Primary Standard Solutions

Standard solutions, that is, solutions with accurately known concentrations, play a leading role in volumetric analysis. The concentrations of these solutions can be determined in two ways. If the solution is prepared by dissolving a weighed amount of a solute in a measured final volume, the concentration is directly calculated. These solutions are called primary standard solutions. However, when a solid is not stable or does not have a known purity, its weight can not be used as an indication of the moles of the solute. The concentration of a solution made with these solids must be determined indirectly by a quantitative reaction with another reagent? a primary standard or a primary standard solution. When this indirect process is required, the new solution is called a secondary standard solution. It should be apparent that the accuracy and precision of the secondary standard solution depends on the accuracy and precision of the primary standard solution. Thus, preparing a primary standard solution requires more than proper quantitative techniques; it requires consideration of precision. Before a solution can be prepared, three questions must be addressed:
1. What concentration range satisfies the requirements of the experiment?
2. How precisely must the concentration of the solution be known?
3. How much solution is required?
In most experiments knowing exactly what concentration you have for a standard solution is more important than having a specific value for that concentration. For instance, if a procedure requires a 1M standard solution, any concentration that would round to this value would be acceptable. That is any solution that had a concentration in the range from 0.950M to 1.499M would satisfy the 1M requirement. In addition, if the exact solution concentration were known to the significant figures given here, then its precision could be assumed to be 0.1%.
However, it is often difficult to achieve this level of precision, particularly for dilute solutions. Even with carefully manufactured volumetric glassware and precision analytical balances, uncertainty in the measurements will affect the precision with which the concentration of the final solution is known. For example, if the sensitivity, (absolute error) of an analytical balance is ±0.2 mg, any sample weighed on this balance would have this absolute level of uncertainty. The relative uncertainty, however, would change with the sample weight. That is, if the sample weighed 10mg, the uncertainty in the weight would be 2.0% (0.2mg/10mg), but if the sample weighed 1.000g, the uncertainty would be 0.02% (0.2mg/1000mg).
Similarly, each piece of volumetric glassware has a tolerance specified by the manufacturer. The National Bureau of Standards has established tolerance standards for the manufacture of class A volumetric ware, which is used for research and accurate analytical work. The table below shows the tolerances (or absolute uncertainty) for equipment frequently used in general chemistry courses.
Capacity in mL NBS Tolerances for Class A Volumetric Glassware (mL)
volumetric flasks transfer pipets burets
1 – 0.006 –
5 – 0.01 –
10 0.02 0.02 0.02
25 0.03 0.03 0.03
50 0.04 0.05 0.05
100 0.08 – –
200 0.10 – –
250 0.12 – –
Clearly, the larger the volume and the amount of material used, the smaller the uncertainty in knowing the concentration of the final solution. When designing a solution in the experiment, you must always remember to consider the contribution of errors from using various glassware and equipment. However, one further issue remains to be considered in the experimental design. At the same time as scientists consider the accuracy and precision of their measurements, proper practice mandates that waste is minimized. This is essential for hazardous materials. Thus, you must always determine in advance how to make the minimum amount of the solution of the required concentration and precision.
Hyperlink Resources
Standard solutions preparation tutorial
– Tutorial on how to prepare standard solutions
HTML Tutor
– A very simple and brief tutorial on how to use HTML tags for text formatting.
UCLA Student Code of Conduct Including the Definitions of Plagiarism
– The Student Guide to Academic Integrity briefly defines plagiarism and describes some activities that are included in this definition.
Assignment Submission to TurnItIn on MyUCLA – Plagiarism Prevention
– This is a short tutorial on how to submit your text to TurnItIn on MyUCLA (courtesy of the UCLA Engineering Department).
Guidance for Writing Your Text
Besides considering the information provided under “Source Materials”, you should also consider the concepts used in the following question.
If a precision of +/-0.5% is required for a concentration of the solution that is 0.015 M, how many significant figures must be reported?
The answer to this question should help you when designing the required solution for the assignment (refer to next section). This question is similar to the study question #3 assigned in your pipet calibation assignment pre-lab report.
Writing Prompt
Using an analytical balance that has a sensitivity of 0.3mg and any of the volumetric flasks in the table above, design a procedure to prepare at least 50 mL of a 1.5×10-2M solution of sodium chloride with a precision of +/-0.5%. You may assume that the sodium chloride is both pure and dry.
When you have designed the procedure for preparing the solution outlined above and have answered the guiding questions outlined under “Source Materials”, write a brief essay of the desired length describing what and why you chose your specific sample weight and volumetric flask. When writing the essay, you should think through all the information provided under “Source Materials”. Be sure to also include the range of weights of NaCl that will give an acceptable concentration for the volumetric flask you are using. If your final solution is more than 50 mL, explain why. The content of the essay should be focused on the concepts and explanations rather than simply outlining a detail experimental procedure.
Your text should also include both a topic sentence and a summary sentence. A topic sentence should introduce the concepts and ideas relevant to the rest of the essay. A summary sentence should summarize the ideas and concepts discussed in the essay. You should keep in mind that you are writing an integrated essay with logical flow rather than simply answering a list of questions in a certain particular order.
Note: Formatting from word processors is NOT preserved when you copy and paste into the text box below. Refer to the HTML Tutor in the “Source Material Resources” section to learn how to format your essay with subscripts and superscripts, as well as line returns and paragraphs.
ALWAYS check your formatting by using the “Preview Text” button below. This will show you exactly how your text will appear to reviewers.
If you plan to use WORD to write your text, please keep in mind that WORD file contains hidden symbols and characters that are necessary for WORD file to function but it will increase the word count of your text. Quite often, this will cause the word count of the text to exceed the limit set by the assignment. The online writing program will not accept your text if the word count either exceed or below the limit set for the assignment.
Always SAVE the text within the online CPR writing program interface. Saving the text within the CPR program does not automatically submit the text. You must submit the final version of your text to the CPR program within the deadline set for the assignment.
IMPORTANT: Text Submission to TurnItIn on MyUCLA ? Plagiarism Prevention

UCLA Student Code of Conduct

Instructions for Assignment Submission to TurnItIn on MyUCLA

After you submit the final version of your text to the CPR online writing assignment program, you will also need to submit your text to TurnItIn on MyUCLA. Click on the link “Instructions for Assignment Submission to TurnItIn on MyUCLA” for instructions.

You MUST submit the final version of your text to BOTH the CPR online writing assignment program AND to TurnItIn on MyUCLA.

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Required Text Entry Length: 250 to 400 words
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Directions
Using an analytical balance that has a sensitivity of 0.3mg and any of the volumetric flasks in the table above, design a procedure to prepare at least 50 mL of a 1.5×10-2M solution of sodium chloride with a precision of +/-0.5%. You may assume that the sodium chloride is both pure and dry.
When you have designed the procedure for preparing the solution outlined above and have answered the guiding questions outlined under “Source Materials”, write a brief essay of the desired length describing what and why you chose your specific sample weight and volumetric flask. When writing the essay, you should think through all the information provided under “Source Materials”. Be sure to also include the range of weights of NaCl that will give an acceptable concentration for the volumetric flask you are using. If your final solution is more than 50 mL, explain why. The content of the essay should be focused on the concepts and explanations rather than simply outlining a detail experimental procedure.
Your text should also include both a topic sentence and a summary sentence. A topic sentence should introduce the concepts and ideas relevant to the rest of the essay. A summary sentence should summarize the ideas and concepts discussed in the essay. You should keep in mind that you are writing an integrated essay with logical flow rather than simply answering a list of questions in a certain particular order.
Note: Formatting from word processors is NOT preserved when you copy and paste into the text box below. Refer to the HTML Tutor in the “Source Material Resources” section to learn how to format your essay with subscripts and superscripts, as well as line returns and paragraphs.
ALWAYS check your formatting by using the “Preview Text” button below. This will show you exactly how your text will appear to reviewers.
If you plan to use WORD to write your text, please keep in mind that WORD file contains hidden symbols and characters that are necessary for WORD file to function but it will increase the word count of your text. Quite often, this will cause the word count of the text to exceed the limit set by the assignment. The online writing program will not accept your text if the word count either exceed or below the limit set for the assignment.
Always SAVE the text within the online CPR writing program interface. Saving the text within the CPR program does not automatically submit the text. You must submit the final version of your text to the CPR program within the deadline set for the assignment.
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Primary Standard Solutions