Access over 20 million homework & study documents

search

Chemistry

Content type
User Generated
Rating
Showing Page:
1/3
Laboratory Standards
With most of the instruments used in the lab, standards are necessary to translate the values provided
by the instrument (e.g. absorbance) into usable information (e.g. concentration.) Analytic procedures
and instruments do not measure the amount of an Analyte directly but instead measure some property,
quality or attribute of the analyte. The spectrophotometers measure the light absorbed by analyte1. pH
meters measure millivolts. In order for these instrument readings to related to concentration, solutions
of known concentration (standards) must be used.
An analogy will help to illustrate the above idea. A person viewing a photograph of an unfamiliar object
has no way of knowing its size unless a familiar object is placed nearby. However, if a ruler is place by
the object, the observer can determine the objects size Standards perform the same function as the
ruler. Standards are reference points by which the actual concentration of an analyte can be
determined..
Standards serve another function aside from relating one parameter to another. Standards indicate
how well a procedure or instrument is working. This is accomplished by graphing the values given by the
instrument against the concentration of the standards. The line obtained is then judged with respect to
optimum linearity, slope and y-intercept. The result is an equation with an r2 value to show how well
the data fits the equation. An r2 of 1 indicates a perfect fit. An r2 of .70 would indicate a poor fit.
Deciding the concentration of the standards to be used is an important part of an analytic procedure.
Two is the minimum number of standards needed for an analysis. Using three or more standards
improves accuracy and establishes the nature of the standard curve i.e. straight of curvilinear. Usually,
three standards and a blank are sufficient for most procedures. The values of the standards should
bracket the values expected from your samples.
Example: If you expect your sample solutions to have a Ca concentration between 110 and 140 ppm,
then your standards should be 100ppm and 150 ppm Ca.
Samples that give readings greater than the standard values need to be diluted.
Equal in importance to the concentration of a standard, is its matrix2. The matrix of a standard should
match the matrix of the sample being analyzed as close as possible. For example, if samples are
extracted with .5 N HNO3 then the standards should be made in .5 N HNO3. The only thing that should
be different about samples and standards should be the concentration of the analyte.
Preparation of Standards Working standards 3 are usually made by diluting a solution of higher
concentration with the sample matrix. This higher concentration solution is called a primary standard.

Sign up to view the full document!

lock_open Sign Up
Showing Page:
2/3
Primary standards can be purchased from scientific supply houses or prepared in the laboratory. A
primary standard should be prepared from oven dry reagent grade
Graduate Laboratory Fresno State, College of Agricultural Sciences and Technology
File name: Laboratory Standards Edited:7-5-13
chemicals. Use the following formula to calculate the amount of reagent needed to make solution.
Primary Standard Calculations Grams of reagent needed = (MW of reagent / MW of analyte x moles of
analyte in reagent) x grams of analyte needed.
Example: 1 gram of Na is needed from Na2SO4
MW Na2SO4 = 142 g / mole, MW Na = 23, moles of Na per mole of Na2SO44 = 2
(142g Na2SO4 / 23g Na x 2) x 1g = 3.09g Na2SO4
Example: Preparation of 1 liter of a primary standard of 1000 ppm* Sodium.
Calculations: (1000 mg Na / L) x 1L x (58.5 g NaCl / 23g Na) ** = 2.543g NaCl * ppm = mg/L ** M.W.
NaCl = 58.5g / mole , M.W. Na = 23g / mole
1. Oven dry NaCl for at least 2 hours at 105C and place in desiccator to cool.
2. Weigh 2.543 g of NaCl and pour into 1L Volumetric. Fill flask half way with DI water and mix until NaCl
is dissolved. Add DI water to 1 L mark. Seal top with Parafilm and invert 30 times to mix solution.
3. Pour solution into labeled bottle. (Solutions should not be stored in volumetric flasks.)
Secondary Standards Standards prepared by diluting the primary standard are called secondary
standards and are usually your working standards. A small portion or aliquot of the primary standard is
diluted with the sample matrix to make secondary standards. Use the following formula to determine
how much primary standard is needed to make a secondary standard.
C1V1= C2V2
C1 = Concentration of primary standard, V1= Volume of Primary standard
C2= Concentration secondary standard, V2= Volume of Secondary standard
Usually you need to calculate the volume of primary standard needed because you know the
concentration of the primary and secondary standards and the volume of the primary standard. To
solve for the volume of primary standard needed, use the following equation.
V1 = C2V2/C1
Graduate Laboratory Fresno State, College of Agricultural Sciences and Technology

Sign up to view the full document!

lock_open Sign Up
Showing Page:
3/3

Sign up to view the full document!

lock_open Sign Up
Unformatted Attachment Preview
Laboratory Standards With most of the instruments used in the lab, standards are necessary to translate the values provided by the instrument (e.g. absorbance) into usable information (e.g. concentration.) Analytic procedures and instruments do not measure the amount of an Analyte directly but instead measure some property, quality or attribute of the analyte. The spectrophotometers measure the light absorbed by analyte1. pH meters measure millivolts. In order for these instrument readings to related to concentration, solutions of known concentration (standards) must be used. An analogy will help to illustrate the above idea. A person viewing a photograph of an unfamiliar object has no way of knowing its size unless a familiar object is placed nearby. However, if a ruler is place by the object, the observer can determine the objects size Standards perform the same function as the ruler. ...
Purchase document to see full attachment
User generated content is uploaded by users for the purposes of learning and should be used following Studypool's honor code & terms of service.

Anonymous
Awesome! Perfect study aid.

Studypool
4.7
Trustpilot
4.5
Sitejabber
4.4