Final answer:
The volume of HCl is higher for titrating the blank because no fat is present to react with the base, meaning all the base must be neutralized by the HCl, as opposed to the sample where some base is consumed in the reaction with the fat.
Step-by-step explanation:
The reason why the volume of HCl needed to titrate the blank is higher than the volume needed to titrate the sample when determining the saponification value has to do with the presence of residual fat in the sample. During the saponification process, a known quantity of a strong base, such as NaOH, is used to hydrolyze the fat present in the sample. After hydrolysis is complete, any unreacted base is titrated with a strong acid like HCl to determine how much base was used to hydrolyze the fat.
When performing a titration calculation, the blank (a sample without fat) is titrated to establish a baseline for the amount of acid needed to neutralize the set amount of base. Because there is no fat to react with the base in the blank, all of the added base must be neutralized by the HCl, requiring more HCl. On the contrary, in the actual sample with the fat, some of the base reacts with the fat, hence less HCl is needed to reach the same endpoint.
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