Phthalo Green: How Can You Tell If It's Always the Same?

Published: July 01, 2003, By Dr. Richard M. Podhajny, Ph.D., Contributing Editor

Recently, a question came in from one of our readers (thank you, Joe Woods) as to how one can assure the copper phthalocyanine green pigment purchased from two suppliers is the same chemically.

Pigment Green 7 has a CAS Registry Number of 1328-53-6. It is commonly referred to as Phthalo Green and has a bright bluish green color.

Copper phthalocyanine green pigment is a unique product that is essentially copper phthalocyanine blue pigment in which most of the hydrogen atoms have been replaced with chlorine atoms. The commercial chemical composition of copper phthalocyanine green can vary due to the number of chlorine atoms. They range between 14 chlorine atoms to a maximum of 16. As a result, it is possible to obtain the same pigment from two different suppliers with the same CAS number but with differences in chemical composition.

Copper phthalocyanine green is produced commercially by the direct chlorination of copper phthalocyanine blue pigment as a melt of sodium chloride and aluminum chloride. Chlorine is introduced at an elevated temperature and the melt cooled. Grinding the pigments in the presence of soluble salts, which can be washed out in the purification process, can produce particles of uniform size and maximize color strength.

Chlorine atoms are electronegative and have a strong effect on the electron distribution within the phthalocyanine molecular structure. Their presence shifts the color from blue associated with copper phthalocyanine blue to a bright green in this chlorinated copper phthalocyanine blue structure.

In combination with copper phthalocyanine green 36 (copper phthalocyanine pigment that contains chlorine and bromine atoms), a broad tonal range of phthalocyanine to the green-yellow end of the visible spectrum is allowed.

This chemical structure is extremely stable and chemically resistant. This pigment is highly resistant to acids, alkali, solvents, heat, and UV.

Due to these outstanding properties, copper phthalocyanine green is used in inks, coatings, and most plastics because of its high transparency. These pigments inherently are insoluble and show no apparent migration in food packaging.

If you are planning to source this material from outside the US, special attention should be made to assure there are no trace PCBs that may be present due to production and purification differences.

It is not easy to differentiate copper phthalocyanine greens from each other unless you have access to good analytical tools. Obviously, if you can measure the chlorine between samples, you can distinguish how close they are to each other.

The level of chlorine (Cl) can range from 46.9% (for Cl₁₄) or 48.7% (for Cl₁₅). Higher levels of chlorine are possible (Cl₁₆), but due to difficulty in the synthesis and chemical stability, they usually are not available commercially.

It also may be possible to distinguish different copper phthalocyanine greens through thin-layer chromatography. Again, this requires analytical experts to differentiate the thin-layer chromatography pattern with appropriate solvent. Due to the low solubility of these products, this is a challenge.

Perhaps another approach would be to use FTIR infrared spectroscopy. However, while this analytical approach may be suitable, it requires considerable analytical expertise.

If a laboratory has access to NMR and a suitable deuterated solvent, then NMR would clearly pick out the protons in the case of Cl₁₄ or Cl₁₅ copper phthalocyanine green and differentiate their structures.

As you can see, the answer to our reader's question gets complicated. I suspect the best way to handle this is to obtain a Certificate of Analysis from the pigment supplier with the percent of chlorine level present.

Copper phthalocyanine green is an outstanding pigment that cannot easily be replaced. It is the green pigment standard, which is used in the printing ink and packaging industry.