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## Hot answers tagged analytical-chemistry

37

Sodium flame is yellow, but all the light is due to two lines in the yellow region. If you look a sodium flame through a purple glass (cobalt glass), the yellow color is absorbed, and you do not see the sodium flame any more. The flame looks dark and colorless through a cobalt glass. On the contrary iron flame looks yellow, but it is made of a huge number of ...

27

Definition of refractive index: $n=\frac {c}{v}$ If n>2 then this means that the velocity of light in the material is less than half than in vacuum so there must be a strong interaction with the material. One of the highest refractory index in liquids is 1.79(20°C) possessed by a solution of sulfur in methylene iodide($CH_2I_2$). Liquids are less dense ...

27

A property of the harmonic oscillator is that the oscillation frequency, $\omega$, is dependent not only on $k$ (the spring constant) but also on the mass $m$ of the object: $$\omega = \sqrt{\frac{k}{m}}$$ We can crudely model a chemical bond as a two-body harmonic oscillator, which largely obeys the same rule, except that the mass must be replaced with ...

22

Yes, you can use a common stove to test for cations. But a stove is designed to minimize the risk of incomplete combustion (which could lead to production of carbon monoxide), hence its flame always appears as an intense blue flame. Such color contamination could be problematic when testing for cations. In contrast, the combustion (and the color of the flame)...

18

The analytical technique is called inductively coupled plasma optical emission spectroscopy. It was invented by a chemist V. A. Fassel, although it is pure physics. The specimen is digested in an acid, and a fine spray is created. That "spray" is heated to a temperature, which is higher than the surface of the Sun. Most of the elements of the periodic table ...

17

Here are the two half reactions: \begin{align} \ce{[Ag(NH3)2]+ + e- &-> Ag^0 + 2NH3} \\ \ce{RCHO + 3OH- &-> RCO2- + 2H2O + 2e-} \end{align} which together yield the overall reaction $$\ce{2[Ag(NH3)2]+ + RCHO + 3OH- -> 2Ag^0 + RCO2- + 4NH3 + 2H2O}$$ Here is a diagram of the reaction mechanism. The carbonyl group is oxidized in the ...

17

You have three types of errors: The errors in the accuracy of your measurement. Assuming you are reading the water levels consistently (at eye level at the bottom of the meniscus), these should be random. To add random, uncorrelated errors, the standard method is to sum their squares and take the square root. So, for the 0.1 mL average error for the 10 mL ...

16

No, knowing the mass ratios is not sufficient by itself. In the absence of additional information (for example, molar mass) that would only be enough information to determine the empirical formula, which is the formula that contains the smallest integral ratios of atoms. (E.g., the empirical formula of $\ce{C2H6}$ would be $\ce{CH3}$.) There are (...

15

According to Nomenclature for Chromatography (IUPAC Recommendations 1993) [1, pp. 843, 845] (also listed in IUPAC Gold Book), retardation factor is denoted as follows: Column chromatography: $R$ (capital $R$ in italics): 3.7.13 Retardation Factor ($R$) The fraction of the sample component in the mobile phase at equilibrium; it is related to the ...

15

Short answer: The success of the project benefitted from many contributors. According to BIPM and its Avogadro project, there are two spheres (reference). The page equally refers to a publiction about the 2011 $\ce{^{28}Si}$ atom count (paywall) and the report in 2017 (here, open access). From the later paper, you may identify the origin of the two ...

14

Building on my comment, marking the distance that the solvent traveled allows us to calculate the retention factor $R_f$ (or apparently retardation factor, according to IUPAC meddlers who need to rename things with perfectly good names that everyone else uses). The retardation factor is the ratio of the distance traveled by the spot to the distance traveled ...

14

Assuming you mean NaCl - the common "salt" (chemists call lots of things salt!). If you extract the salt by physical means, it's a physics question. Assuming that the "dirt" is not (or poorly) soluble in water, I would simply dissolve the salt in water, filter the liquid, then recrystallize (by evaporation of the liquid) and weigh the resulting crystals. ...

14

Fehling's solution consists of two parts, Fehling's A and Fehling's B. Fehling's A consists of aqueous copper(II) sulfate, while Fehling's B consists of aqueous sodium potassium tartrate ($\ce{NaK(C4H4O6)}$). Benedict's reagent consists of sodium carbonate, sodium citrate ($\ce{Na2(C6H6O7)}$), and copper(II) sulfate dissolved in water. It's pretty clear now, ...

14

To complement @blu_potatos's answer, there are some other contenders having refractive index greater than 2 Arsenic di/trisulfide and sulfur and/or selenium and/or mercuric sulfide in arsenic tribromide solvent having refractive index ranging from 2.0 to 2.07 (arsenic tribromide seem to attack the lead-glass prisms of refractometers, corroding metal, ...

13

The phenomenon that is witnessed during a flame test is an "atomic emission". This statement may seem inappropriate, since it is a solution of metal ions (and not atoms) that is tested. The reason for calling it atomic emission lies in the process occurring in the flame. One of the steps of the process is an atomization step. That is, the flame converts the ...

13

You asked to keep it simple, which is why I’m skipping explaining some details. Check the bottom of the answer to see the parts with superscript numbers explained. The signal at $7\,\mathrm{ppm}$ is not a quartet. It would only be a (typical; there are exceptions, however not in typical organic chemistry) quartet if both of the following two conditions are ...

13

This is a so-called "Pearson's square" or "Box method" of balancing ratios, originally used extensively in dairy industry (at least since 1920s judging from Google Books search). Earlier the similar approach has been used in sugar industry by using "Cobenz diagrams" aka spider diagrams. Widely popularized in Soviet books for analytical chemistry at least ...

13

It might contain $\ce{NaNO3}$ as well as $\ce{KNO3}$. This paper by Foong et. al. cites some thermal measurements, such as melting point and heat capacity, that you could make with a thermometer and heater. Note that there are inflections in the heat capacity curve as the different components ($\ce{NaNO3}$, $\ce{KNO3}$ and the eutectic of the two) melt. If ...

13

Referring to your comment on Buttonwood's answer: Unfortunately, in this case I only have 1D HNMR and IR available to analyse the product If this is really the case, then you are essentially out of luck. It is not possible to extract much information from a series of overlapping multiplets, which may well be coupled to each other (these strong coupling ...

12

Modern day scientists rely on technology and advanced nuclear physics. But in absence of these advances, how were ancient scientists able to discover them? In many cases, they weren't - the ancient... maybe we can't call them scientists. Let's call them "natural philosophers." The ancient natural philosophers (up until about 1700) only knew about a handful ...

12

Structure elucidation via degradative studies and decomposition chemistry has sadly all but passed from the chemists' toolbox, but this was the method once used to determine molecular structures up until about half a century ago. One of my very favourite examples that I use to teach structure determination by NMR is the story of strychnine. Strychnine (below)...

11

Take some potato starch dissolved in water (you can easily get it from the water when you boil potatoes), and put a tablet into the water. If the water becomes bluish black, there is iodine in the tablet. Yes, there are testing labs that will determine the amount of iodine in the tablets which will cost somewhere around \$100 to \$200.

11

When phosphoric acid is added to an aqueous solution of iron (III), the ligands in the yellow complexes $\ce{[Fe(OH)(H2O)5]^2+}$ and $\ce{[Fe(OH)2(H2O)4]+}$ ( $\ce{[Fe(H2O)6]^3+}$ are weakly colored but fairly acidic), are successively substituted by $\ce{H2PO4^-}$ and $\ce{HPO4^2-}$. This yields colorless hydrogenphosphatoiron(III) complexes, like $\ce{[Fe(... 11 Virtually every analytical chemistry textbook's chapter on anion determination mentions that Devarda's alloy is very brittle and can be easily pulverized in a mortar, unlike$\ce{Al}$and$\ce{Zn}$metals. This is quite handy property as for qualitative analysis of nitrates in a heterogeneous media the reducing phase should be ground to fine powder in order ... 11 Most commonly they work by pulling air through a UV absorption cell and simply measuring the absorption at 254 nm. Often there is a pre-concentration step involved. This usually involves a gold-plated region prior to the UV cell. At room temperature mercury adsorbs to the gold, then after some period of time the gold-plated region is flash heated to ... 11 There are many ways to detect mercury, and you can find a more detailed analysis of these methods, along with examples of these detectors and their limitations, here. Gold film sensors Gold Film Sensors were the first reliable forms of mercury detectors due to gold’s affinity for elemental mercury [...] When a mercury rich air sample passes over a thin ... 11 With unsubstituted phenol ($\ce{PhOH}\$) dark-brown precipitation is formed: $$\ce{(NH4)2[Ce(NO3)6] + 2 PhOH -> [Ce(NO3)4(PhOH)2] + 2 NH4NO3}$$ Other phenols should produce wine-red solution as expected [1, p. 478]. Also, this and other phenol tests are discussed in [2]. Pavia, D. L. A small-scale approach to organic laboratory techniques, 3rd Ed.; ...

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