Solutions Preparation#
Introduction#
This is not actually an experiment. Rather it is a set of information to be used in preparing standard solutions of known concentrations for use in calibrating chemical instruments. Virtually all analytical instruments require frequent (usually daily) calibration to insure precision and accuracy in analysis. Typically calibration is accomplished by making a set of standard solutions (solutions of known analyte concentration) and measuring the instrumental response to those solutions. It is imperative that the standard solutions be made accurately and that requires careful solutions preparation technique. The process of making standard solutions involves two steps: calculations and actual solution preparation. The complexity of each of the steps depends on the analyte and the required concentrations.
Calculations#
Before making standard solutions the analyst must carry out some calculations to establish what steps will be done to produce the desired solutions. Usually a concentrated stock solution is made and then diluted by different amounts to produce the various standard concentrations.
Concentration Units and Conversions#
We will use two solution concentration units, molarity (M) and ppm, and one concentration unit for solids (wt/wt %). The units will be defined and a quick method for converting from molarity to ppm and vice versa will be presented.
Molarity#
Molarity is defined as moles of solute per liter of solution and is given the symbol M. This unit is typically used for medium to high concentration solutions. Molarity describes what is actually in the solution. A solution made by dissolving one mole of NaCl (a strong electrolyte which is completely dissociated) in 1 L of solution can properly be said to be 1 M in Na+ or 1 M in Cl-. The Formality, F, of the solution tells what went into the solution and has the same units as molarity. The above solution has a formality of 1 F in NaCl. Formality and molarity are usually used interchangeably, and we will be guilty of that simplification in this course, i. e. the above solution could be said (technically incorrectly) to be 1 M in NaCl. You may frequently see concentrations expressed in millimolarity, mM. A 1 mM solution is 0.001 M.
Parts-per-million#
Parts-per-million, ppm, is defined as micrograms of solute per gram of solution. This unit is used for solutions that are relatively dilute. In dilute aqueous solutions the approximation is made that 1 gram of solution occupies 1 mL, thus changing ppm to 1 microgram per mL (or 1 mg/L) of solution. This approximation is good as long as the solution is near room temperature and the solution is dilute enough that the solute does not change the density from that of pure water. Note that a 1 ppm-solution of NaCl is not 1 ppm in Na+ (it is about 0.4 ppm Na+).
Conversion from M to ppm#
Converting from one of the above units to another can be handled by remembering one concept. The concentration of a 0.001 M (1 mM) solution in ppm is equal to the number of grams of solute per mole of solute. An example is a 0.001 M solution of NaCl is 58.4 ppm NaCl (NaCl is 58.4 g/mole). A 0.001 M solution of Na+ is 23.0 ppm Na+.
wt/wt %#
Wt/wt % is defined as grams of solute per 100 g of sample, and can be calculated as (g solute/g sample) Γ 100 %. This unit is usually used to describe solid solutions or mixtures. The concentration of analyte must be relatively high for this unit to be useful.
Dilution Calculations#
It is necessary to perform calculations to establish how standard solutions will be made. Three types of calculations will be presented: making a stock solution from a solid, doing a one-step dilution, and doing a two-step dilution. There are several limitations that must be kept in mind when establishing solutions preparation steps:
When weighing out solid you should weigh out at least 1 gram and not more than 10 grams. This ensures less relative error, good precision and keeps costs reasonable.
Volumetric flasks come in sizes from 10 mL to 2 L. These should be used for all cases where a specific volume of solution is to be made.
Volumetric pipets come in sizes from 1 to 100 mL. Volumetric pipets or burets should be used for all dilutions. Micropipets, available in sizes from 10 ΞΌL to 5 mL may also be used. Keep in mind that less relative error is encountered as the measurement size goes up.
Another decision that must be made at this point is what the primary standard solid will be. There are several sources for finding what solid to use:
If you have an elemental analyte and can obtain a pure sample of the analyte that is the best starting material. This is possible for many metals, e. g. copper and iron.
Standard methods of analysis books are published by several groups (Environmental Protection Agency, Association of Official Analytical Chemists, American Society for Testing Materials). These references will provide information on primary standards and other matters regarding analysis.
Instruments often have methods books supplied, which describe methods and materials for standards preparation. These are best since they are specific for that application.
Making a stock standard solution#
A stock solution is one that has a known concentration of analyte but is more concentrated than the standards that will be used for calibration. This process requires three steps:
Calculate the number of moles of analyte needed to make the required volume of the desired concentration.
Calculate the mass of solid needed to give the correct number of moles of analyte using the formula weight of the solid.
Weigh out the solid, dissolve it in a small amount of solvent, quantitatively transfer the solution to a volumetric flask and dilute to the mark.
One-step Dilution#
The following steps should be used to do a one-step dilution of a stock solution to produce a more dilute standard solution:
Calculate the amount (moles or grams) of solute needed in the required volume of the desired concentration.
Calculate the volume of stock solution needed to give that amount of solute.
Volumetrically transfer the necessary volume of stock solution to a volumetric flask and dilute to the mark.
If the required volume of stock solution is less than 1 mL you will need to do a two-step dilution.
Two-step Dilution#
The following steps should be done to perform a two-step dilution:
Perform step 1 from the one-step dilution.
Do a volumetric 1 to 10 or 1 to 100 dilution of the stock solution.
Calculate the volume of the diluted stock solution required to give the necessary amount of solute.
Volumetrically transfer the required volume of diluted stock solution to a volumetric flask and dilute to the mark.
Making Standard Solutions#
Handling Primary Standards (see pertinent pages in Harris)
Drying and weighing procedures
Some primary standards need to be dried before weighing and some donβt. Be sure to consult the reference that identified the primary standard to find out drying temperature and time. The chemical formula of the primary standard may be changed (particularly the number of waters of hydration) by drying so be sure you know what youβre doing before you dry the standard.
When weighing the primary standard, weigh by difference using an analytical balance into the vessel in which you intend to dissolve the solid.
Dissolution of the Primary Standard
Usually the source reference will provide information about how to dissolve the solid standard. If no dissolution information is provided check the solubility of the compound in the CRC Handbook or Merck Index before proceeding. Start with mild conditions and use more drastic measures (acid and/or heat) if necessary. The standard solvent should match the matrix of the unknown as closely as possible.
