When dyeing protein fibers with acid dyes, in addition to the direct influence of the composition, chemical properties, and physical state of the fiber and dye itself, external conditions such as the dye bath's pH, temperature, and the addition of electrolytes also directly impact the dyeing process.

1. Dye Bath pH

When the solution pH is low, wool becomes positively charged, increasing the opportunities for wool to bind with inorganic anions (chloride or sulfate ions) and dye anions, making it easier to dye.

Therefore, controlling the pH of the dye bath controls the amount and rate of dye absorption by the wool. Adding acid during the dyeing process increases the hydrogen ion concentration in the dye bath, making the wool easier to dye. To achieve even dyeing, it can be added in stages.

Different acids are used depending on the type of dye. For example, sulfuric acid is used for strong acid baths, acetic acid for weak acid baths, and ammonium acetate or ammonium sulfate for neutral baths.

2. Electrolyte Addition

The effect of adding electrolytes during wool dyeing is related to the pH of the dye bath. That is, when the dye bath pH is below the wool's isoelectric point, the wool and dye primarily bond through salt bonds, and the addition of electrolytes slows dyeing. When the dye bath pH is above the wool's isoelectric point, the wool and dye primarily bond through hydrogen bonds and van der Waals forces, and the addition of electrolytes accelerates dyeing.

3. Temperature

The relative molecular mass of acid dyes ranges from approximately 300 to 800. At normal dye concentrations and room temperature, dyes rarely aggregate. However, acid dyes dyed in weak acid or neutral baths do aggregate at room temperature, and only higher temperatures, or even boiling dyeing, can reduce the dye's aggregation tendency.

The greater the dye's aggregation tendency, the more significant the effect of temperature. Increasing the temperature can also cause the wool to expand, facilitating dye diffusion into the fiber. Above 50°C, increasing the temperature significantly increases fiber expansion.

The wool's scale layer significantly impedes dye diffusion, so boiling and prolonged dyeing are necessary. In order to avoid uneven dyeing, the dyeing temperature should be low at the beginning of dyeing and then gradually increased to boiling. Controlling the temperature increase is very important for uniform dyeing.