=184. Estimation of Ash.=—When the drying is accomplished in a flat platinum dish, the same sample may serve for incineration. Otherwise the incineration may be accomplished in another portion of the sample by following directions already given.[149] =185. Nitrogen.=—Even very pure samples of starch may contain a little nitrogen which is most conveniently determined by moist combustion.[150] As a rule, in commercial starches of good quality, the quantity of pure starch may be considered to be the remainder after subtracting the sum of the weights of water, ash and nitrogen multiplied by 6.25, from the original weight of the sample taken. _Example_:— Per cent of moisture found 12.85 ” ” ” ash found 0.08 ” ” ” nitrogen × 6.25 0.27 ----- Sum 13.20 Per cent of pure starch in sample 86.80 Samples of starch usually contain also traces of fat and fiber, and these when present in weighable quantities, should be determined and proper deductions made. =186. Hydrolysis with Acids.=—The acids commonly chosen for hydrolyzing starch are sulfuric and hydrochloric. The former has the advantage of being more easily removed from the finished product but the latter performs the work with less damage to the sugars formed. For commercial purposes sulfuric and for analytical practice hydrochloric acids are commonly employed. [Illustration: FIGURE 47 (BIS). MAERCKER’S HYDROLYZING APPARATUS FOR STARCH.]