Purple Shampoo Insight

Dear Valerie: A couple years ago you answered a Dear Valerie question about purple shampoo. I started digging into the science and was curious about Acid Violet 43 versus Ext. Violet 2. From what I have read, Ext. Violet 2 is purer and meets FDA standards, whereas Acid Violet does not. Other than that, they are the same molecule. If this is the case, why would a brand choose to use Acid Violet 43? Does it have better affinity or color? I also wanted to better understand why purple shampoos leave hair feeling so dry. This problem seems to be common, so I am curious there aren’t more purple conditioners on the market. 

—Pretty in Purple

Dear Pretty in Purple,

At its core, Ext. D&C Violet 2 and Acid Violet 43 are the exact same chromophores. They share the same molecular structure and exhibit relatively the same color. The former bears a United States nomenclature convention, while the latter is the naming convention more commonly used in other parts of the world. However, the nomenclature usage is not necessarily tied to where in the world the product is sold.

The distinction between the two names is surprisingly less about chemistry or geography, and more about regulatory classification. Colorants are one of the most tightly regulated ingredient categories in the world, with the United States maintaining some of the strictest requirements around their use in foods, drugs and cosmetics. Nonplussed? It’s true. Each color additive must be approved by the FDA and its permitted use conditions are outlined within Title 21 of the Code of Federal Regulations (CFR).

In the case of Ext. D&C Violet 2, the applicable regulation is found in 21 CFR 74.2602a. This regulation establishes the specifications the colorant must meet in order to comply with FDA safety requirements, including identity testing, purity standards and impurity limitations. For example, the colorant must be at least 80% pure, heavy metals cannot exceed specified ppm limits, and impurities such as 1-Hydroxy-9,10-anthracenedione, 1,4-Dihydroxy-9,10-anthracenedione and p-toluidine are restricted to 0.2%, 0.2% and 0.1%, respectively. The European Union restrictions, for the most part, parallel these impurity profile requirements.

The “Ext.” designation itself is also significant. It refers to “external” use, meaning the color additive is approved only for externally applied drugs and cosmetics and not for food use. When used in cosmetic products, labeling must also comply with safe-use requirements outlined in 21 CFR 70.25.

However, one of the most unique aspects of U.S. color regulation is not necessarily the impurity profile or labeling regulation itself, but the FDA batch certification process. Under this framework, every manufactured lot of a certified color additive entering U.S. commerce must be submitted for FDA certification to confirm it complies with the required specifications. Any batch that fails to meet those specifications cannot legally be used. The FDA even maintains a public repository summarizing certified color additive batches and can be found by searching on Google “FDA Color Certification Reports.”

When using the Ext. D&C Violet 2 nomenclature, this implies the colorant has undergone the FDA certification process and complies with the specifications outlined in its CFR monograph.

Not all colorants require FDA batch certification; in fact, the FDA maintains a category of color additives exempt from certification, such has hair colorants under the coal-tar hair dye exemption. In practice, this means that if a colorant is intended solely for the purpose of coloring hair, it is not subject to follow the same certification pathway, and thus is not subject to the purity criteria. This is where Ext D&C Violet 2’s use in hair color regulation becomes particularly interesting because you can use non-certified material, but it shouldn’t bear the INCI Ext D&C Violet 2. 

As a result, the nomenclature “Acid Violet 43” is often used in hair colorants, particularly if the product is sold in the EU with a global label where the FDA naming system does not apply. However, this doesn’t mean the colorant itself doesn’t meet the purity criteria outlined by the FDA or the EU. A manufacturer can still be using a D&C certified colorant that meets FDA purity criteria and simply disclosing on the label Acid Violet 43. 

Of course, there is the possibility the manufacturer is using a dye from a supplier that doesn’t meet purity criteria, and this is quite problematic from a safety perspective. If you are manufacturing a purple shampoo—or semi-permanent hair color—it is important to use colorants that meet FDA or EU purity criteria and disclose those impurities on the certificate of analysis that comes with each lot of colorant. In my opinion, this is absolutely critical to ensure consumer safety, a basic requirement for selling products anywhere in the world.

Regarding purple shampoo performance, I agree that there is a lot more love purple shampoo can give to hair in terms of conditioning. The challenge is twofold.

Acid violet 43 is a large anthraquinone dye. Due to its size and charge, its penetration into the hair fiber is minimal, and primarily binds to the exterior of the hair cuticle. This “coating” changes the perception of hair feel to the consumer. The second consideration is the emphasis on the dye’s anionic character. It’s not possible to use many cationic conditioning aids because of the positive charge of the conditioning agent may complex with the dye. It’s also for this very reason you don’t see many conditioning products containing acid dyes on the market. 

Lastly, all colorants compete for binding sites on the hair, regardless of charge. If a plethora of conditioning agents were bound to the hair cuticle, there is no space for the dye to bind, either through ionic site availability or steric hindrance. This can lead to weak color uptake or uneven color uptake.

For more answers to your questions from Valerie George, click here.

Valerie George

[email protected]

Valerie George is a cosmetic chemist, science communicator, educator, leader, and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care at her company, Simply Formulas, and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist or showcasing her favorite ingredients to small brands and home formulators at simply-ingredients.com