Pipettes specifically used for delivering small volumes of viscous solutions like protein are:

• A. Serological measuring pipettes
• B. Mohr pipettes
• C. Volumetric transfer pipettes
• D. Ostwald-Folin transfer pipettes

Answer: (D)

Ostwald-Folin transfer pipettes are specifically designed for delivering small volumes of viscous solutions, such as those containing proteins. This type of pipette has a unique design that includes a wider opening and a smaller bore compared to other pipettes, allowing for the accurate and efficient transfer of thicker liquids without the risk of clogging. The tapered end also helps in creating a better seal when immersed in the sample, which is crucial for minimizing loss of viscous materials during pipetting.

Moreover, the Ostwald-Folin pipette is calibrated to deliver a specific volume, which is essential when working with concentrated protein solutions where precise measurements are critical for experimental accuracy. This makes it the preferred tool for applications in clinical and biochemical laboratories where precision with small amounts is necessary.

In contrast, other types of pipettes like serological and Mohr pipettes are more suited for less viscous solutions and general liquid handling, whereas volumetric transfer pipettes are designed for exact volume measurements typically for non-viscous liquids. Therefore, Ostwald-Folin transfer pipettes stand out as the appropriate choice for handling viscous solutions.