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Accuracy, Precision, Mean and Standard Deviation 14 asic Calculations B
There are certain basic concepts in analytical chemistry that are helpful to the analyst when treating analytical data. This
section will address accuracy, precision, mean, and deviation as related to chemical measurements in the general field of
analytical chemistry.
(Cl, F, Br, and I), thiosulfate, arsenate, and sulfide to name the most common. However, the major problem with trace Pb
analysis is contamination from the apparatus and atmosphere. Pb is used in industry in plumbing (pipes), solder, gasoline
(significantly curtailed), drying agent for oils, glass, plumber’s cement, covering of steel to prevent rust, as a pigment in paint
(significantly curtailed), hair dye and as a pigment in plastics.
• Environmental contamination from airborne particulates is still a major concern in certain regions/laboratories depending
upon location and age. When tetraethyl lead was widely used as an octane booster it was impossible to avoid environmental
contamination in an open digestion apparatus. Open digestions in hoods where large volumes of air pass over the apparatus
are of most concern. Closed container digestions or clean rooms / hoods are suggested to avoid this source of contamination.
• Avoid the use of any type of glass in sample preparations for Pb. Use quartz or fused silica and perform a sufficient number
of blanks to define the degree of contamination.
• Avoid the use of any plastic with an inorganic pigment. Here Pb is only one of many concerns.
• Teflon containers should be carefully leached with dilute nitric acid before use and blanks performed to confirm freedom
from Pb contamination. Be particularly suspicious of Teflon that has been used in sample preparations where Pb was a major,
minor or trace component.
Pb elemental data*
Chromium (Cr)
The major difficulty that I have experienced with Cr is that it often exists in forms that are difficult to put into solution.
Chromite 2O 3), chromic oxide, pigments, stainless steel and ferro-chrome all present a challenge but the hexavalent
chromium oxides are the most difficult. If the oxide has been ignited (pigments) the refractory nature is such that an analyst
confronted with the task of bringing about solution will never forget the experience. The most common approach is to
perform a fusion. Fusions that have been used include but are not limited to potassium and sodium bisulfate, carbonate
(sodium or potassium), sodium peroxide, NaOH / KNO 3, and NaOH / Na2O2. In addition, the fusion will not be complete
unless the chrome is finely divided and mixed with the flux.
• Know you sample to the fullest extent possible. The possible chemical forms of Cr should influence the sample preparation
technique employed.
• If your sample is an inorganic pigment containing Cr then you know that you have an extremely refractory material to dissolve.
• If you are unfamiliar with your sample type a literature search is strongly suggested.
• Method validation using a CRM containing Cr in the suspected or known chemical form(s) is vital. The importance of
CRMs prepared from ‘real world’ materials is critical (i.e., synthetic CRMs are likely to contain easily dissolved compounds).
• Avoid mixing water-soluble hexavalent chrome with Ba or Pb to avoid loss of Cr Pb and Ba as the insoluble chromates.
Cr elemental data*
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