Why can impurities cause errors in gravimetric analysis?

In gravimetric analysis, the accuracy of the final result depends on isolating a constituent that is 100 percent pure and matches the expected chemical formula. Impurities cause errors because they alter the mass of the isolated substance, leading to an incorrect computation of the constituent's original amount in the sample.1

An impurity acts as an interference—a substance other than the analyte that is either measured by the analytical method or prevents the accurate measurement of the desired material. When a precipitate is formed, it must be washed free of soluble impurities to ensure the final weight reflects only the target constituent. If these impurities are not successfully removed through processes like filtration or selective precipitation, they remain part of the weighed sample, introducing error into the quantitative determination. Because achieving a perfectly pure precipitate is technically difficult, all gravimetric methods are subject to some degree of error related to the purity of the isolated constituent.213