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editIn analytical chemistry, a standard solution is a solution containing an accurately known concentration. Standard solutions are prepared by dissolving a solute of known mass into a solvent to a precise volume, or by diluting a solution of known concentration with more solvent. A standard solution ideally has a high degree of purity and is stable enough that the concentration can be accurately measured after a long shelf time.[1]
Making a standard solution requires great attention to detail to avoid introducing any risk of contamination that could diminish the accuracy of the concentration. For this reason, glassware with high degree of precision such as a volumetric flask, volumetric pipette, micropipettes and automatic pipettes are used in the preparation steps. The solvent used must also be pure and readily able to dissolve the solute into a homogenous solution.[1]
Standard solutions are used for various volumetric procedures such as determining the concentration of solutions with unknown concentration. The concentration of a standard solution is normally expressed in units of moles per litre (mol/L, often abbreviated to M for molarity), moles per cubic decimetre (mol/dm3), kilomoles per cubic metre (kmol/m3), grams per milliliters (g/mL), or in terms related to those used in particular titrations (such as titres).
Figure 1: Experimental setup for creating standard solutions
Types of standard solutions
editStandard solutions can be categorized by the type of analyte used to prepare them. These analytical standards can either be a primary standard or secondary standard.
Primary standards
editPrimary standards are compounds with known stoichiometry, high purity, and high stability under standard conditions. The compound must not be hydroscopic to have a mass that accurately represents the exact number of moles when weighed. These characteristics make primary standards reliable for preparing standard solutions with an accurate concentration just by knowing the amount of compound and solvent used. Primary standard solutions are commonly used to determine the concentration of secondary standard solutions through titration. An example of a primary standard is potassium dichromate.[2]
Secondary standards
editSecondary standards do not satisfy the requirements for a primary standard. A standard solution created from a secondary standard cannot have its concentration accurately known without stoichiometric analysis against a primary standard. An example of a secondary standard is sodium hydroxide, a hydroscopic compound that is highly reactive with its surroundings. The concentration of a standard solution made with sodium hydroxide may fluctuate overtime due to the instability of the compound, requiring for calibration using a primary standard before use.[2][3]
Methods of standardization
editStandard solutions are commonly used for standardization processes in quantitative analysis to minimize error and maintain accuracy in the results.
External standards
editThis is the most common method of standardization which requires one or multiple standards, each containing a known concentration of the same analyte. External standards are analyzed separately from the sample unlike other methods of standardization, hence the name "external". When concentrations of a set of external solutions are plotted against a measured value such as the absorbance of each external solution, a normal calibration curve can be obtained. Multiple samples with unknown concentrations can then be analyzed using this calibration curve which make it a useful tool. The external standardization method can introduce determinate error if the matrix of the unknown solution differs drastically from the external standard. This issue can be accounted for by replicating the matrix of the unknown solution in the external standard with a process called "matrix matching". [2][4]
Internal standards
editA series of internal standards contain the same concentration of a chemical called the internal standard and different concentrations of the analyte.The internal standard should be chemically similar to the analyte, so that the two receive the same treatment during measurement. Internal standards are used to correct for loss of analyte during sample preparation, for example when the analyte is in a volatile solvent. If both the internal standard and the analyte lose solvent proportionally, their signals will remain identical and the ratio of their signals can be measured.
Plotting the ratio of the analyte signal to the internal standard signal against the analyte concentration results in a calibration curve. Similar to the external calibration curve, the internal calibration curve also allows to calculate the concentration of analyte in an unknown sample.[5]
Standard additions
editIn the standard addition method, a standard (usually in the form of a solution) with a known concentration is added in increasing incerments to a set of solutions containing the same unknown analyte.The matrix for these solutions are identical which eliminates the matrix effect from changing the signal of the analyte when measured. For this reason, the graph produces a linear slope when a calibration curve is plotted. The concentration of the unknown analyte can then be measured after determining the value of the x-intercept.[6]
Applications
editNMR Spectroscopy
editA commonly used internal standard for calibrating chemical shift signals in 1H and 13C NMR spectroscopy is Tetramethylsilane (TMS). TMS is an organosilicon compound where the silicon is bound to four methyl groups. Because all the methyl protons are chemically equivalent, the 1H peak is a strong, upfield singlet. This peak is assigned a value of 0.0 ppm and all other peaks are reported relative to this TMS signal as a chemical shift (ppm) value.[7] A similar approach is also taken for the carbons of TMS to produce a 13C NMR spectra. (Not transferring to the article)
Chromatography
editInternal standards are used in GC/MS and LC/MS to control for variability introduced by injection, sample preparation and other matrix effects. The ratio of peak areas between the internal standard and analyte is calculated to determine analyte concentration.[8] A common type of internal standard is the use of isotopically labelled analog of the analyte. These analogues include one or more atoms of 2H, 13C, 15N and 18O incorporated into their structure.[9]
Example of preparing a series of standard solutions
editSuppose the concentration of glutamine in an unknown sample needs to be measured. To do so, a series of external standard solutions containing glutamine is prepared to create a calibration curve. A table summarizing a method for creating these solutions is shown below:
| Concentration of glutamine stock solution (g/mL): | 7.50 x 10-3 | ||
|---|---|---|---|
| Solution | Glutamine added (mL) | Dilute to mark with: | Resulting Concentration (g/mL) |
| 1 (blank) | 0 | Deionized water in
25 mL Volumetric Flask |
0 |
| 2 | 1 | 3.00 x 10-4 | |
| 3 | 2 | 6.00 x 10-4 | |
| 4 | 3 | 9.00 x 10-4 | |
| 5 | 4 | 1.20 x 10-3 | |
Here, a stock solution of glutamine is added in increasing increments with a high-accuracy instrument, such as a volumetric pipette, and diluted to the same volume in volumetric flasks. The resulting concentration is calculated using the formula for molar concentration. The result is four standard solutions with varying known concentrations, along with a blank, prepared for instrument calibration.
References
edit- ^ a b Christian, Gary; Dasgupta, Purnendu; Schug, Kevin. Analytical Chemistry, 7th Edition (7th ed.). ISBN 978-0-470-88757-8.
- ^ a b c Harvey, David (2016). "Analytical Chemistry 2.1". dpuadweb.depauw.edu. p. 148.
- ^ Bassett, J; G H, Jeffery; Mendham, J; Denney, R C (1989). Vogel's Textbook of Quantitative Chemical Analysis (5th ed.). Longman Scientific & Technical. p. 260. ISBN 0582446937.
- ^ Harvey, David (2000). Modern analytical chemistry. Boston: McGraw-Hill. pp. 158–160. ISBN 978-0-07-237547-3.
- ^ Harvey, David (2000). Modern analytical chemistry. Boston: McGraw-Hill. pp. 167–170. ISBN 978-0-07-237547-3.
- ^ Ramaley, Louis; Wentzell, Peter; Doucette, Alan; Guy, Robert. "An Introduction to Analytical Chemistry" (PDF) (8.1 ed.). pp. 110–115.
- ^ Mohrig, Jerry R.; Noring Hammond, Christina; Schatz, Paul F. (2010). Techniques in Organic Chemistry (3rd ed.). W. H. Freeman and Company. pp. 321–322. ISBN 978-1-4292-1956-3.
- ^ Kealey, David; Haines, P J (2002). Instant Notes in Analytical Chemistry (1st ed.). London: Taylor & Francis (published 2002-05-31). p. 154. ISBN 9780429258428.
- ^ Grocholska, Paulina; Bąchor, Remigiusz (2021-05-18). "Trends in the Hydrogen−Deuterium Exchange at the Carbon Centers. Preparation of Internal Standards for Quantitative Analysis by LC-MS". Molecules. 26 (10): 2989 – via MDPI.