Which ingredient is not commonly found in shampoo formulations?

Citric acid is indeed an ingredient used in some hair care products, including shampoos, primarily for its ability to adjust pH levels and provide a slight acidic nature that can help enhance the shine of hair. However, it is not as commonly found in shampoo formulations compared to the other listed ingredients.

Silicone is widely used in shampoos for its smoothing properties, which help to reduce frizz and enhance shine. It provides a protective layer over the hair, making it feel softer and appear healthier. Glycerin is a humectant, commonly found in shampoos due to its ability to attract moisture to the hair, ensuring it remains hydrated and manageable. Sodium chloride, or table salt, is often included in shampoos to adjust viscosity and improve the product's texture.

In contrast, while citric acid plays a role in some formulations, it does not dominate ingredient lists as frequently as the others, which are more integral to the overall performance and feel of the shampoo. Thus, when assessing common components in shampoo formulations, the other ingredients tend to be more prevalent than citric acid.