Passage Bone growth and remodeling during fracture healing are often slow and have potentially poor outcomes, including malunion or nonunion at the fracture site. Magnesium bone implants can aid in bone formation and safely biodegrade in the body according to the following reaction:Mg(s) + 2H 2 O(l) → Mg(OH) 2 (s) + H 2 (g)Reaction 1 Corrosion of magnesium metal in aqueous mediumHowever, magnesium metal tends to degrade too quickly and forms hydrogen gas pockets in the body. Corrosion is slowed as the metal naturally forms a protective layer of magnesium hydroxide in a process called passivation. Magnesium hydroxide is less soluble than magnesium metal, and its dissolution is described by the endothermic reaction: Reaction 2 Solubility equilibrium of Mg(OH) 2 in water at 25°CAlthough passivation slows down the corrosion rate, it must be slowed even further to adequately facilitate bone reformation.Researchers are interested in the differences in the corrosion rates of magnesium metal protected by different biocompatible coatings. They compared 10-mm magnesium disks coated with compounds of various cation and anion combinations, with solubility results shown in Table 1. The coated disks were immersed in simulated body fluid (SBF) at room temperature for 50 hours.Table 1 Solubility for Biocompatible Inorganic Salts Given in mmol/100mL of Pure Water at 25°C Adapted from Huan ZG, Leeflang MA, Zhou J, Fratila-apachitei LE, Duszczyk J. In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys. J Mater Sci Mater Med. 2010;21(9):2623-35. -Throughout the experiment, researchers monitored the amount of magnesium that dissolved at various time points. Which of the following is NOT a valid test for the rate of magnesium corrosion?

Question

Quizplus · Accepted Answer

11ecba2d_11a9_3840_a3bf_597d2b9bcfd5_MD0008_00 When determining the corrosion rate of the magnesium disks, the experimental technique cannot affect the outcome (ie, the corrosion rate) or it will skew the results. HCl is a strong acid and reacts with magnesium metal to form H₂ gas and aqueous MgCl₂ (which dissociates into Mg²⁺ and 2 Cl⁻ ions) according to the equation: Mg(s) + HCl(aq) → Mg²⁺(aq) + 2Cl⁻(aq) + H₂(g) Furthermore, the addition of acid will neutralize hydroxide ions in solution, which will shift the equilibrium toward Mg(OH)₂ dissolution, and HCl will directly react with Mg(OH)₂ as follows: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l). As a result, the addition of HCl will increase the corrosion rate both by directly dissolving Mg metal and by dissolving the passivating Mg(OH)₂ coating. Therefore, titrating the beaker contents with HCl during the experiment is not a good way to determine the rate of corrosion because the acid alters that rate. (Choice A) Hydrogen gas is evolved when magnesium metal corrodes into Mg(OH)₂ and H₂ gas. Therefore, measuring the amount of H₂ gas evolved over time would give a good estimate of the corrosion rate of the magnesium disks. (Choice B) Magnesium disks will lose mass during corrosion as magnesium ions are released into solution. Therefore, determining the mass of the disks at various time intervals during the experiment would be a good way to determine the rate of corrosion. (Choice C) Magnesium ions form as the metal dissolves, so direct measurement of the concentration of Mg²⁺ in solution over time would be a good way to determine the rate of corrosion. Educational objective: In a good experimental design, the method of measurement must not alter the experimental outcome.

Passage Bone Growth and Remodeling During Fracture Healing Are Often Slow