Handling, Preparation and Storage of Standards

Preparation

Weight ≠ Volume

Standard chemical solutions can be prepared to weight or volume. The elimination of glass volumetric flasks may be necessary to eliminate certain contamination issues with the use of borosilicate glass or to avoid chemical attack of the glass. It is often assumed that 100 grams of an aqueous solution is close enough to 100 mL to not make a significant difference since the density of water at room temperature is very close to 1.00 (0.998203 at 20.0 °C). Diluting / preparing standard solutions by weight is much easier. Still, the above assumption should not be made. The problem is that trace metals standards are most commonly prepared in water + acid mixtures where the density of the common mineral acids is significantly greaten than 1.00. For example, a 5% v/v aqueous solution of nitric acid will have a density of ~1.017 g/mL which translates into a fixed error of ~1.7%. Higher nitric acid levels will result in larger fixed errors. This same type of problem is true for solutions of other acids to a degree that is a function of the density and concentration of the acid in the standard solution as described by the following equation (to be used for estimation only):

d_S = [(100-%) + (d_A)(%)] / 100

Where:

d_S = density of final solution
% = The v/v % of a given aqueous acid solution
d_A = density of the concentrated acid used

For example, lets estimate the density of a 10% v/v aqueous solution of nitric acid made using 70% concentrated nitric acid with a density of 1.42 g/mL.

D_S = [(100-%) + (d_A)(%)]/100 = [(100-10) + (1.42)(10)]/100 = (90 + 14.2)/100 = 1.042 g/mL

Acid Content

Another area of confusion is the expression of the acid content of the solution. We all agree that it is important to matrix match the standard and sample solutions to avoid a fixed error in the solution uptake rate and/or nebulization efficiency sometimes referred to as a matrix interference. If a solution is labeled as 5% HNO₃ what does this mean? If we take 5 mL of 70% concentrated nitric acid and dilute to a volume of 100 mL then this is 5% HNO₃ (v/v) where the use of 70% concentrated acid is assumed. However, nitric acid can be purchased as 40%, 65%, 70%, and > 90%. Therefore, note the concentration of the concentrated acid used if different from the 'norm' as well as the method of preparation i.e. v/v or wt/wt or wt/v or v/wt. The wt. % concentrations of the common mineral acids, densities, and other information are shown in the following table:

Table 3.3: Wt. % Concentrations

Acid Content in Molarity

It is important to know what the concentration units of the concentrated acid being used mean. Taking 70% concentrated nitric acid as an example means that 100 grams of this acid contains 70 grams of HNO₃. The concentration is expressed at 70% wt./wt. or 70 wt. % HNO₃. Some analysts prefer to work in matrix acid concentrations units of Molarity (moles/liter). To calculate the Molarity of 70 wt. % nitric acid we calculate how many moles of HNO₃ are present in 1 liter of acid. Lets say that we tare a 1 liter volumetric flask and then dilute to the mark with 70.4 wt. % HNO₃. We would then measure the weight of the solution to be 1420 grams. Knowing that the solution is 70.4 wt % would then allow us to calculate the number of grams of HNO₃ which would be (0.704)(1420g) = 999.7 grams HNO₃ per liter. Dividing the grams HNO₃ by the molecular weight of HNO₃ (63.01 g/mole) gives the moles HNO3 / L or Molarity which is 15.9 M. The above logic explains the following equation used for calculating the Molarity of acids where the concentration of the acid is given in wt %:

[(% x d) / MW] x 10 = Molarity

Where:

% = wt. % of the acid
d = density of acid (specific gravity can be used if density not available)
MW = molecular weight of acid

Using the above equation to calculate the Molarity of the 70 wt % nitric acid we have:

[(70.4 x 1.42) / 63.01] x 10 = 15.9 M

Dilutions of the concentrated acid to prepare specific volumes of specified Molarity can be make using the (mL_A)(C_A) = (mL_B)(C_B) equation.

Avoiding Precipitates

In the preparation of mixtures of the elements, it is good to avoid the formation of precipitates. It is common to form precipitates when concentrates of elements that are considered compatible (see part 1 of this series) are mixed. Many precipitates are not reversible (i.e., will not go into solution upon dilution). It is therefore best to add all of the acid and most of the water to the volumetric flask or standard solution container (dilutions to weight) before adding the individual element concentrate aliquots. Mixing after each aliquot addition is strongly advised. When diluting to volume it is often found that the solution is above room temperature. Therefore allow the solution to cool to room temperature and adjust to the mark with DI water. It is best to prepare the dilution the day before needed to allow for proper volume adjustment.

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