Volume end point

Dextrin Polyethylene glycol 400 Use 5 mL of 2% aqueous solution of chloride-free dextrin per 25 mL of 0. IM halide solution. Prepare a 50% (v/v) aqueous solution of the surfactant. Use 5 drops per 100 mL end-point volume. 

Relationship Between End Point Volumes and Sources of Alkalinity... 

What is the concentration of H2SO4 in a sample whose end point volume is 0.157 mb ... 

Plot the titration curve and determine the end-point volume and concentration of the original lead nitrate solution. 

Figure 6.4 A schematic of the first derivative of the pH curve in Figure 6.3. The end-point volume is determined as the volume at the peak. A first derivative plot such as this can yield a more accurate end point than drawing parallel lines on Figure 6.3...

During a redox reaction, a potentiometric titration can be employed to determine a concentration of analyte rather than an activity, since we are only using the emf as a reaction variable in the accurate determination of an end point volume. For this reason, an absolute value of reference electrode need not be known, as we are only concerned with changes in emf. It is, however, advisable to titrate at high ionic strength levels in order to minimize fluctuations in the mean ionic activity coefficients. 

Replicates of six samples per each level of concentration were prepared for each compound. Each standard was prepared and titrated immediately. The data sets of end-point volumes and milligrams of the standards were treated statistically, where the standard deviations and regression analysis were performed. 

For consistency, all the 0.001 N sodium thiosulfate end-point volumes were converted to 0.005 N end-point volumes as follows ... 

Six filters for each level of concentration were spiked using variable volume pipettes. The following volumes 13, 25, 50, 100, and 200 yL of the 10 mg/mL solution yielded 0.13, 0.25, 0.50, 1.00 and 2.00 mg of sodium dichloroisocyanurate dihydrate on a filter, respectively. When the filter was dry, the "filter extraction" section was followed. The end-point volumes were converted to milligrams from the calibration curve and the percent recoveries were calculated. 

The solution to the problem was discovered when a titrated sample (clear solution) was left on the bench and, after a period, it started changing back to a faint yellow color. We hypothesized that air oxidation may have caused that effect and, consequently, air may have interfered with analysis. Standard samples prepared and purposely delayed during the analysis showed that end-point volumes were larger, indicating that some of the iodide ions turned into free-iodine by air oxidation which, in turn, required more thiosulfate for titration and, therefore, larger end-point volume. The following chemical equations obtained from the literature 8) show what happens before, during, and after titration. The reaction of a chlorinated isocyanuric acid compound with potassium iodide in acidic pH is ... 

To obtain the end-point volume, the value of V where [Cl ] = 0, you need to calculate the x intercept. The x intercept = -intercept/slope = 32.83 mL, the endpoint volume for the titration. 

Subsequent titrations can be speeded up by using the first to approximate the end-point volumes. 

It is necessary to plot the potential only near the end point. Small increments of titrant are added near the end point, 0.1 or 0.05 mL, for example. The exact end point volume need not be added, but it is determined by interpolation of the E versus volume plot. 

There are numerous automatic titrators that employ potentiometric end-point detection. They usually can automatically record the first or second derivative of the titration curve and read out the end-point volume. The sample is placed in the titration vessel, and the titrant, drawn from a reservoir, is placed in a syringe-driven buret. The volume is digitally read from the displacement of the syringe plunger by the electronic driver. Titrators may also employ photometric detection of indicator color changes. An automatic titrator is shown in Figure 14.7. Automatic titrators make volumetric analyses rapid, reproducible, and convenient. While instrumental methods provide many advantages, classical volumetric analyses are still widely used and are very useful, especially for major constituents, for example, in the pharmaceutical industry. 

The end point is the volume at which the second derivative is 0. A graph on the scale of Figure 10-6 allows us to make a good estimate of the end-point volume. 

When the indicator changes color (colorless to pink for phenolphthalein), the end point volume on the burette is recorded. 

Before the end point, there is excess H, so the pH is less than 7 (see Section 6.6 for a discussion of pH and the pH values in acidic and basic solutions). Beyond the end point, there is excess base, and the pH is greater than 7. Since HCl is a strong (completely ionized) acid and NaOH is a strong (completely ionized) base, the pH at the end point is exactly 7. (See Section 6.4 for a discussion of strong and weak acids and bases.) Furthermore, the pH changes very markedly by several pH units with the addition of just a few drops of sodium hydroxide titrant at a volume that is in the immediate vicinity of the end point. This change is reflected by the abrupt change in color of the phenolphthalein indicator from colorless to pink at the end point, where the addition of titrant is stopped, and the end point volume recorded. 

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