Graph standard addition

HIS Standard addition graph. Students performed an experiment like that in Figure 5-4 in which each flask contained 25.00 mL of serum, varying additions of 2.640 M NaCl standard, and a total volume of 50.00 mL. 

Isal Standard addition graph. An assay for substance X is based on its ability to catalyze a reaction that produces radioactive Y. The quantity of Y produced in a fixed time is proportional to the concentration of X in the solution. An unknown containing X in a complex, unknown matrix with an initial volume of 50.0 mL was treated with increments of standard 0.531 M X and the following results were obtained. Prepare a graph of Equation 5-9 and find [X]... 

Figure 15-23 Standard addition graph for ion-selective electrode based on Equation 15-12. [Data from G. LI, B. J. Polk, L. A. Meazell, and D. W. Hatchett, ISC Analysis of Hydrogen Sulfide in Cigarette Smoke," J. Chem. Ed. 2000, 77. 1049.]...

From a separate calibration curve, it was found that 3 = 0.985 in Equation 15-11. Using T = 298.15 K and n = —2 (the charge of S2 ), prepare a standard addition graph with Equation 15-12 and find the concentration of sulfide in the unknown. 

Prepare a standard addition graph with Equation 15-12 and find [COJ in the unknown. 

Blrt Standard addition. Selenium from 0.108 g of Brazil nuts was converted into the fluorescent products in Reaction 18-15. which was extracted into 10.0 mL of cyclohexane. Then 2.00 mL of the cyclohexane solution was placed in a cuvet for fluorescence measurement. Standard additions of fluorescent product containing 1.40 rg Se/mL are given in the table below. Construct a standard addition graph like Figure 5-6 to find the concentration of Se in the 2.00-mL unknown solution. Find the wt% of Se in the nuts and its uncertainty and 95% confidence interval. 

Standard addition. To measure Ca in breakfast cereal, 0.521 6 g of crushed Cheerios was ashed in a crucible at 600°C in air for 2 h.22 The residue was dissolved in 6 M HC1, quantitatively transferred to a volumetric flask, and diluted to 100.0 mL. Then 5.00-mL aliquots were transferred to 50-mL volumetric flasks. Each was treated with standard Ca2+ (containing 20.0 pg/mL), diluted to volume with H20, and analyzed by flame atomic absorption. Construct a standard addition graph and use the method of least squares to find the x-intercept and its uncertainty. Find wt% Ca in Cheerios and its uncertainty. 

In principle there are no differences between calibration procedures for flame and ETA methods although the latter case will take longer, as has already been pointed out. Calibration standards should match the samples as nearly as possible with respect to major components otherwise standard additions must be used. Indeed the method of standard additions will have to be used at some time to check accuracy, so this procedure will usually be tried first. If the standard additions graph is parallel to the direct calibration graph then freedom from matrix interferences would be indicated. The standard additions principle as applied to ETA is now described. 

For more complex solutions and samples where it is not possible to remove the matrix during the ashing step it may be necessary to use the method of standard additions. Some workers advocate that this method should always be run initially to check for interference effects so that the best calibration procedure can be selected. If the standard additions graph and the direct calibration graph were parallel, freedom from interferences in the sample would be indicated, i.e. the element is in the same form in sample and standard immediately before atomisation, or the two forms give the same absorption response. 

Standard addition graph. Tooth enamel consists mainly of the mineral calcium hydroxyapatite, Caio(P04)6(OH)2. Trace elements in teeth of archeological specimens provide anthropologists with clues about diet and diseases of ancient people. Students at Hamline University measured the trace element strontium in enamel from extracted wisdom teeth by atomic absorption spectroscopy. Solutions were prepared with a constant total volume of 10.0 mL containing 0.750 mg dissolved tooth enamel plus variable concentrations of added Sr. 

Standard addition graph. The figure shows standard additions of Cu to acidified tap water measured by an electrochemical method. The standard additions had negligible volume compared with that of the tap water sample, so you can consider all solutions to have the same volume. The signal is the peak height (in jxA), which you will need to measure relative to the baseline in the figure. Find the concentration of Cu in the tap water. 

The method of standard addition was described in Section S-3. Problem 5-19 gives an equation for the uncertainty in a standard addition graph. 

In each experiment, 0.50 mL of blood was treated with Cr(VI) standard and oxidant. After digestion, the solution was brought to a final volume of 20.0 mL prior to stripping analysis. Prepare a standard addition graph for the case of constant final volume. Find the concentration (and uncertainty) of Cr in the 20.0 mL volume and in the original 0.50 mL of blood. 

Another approach to graphing standard addition results is shown in Figure 2.16 (right) the signal for the unspiked test sample is marked off on... 

In isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) the spike, the unspiked and a spiked sample are measured by ICP-MS in order to determine the isotope ratio. Using this technique, more precise and accurate results can be obtained than by using a calibration graph or by standard addition. This is due to elimination of various systematic errors. Isotopes behave identically in most chemical and physical processes. Signal suppression and enhancement due to the matrix in ICP-MS affects both isotopes equally. The same holds for most long-term instrumental fluctuations and drift. Accuracy and precision obtained with ID-ICP-QMS are better than with other ICP-QMS calibration... 

Salgado Ordonez et al. [28] used di-2-pyridylketone 2-furoyl-hydrazone as a reagent for the fluorometric determination of down to 0.2 pg aluminium in seawater. A buffer solution at pH 6.3, and 1 ml of the reagent solution were added to the samples containing between 0.25 to 2.50 pg aluminium. Fluorescence was measured at 465 nm, and the aluminium in the sample determined either from a calibration graph prepared under the same conditions or a standard addition procedure. Aluminium could be determined in the 10-100 pg/1 range. The method was satisfactorily applied to spiked and natural seawater samples. 

Either calibration graphs prepared from standards or the method of standard addition (p. 30) can be used. For the former, the standards should be as similar as possible in overall chemical composition to that of the samples so as to minimize errors caused by the reduction of other species or by variation in diffusion rates. Often, the limiting factor for quantitative work is the level of impurities present in the reagents used. 

Consider the analysis of a soda pop sample for caffeine by the standard additions method. Construct a graph from the following data and report the milligrams of caffeine in one 12-oz can of the soda pop. 

At extreme overpotentials, the current is independent of potential. This maximum current is said to be limiting, that is, current a Cbuik- It is termed the diffusion current, /j. The dependence of la on concentration, drop speed, etc., is described by the Ilkovic equation (equation (6.5)), although calibration graphs or standard addition methods (Gran plots) are preferred for more accurate analyses. 

Fig. 3.2 Sono-square-wave anodic stripping voltammetric traces for an insonated deposition of 60 s at —1.5 V. Traces show background corrected standard additions to sono-solvent extracted laked horse blood solution (test solution 0.05%, by volume-blood). Each 10 (tl addition corresponds to an increase in copper concentration of 0.22 pg/1. Calibration graph shown inset (R = 0.9972) gives concentration of 1.637 mg/1 (reprinted from [64] with permission)...

Both methods of standard addition can be analyzed with the graph in Figure 5-6. The theoretical response to the additions is derived by substituting expressions for [X]f and [S]f... 

If all samples for the standard addition experiment are made up to a constant final volume, as in Figure 5 1, an alternate way to handle the data is to graph the signal /s+x versus the concentration of diluted standard, [S]f. In this case, the x-intercept of the graph is the final concentration of unknown, [X]f, after dilution to the final sample volume. 

A standard addition is a known quantity of analyte added to an unknown to increase the concentration of analyte. Standard additions are especially useful when matrix effects are important. A matrix effect is a change in the analytical signal caused by anything in the sample other than analyte. You should be able to use Equation 5-7 to compute the quantity of analyte in a standard addition experiment. Equation 5-9 is used with multiple standard additions to construct the graph in Figure 5-6, in which the jc-intercept gives us the concentration of analyte. 

A. Li was determined by atomic emission with the method of standard addition. Use a graph similar to Figure 5-6 to find the concentration of Li and its uncertainty in pure unknown. The Li standard contained 1.62 pg Li/mL. 

For illustration purposes, in the following the determination of carbonyl chloride in test sample is described in detail. From the measurements of the calibration samples prepared according to the standard addition procedure a calibration graph is obtained as depicted in Fig. 10-9. 

Figure 10-9 Calibration graph obtained from the standard addition procedure.

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