biodiesel.
  • Empty the vial after the test, over time the biodiesel in the vial will go turbid and the red shift to yellow
  • Do not flip the test a second time, it may not (most likely will not) provide you with the same results after a second shake.

After 10 minutes, first check the biodiesel for turbidity. If it is turbid, then it failed. Look at the red indicator dye to determine if it failed for conversion or aging. Aging will be indicated by a yellowing of the red dye as seen below:

pHLip Test

Turbid Biodiesel with unchanged red indicator on the bottom indicates under converted fuel. The sample on the left is the reference with each sample getting progressively more under converted as you go from right to left.

pHLip Test

If the biodiesel is clear and bright, then inspect the mirror finish between the biodiesel and the red liquid. If you biodiesel is good it will reflect images like a mirror. All of the samples below are of passing biodiesel. The pictures are taken with different lighting and angles to better show the mirror surface.

pHLip Test

There are a few other indications that can be obtained from the test. If the red portion goes turbid, it indicates excessive soap.  If the red dye turns purple, it is contaminated with catalyst.

The Biodiesel pHLip Test is avaiable directly from Cytoculture.

The Methanol Test vs the pHLip Test

There has been some debate on the forums on this subject.  Neither the methanol test or the pHLip test are replacements for GC testing. Both are good homebrew tests.  Understanding their differences may help to understand why sometimes the two tests give different results.



Methanol Test

pHLip Test

How does it work? It works on the principle that biodiesel will dissolve in methanol and WVO will not. It works on the principle that under converted fuel attracts water more than fully converted fuel.
Conversion Results are "Near ASTM"  yes yes
Quantitative test results  no no
Has Academic Research been conducted? no yes, but not published
Has it been calibrated to GC results? limited testing to determine composition of fallout. yes
Length of time in testing  since June 2006 since June 2004
Sensitive to Triglycerides?  very sensitive less sensitive
Sensitive to DiGlycerides  sensitive very sensitive
Sensitive to MonoGlycerides
less sensitive - possibly insensitive
very sensitive
Sensitive to Free Glycerine  unknown - possibly insensitive less sensitive
False test results can be caused by  water, temperature, time, possibly fatty acid composition of methyl esters, possibly saturation level of feedstock oil. how hard you shake, temperature, time.
Works on finished dry biodiesel  yes yes
Works on unwashed biodiesel   
yes - may be some restrictions
Works on washed but wet biodiesel
yes - some debate on this issue
cost per test 4 to 20 cents per test (27/3) $5.00 per test
Tests for "conversion"


Tests for aging or oxidized fuel no yes
Tests for catalyst contamination no yes
Tests for excess soap  no yes


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