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1. The titration approach that involves adding a reagent in excess, then titrating that added reagent to determine the concentration of an analyte is known as:
A. Double titration B. Back titration C. Complex titration D. Dose titration
See Why
Answer: B, back titration.
It is most useful when the primary end point of the initial titration is difficult to detect.
Source: Oxford Dictionary of Chemistry.
NEXT QUESTION
2. In the early days of acid-base titrations, researchers used as an indicator either litmus or which of the following?
A. Effervescence from neutralizing carbonate B. Flavor change from conversion of acetic acid to sodium acetate C. Color change from permanganate D. Precipitation of silver nitrate
Answer: A, effervescence, or bubbling, from neutralizing carbonate.
Adding an acid to a solution of carbonate neutralizes the molecule and produces CO2, which escapes the solution as bubbles. Before 1871, when the color-changing pH indicator phenolphthalein was first synthesized, this effervescence was used to determine and adjust the acidity of vinegar and standardize the product.
Sources: Analytical Chemistry, 2019; Lumen Chemistry; Encyclopedia of Food Sciences and Nutrition, 2003.
3. A coulometric approach to determining the water content of a sample is frequently referred to as ___ titration, after its inventor.
A. Karl Popper B. Karl Fischer C. Karl Fleming D. Karl Auerbach
Answer: B, Karl Fischer.
This titration approach measures a stoichiometric redox reaction between water and iodine that can be measured by coulometry, the measurement of electricity to monitor a reaction. Before the titration end point, divalent iodine is rapidly oxidized to form iodide anions, which conduct electricity. After the titration end point, as more iodine is added but not oxidized, the solution’s conductivity drops.
Sources: Karl Fischer Titration: Determination of Water, 1984; Anal. Chem., 1991.
4. Which of the following can help find a difficult-to-detect titration end point?
A. Calculating the first derivative of the titration curve B. Using a more dilute titrant C. Increasing the reaction temperature D. Performing regression analysis of the end points from multiple experiments
Answer: A, calculating the first derivative of the titration curve.
Titration end points are often detected as the inflection point of a titration curve. Calculating either the first or second derivative of that curve can make an inflection point more apparent. Because this approach requires many data points taken near the equivalence point, when rapid change in the readout occurs, difficult-to-detect endpoints are best measured using an automatic titration system.
Source: Acid–Base Titrations. 2020.
5. Wastewater treatment plants often use redox titration to measure which of the following properties?
A. Turbidity B. Acidity C. Concentration of hydrophobic organic molecules D. Chlorination efficiency
Answer: D, chlorination efficiency.
Because chlorine rapidly reacts with ammonia in wastewater to form chloramines, which are not effective disinfectants, chlorine concentrations do not necessarily reflect disinfection. Instead, treatment plants use colorimetric, iodometric, or amperometric titration to determine whether more chlorine is needed
Sources: Metrohm application note T-217; Current Technology of Chlorine Analysis for Water and Wastewater, 2002.
6. Karl Fischer titration, a powerful tool for determining the water content of a sample, is widely used for quality control of lyophilized drug products. Why does it matter how much residual moisture is present?
A. Residual moisture can cause undesirable clumping B. Water may oxidize the active pharmaceutical ingredient C. Water may hydrolyze components of the drug D. Water throws off molarity calculations when redissolving the drug
Answer: C, water may hydrolyze components of a drug—especially at higher storage temperatures.
Lyophilization can make small-molecule and biologic pharmaceuticals more shelf stable at warmer temperatures. The residual water, a key regulatory quality indicator, can be detected using several approaches to Karl Fischer titration.
Sources: Am. Pharm. Rev., 2019; Am. Pharm. Rev., 2010.
7. Coulometric titration of sweat can be used to diagnose which of the following conditions?
A. Cellulitis B. Ichthyosis C. Cystic fibrosis D. Diabetes mellitus
Answer: C, cystic fibrosis.
The disease affects the function of a chloride channel, increasing the chloride concentration in sweat and other secretions. Although DNA tests from blood screening are now routine, diagnostic titration instruments known as chloridometers, which use coulometric titration to measure chloride ions in as little as ten microliters of sweat, remain the gold standard to confirm a cystic fibrosis diagnosis.
Sources: Anal. Bioanal Chem., 2020; ELITech Group; “Sweat Test,” Cystic Fibrosis Foundation.
8. Which of the following titrants can be used in a thermometric titration assay to determine the sodium content of food samples?
A. Potassium thiocyanate B. Silver chromate C. Ferric ammonium sulfate D. Aluminum nitrate
Answer: D, aluminum nitrate.
Its reaction with sodium is exothermic; when the sodium in a sample has been exhausted, the temperature increase stops.
Sources: Microhm application note H-112; Acid–Base Titrations. 2020.
9. Potentiometric titration can be used in drug design and synthesis to achieve all but which one of the following?
A. To test the purity of starting reagents B. To determine the salt ratio of a multiprotic compound C. To calculate the efficiency of synthesis D. To predict whether a compound will cross the blood-brain barrier
Answer: D, to predict whether a compound will cross the blood-brain barrier.
Potentiometric titration is widely used to measure the concentration of starting materials and active pharmaceutical ingredients, as well as to characterize candidate compounds’ pKa, ionization state, and salt ratio. Although those properties affect drug absorption, predicting how drugs will be absorbed and metabolized in the body from their chemical properties is still an active area of research.
Sources: Am. Pharm. Rev., 2012; Nat. Rev. Drug Discovery, 2015.
10. Automated titration can be used to monitor which of the following industrial solutions?
A. Solvents for carbon dioxide capture and reuse B. Solutions for paper pulping C. Pickling baths to finish steel D. All the above
Answer: D, all the above.
Because automated titration systems detect endpoints more accurately, reduce human exposure to potentially hazardous reagents, and can be built into continuous-flow systems, they are used in these and many other industrial-scale processes.
Sources: J. CO2 Util., 2017; Org. Process Res. Dev., 2019; Metrohm application notes PAN-1004 and PAN-1019.
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