In my experiment, I am trying to quantify the nitrate absorption by duckweed. I need a titration method to measure the left over nitrate levels in the solution. However, this is to be done in a school environment which limits specific chemicals I have access to. I thought of using sodium thiosulphate in acidic conditions to reduce nitrate into nitrogen oxide. But I'm not too sure about what indicator I can use. Appreciate your help guys. Thanks in advance.
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$\begingroup$ It is usually done by a distillation method after reducing nitrate to ammonia by Dewarda's alloy (Cu,Zn,Al) at alkaline pH. $\endgroup$– PoutnikCommented Jun 18 at 12:05
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1$\begingroup$ Poutnik, this is rather old method. Most methods use colorimetric reagents and a spectrophotometer. $\endgroup$– ACRCommented Jun 18 at 14:39
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$\begingroup$ What is your working concentration(s) of nitrate? $\endgroup$– Mathew MahindaratneCommented Jun 18 at 21:47
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$\begingroup$ Is this help you?: chemistry.stackexchange.com/a/67369/61578 $\endgroup$– Mathew MahindaratneCommented Jun 18 at 22:48
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1$\begingroup$ @ACR You are right. Another option is based on sample evaporation, nitration of salicylic acid in H2SO4 and measuring of nitrosalicylate at alkalic pH at 410 nm. $\endgroup$– PoutnikCommented Jun 19 at 2:42
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I thought of using sodium thiosulphate in acidic conditions to reduce nitrate into nitrogen oxide..
I think you mean nitric oxide in this sentence. That is the qualitative test for nitrate ion test perform in undergraduate general chemistry labs (recall Brown Ring test). The Brown Ring test (or nitrate test) is a chemical procedure for determining the presence of nitrate ions in a sample. It’s a standard qualitative test for confirming the presence of nitrate ions in any solution that results in the formation of a brown-colored ring in the interface of two layers generated by the addition of conc. sulfuric acid to the mixture of ferrous sulphate and nitrate containing solution:
According to Wikipedia, the reactions towards the brown ring is given as follows:
$$\ce{2HNO3 + 3H2SO4 + 6FeSO4 -> 3Fe2(SO4)3 + 2NO + 4H2O} \tag1$$ $$\ce{[Fe(H2O)6]SO4 + NO -> [Fe(H2O)5(NO)]SO4 + H2O} \tag2$$
The cation of the brown color product complex, $\ce{[Fe(H2O)5(NO)]SO4}$ is recently found to be $\ce{[Fe^{III}(H2O)5(NO^-)]^2+}$, and not $\ce{[Fe^{I}(H2O)5(NO^+)]^2+}$ as usually quoted in undergraduate text books (Ref.1). Regardless, the point I wanted to make here is this reaction can be used to quantitatively determine the nitrate concentration of a given solution as the authors of Ref.1 have done to find the kinetics of this reaction:
Note that the absorption spectral changes at $\pu{451 nm}$ have been recorded for the reaction of $\pu{3 × 10^{-3} M}$ $\ce{[Fe^{II}(H2O)6]^2+}$ with $\ce{NO}$. Experimental conditions used here are $\pu{0.2 M}$ acetate buffer $(\mathrm{pH} = 5.0)$ at $\pu{25 ^\circ C}$. The UV/vis spectrum of a degassed solution of $\ce{[Fe^{II}(H2O)6]^2+}$ after its exposure to excess $\ce{NO}$ displays a significant increase in absorbance giving band maxima at $336, 451,$ and $\pu{585 nm}$ with $ε = 440, 265,$ and $\pu{85 M-1 cm-1}$ respectively. It is also noteworthy that the UV/vis spectrum of $\ce{[Fe^{II}(H2O)6]^2+}$ measured in $\pu{0.2 M}$ acetate buffer at $\mathrm{pH} = 5.0$ exhibits no absorption band between $250$ and $\pu{800 nm}$ region, and therefore can use as the blank (zero concentration solution). OP should create a calibration curve with known nitrate solutions before measuring unknown solutions.
References:
- Alicja Wanat, Thorsten Schneppensieper, Grażyna Stochel, Rudi van Eldik, Eckhard Bill, and Karl Wieghardt, "Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of Nitric Oxide to Aquated Iron(II). An Undergraduate Text Book Reaction Revisited," Inorg. Chem. 2002, 41(1), 4–10 (https://doi.org/10.1021/ic010628q).
answered Jun 19 at 2:03
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$\begingroup$ Mathew, this is not a quantitative method for nitrate determination in water or soil samples. The OP's idea of using thiosulfate is not conceptually right. Thiosulfate will not react with nitrate ion. I have a reference but the OP needs to show some effort with hints in the comments. $\endgroup$– ACRCommented Jun 19 at 2:47
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$\begingroup$ @ACR: I understand it is a qualitative test. But sited reference simply use it to determine spectrophotometrically the time dependent concentrations. I thought you can use it with necessary changes to find sought leftover concentrations since it seems highschool project (may be present in a junior science fair). $\endgroup$ Commented Jun 19 at 8:22
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$\begingroup$ @MathewMahindaratne Appreciate your time into this answer. However, my school does not have the means of observing light absorption. We don't have any specialised equipment thats why I was looking for a titration method although outdated. Thanks for your answer though. $\endgroup$ Commented Jun 23 at 3:59