I will participate in a practical course titled "Synthesis of Coordination Compounds." One of the materials needed to synthesize sodium trioxalatoferrate (III) is sulphuric acid with the concentration of 6 N (normality). Can someone explain the possible reasons the producers of H2SO4 with said concentration use normality instead of molarity for the unit of concentration? Can someone enlighten me on why equivalent concentration is sometimes used over molarity in lab products? Thanks.
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6$\begingroup$ Because some chemists are still mentally in 20th century? $\endgroup$– MithoronCommented Feb 3, 2023 at 12:40
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5$\begingroup$ For similar reasons why some people in scientific community still use imperial/"US" units. $\endgroup$– PoutnikCommented Feb 3, 2023 at 13:31
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2 Answers
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Normality and equivalents are obsolete concepts today. Mole and molar units are the only units used today. But these old units are still used when titrating a solution containing one (or several) unknown acids, like in waste waters or acidic rain for example, if nobody is able to do more measurements.
Let's consider an example. Suppose $250$ mL acidic waste water has required $36.2$ mL $0.1$ M (or $0.1$ N) $\ce{NaOH}$ to be neutralized. You can state that the sample of waste water contains $0.00362$ mol monoacid $\ce{HX}$ like $\ce{HCl}$, or $0.00181$ mol diacid $\ce{H2X}$ like $\ce{H2SO4}$, or some other value in case of a mixture. You cannot be more precise. Of course this "double" result cannot be printed in a report, and may be criticized by politicians, not aware of the problem.
But if you don't know the composition of acids in solution, you may state that the sample contains $0.00362$ equivalent of acid, once and for all. This a precise result.
And the acidic concentration (or normality) of your waste water can be calculated, even if you don't know which acid is in solution, and even if your solution is a mixture of different acids : $\ce{c = 0.00362 eq/0.25 L = 0.01448 eq/L = 0.01448 N}$. And this is an acceptable result to be published in a report or in front of non-chemists.
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1$\begingroup$ Normality is strongly deprecated, but not yet obsolete. Obsolete means not used anymore. $\endgroup$– PoutnikCommented Feb 4, 2023 at 6:43
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Normality is not obsolete at least in practice, it is used in various parts of the world including the USA/India, which means several billion people are familiar with this unit. Unfortunately, it has become a trend to bash historical ideas/notions and theories. Old does not mean that it is useless. Indeed, normality does cause a lot of conceptual difficults to beginning students but so does molarity, but once normality is understood along with its limitations, normality is/was not a bad concept after all. This unit is still used in industrial settings, pharma in some applications. All in all it is good to be familiar with it. Very soon we will start hearing that molarity is an old unit because we read that on the web and IUPAC website. Molarity is not going anywhere anytime soon!
The sole reason normality is useful is that all volumetric titration calculations are reduced to a very simple formula (CbVb=CtVt). Here b indicates the titrant in the burette and t indicates the substace being titrated... no thinking is required on the operator's part once the Standard Operating Procedure has been set for an analysis. Just plug and chug. This reduces the chances of mistakes or even forgetting the use of stoichiometric ratios. Other uses of normality are indicated by Maurice. You can titrate fruit juices without even knowing what acids are being titrated.
In your particular case, there is no specific reason to use normality but most likely the procedure was adapted from an older reference and they followed it verbatim. Normality use is being discouraged as concentration unit but it has not been wiped out from the face of the Earth.
