How to detect the composition of matter in an unknown solution?

Kaixin micro test

If you are Holmes in the 21st century, you will find a bottle of unlabeled (water) solution at the crime scene. How do you identify its ingredients?

For the identification of the composition of the unknown solution, I would like to say that the comrades who have worked in the laboratory have done this kind of work more or less, because most people have poor experimental habits, the most important thing is to often do not label!! !

Is there any good way to make up the composition of the substance in the unknown solution? Some people will say: "Go to the Beijing No. 10 line and have a security check!"... Why? Why not go to Line 1?

"Come, come on GC-MS!"

As soon as I came up with GC-MS, I can only say, good! You are a local tyrant! But can GC-MS really give a satisfactory answer? Besides, if the solution contains a little inorganic salt, it may Directly waste the instrument. If you have a high concentration of HF, then it is estimated that you can only sorrow ... ... money can not be so capricious!

Ok, no kidding, focus! Someone said seriously: This is an impossible task! But Xiaobian I think we have been analyzing chemistry for so many years, if you can't make it, it is still very hurtful!

I remember that I learned in high school chemistry. For unknown solutions, smelling is actually the most common method. Do you have such a brother in your lab: he is responsible for handling waste liquid, and he has encountered no waste liquid components on the waste bottles of some students. Just unscrew the cover and smell it... It’s also quite a fight! Pay tribute!

Return to the topic! First of all, we have to imagine that this bottle of solution may have both inorganic and organic matter. Then we have to figure out how to go through a series of testing methods to quantitatively determine the substances contained in the solution.

At present, there are many means for detecting substances, and most of them can only detect some substances, which have their limitations. Spectroscopic analysis theoretically enables the simultaneous detection of substances in the form of molecules in liquids. Spectral analysis techniques such as Fourier transform infrared spectroscopy, near infrared spectroscopy, ultraviolet-visible spectroscopy, photoacoustic spectroscopy, and the like.

With the development of laser technology, laser Raman spectroscopy as a fast, non-destructive, efficient and accurate detection method, suitable for liquid substance detection. Since the Raman shifts produced by different molecules are different, the matter can be known by the frequency shift of the spectrogram.

Anyway, for a bottle of solution that doesn't have any information, you have to have this awareness: it is basically impossible to measure with a single analysis method!

Option One:

How to simply judge whether the ingredients are organic or inorganic?

1. If the substance is not a polymer or a substance with a large molecular weight, generally the small molecule organic matter should melt at 300 °, and the EDX energy analysis spectrometer can be used to measure the contained elements;

2. Look under the UV lamp, if the color can be organic, and vice versa.

Liquid and liquid extraction is carried out. Extraction removes organic components from the water and provides most of the organic ingredients. The organic phase was then analyzed by GC-MS and the inorganic elements were analyzed using ICP-MS.

Option II:

First of all, judging from the appearance, looking at the color, with or without odor, with or without sediment, whether it is flammable, whether it is corrosive (for metal, glass), whether it is volatile.

Secondly, the method of distillation, condensation, and boiling is adopted to collect GC-MS for gas that does not condense, LC-MS for condensed liquid, XRD for remaining crystals, and ICP-MS after dissolution of crystal acid;

Finally, anyway, it is either organic matter or inorganic matter. Analyze the comparison. (Do not use up all the samples in each experiment. Try to use an oxygen-free environment when distilling. If it is heated and discolored, it will provide clues after cooling. ).

third solution:

We assume that it is an aqueous solution, colorless, odorless, and not particularly viscous. Does not react with the glass container. There is no soapy water foam in the mammoth bottle. A simple physical observation can sort out a lot of information before you start doing chemical analysis. Every step of chemical analysis is time-burning, so it is also a quick and inexpensive method. More importantly, it is necessary to consume the test solution as little as possible because it is a very limited resource. Therefore, non-destructive testing should be used first.

First of all, of course, the pH is measured.

Just a drop of water, and a low-cost test strip, you can see which property it is. Red is acid, blue is alkali, white is oxidant. Of course, if it turns black, it may be concentrated sulfuric acid. . .

Then measure the conductivity and the reduction potential.

This is also very important in the early days, because it can tell you how much electrolyte is inside, basically it is equal to telling you whether there is inorganic solute. The redox potential can help you estimate if there are oxides inside. This test is very simple and very fast.

Then use UV and fluorescence spectra. Although UV spectroscopy cannot be used for qualitative detection, it can be used to exclude many organic substances. For example, aromatic compounds and coordination compounds will be absorbed in specific segments. In addition, if you are lucky, there is fluorescence emission, you can directly find the compound.

The two methods give a small amount of information, and the choice is preferred because the two methods are low cost and non-destructive.

TGA/DSC Synchronous Thermal Analyzer

: This is a step towards the essence. Just put a drop of solution on the platinum tray and slowly heat it to check the weight loss curve and the heat absorption curve. This test can tell you the concentration of the solution, whether there are volatile solutes, and whether there are salts that will be decomposed by heat. The disadvantage is that the time may be long. The decomposed solid in this step can be used to measure the elemental composition by SEM-EDX.

ICP-OES

This test is expensive and time consuming, but basically detects most of the elements on the periodic table. After the elemental composition is detected, it is necessary to prepare a standard sample for quantitative determination, which takes a considerable amount of time.

GC-MS/HPLC/capillary electrophoresis

: When you get to this step, you should have a general understanding of the solute in the solution. The difficulty with these methods is that the experimental conditions are determined by the composition of the liquid to be tested. Regardless of GC or LC, with the correct Column, the correct flow rate is very important. If you don't know what to analyze, it is difficult to choose conditions. The conditions are not correct, the peaks of different substances may be stacked and cannot be analyzed.

There are also some methods that can be used as an auxiliary method. For example, COD and TOC are mainly used to determine the content of organic matter, but it is impossible to know which compounds. ATR-IR infrared spectroscopy can be used to detect functional groups, but it is also not possible to perform qualitative analysis alone. Cyclic voltammetry, to determine whether there is a redox pair, and to add iron ions, see if there is coloration and so on.

Looking at the three options, what kind of reliable do you think? Many people will say that the third type is obviously reliable, and the number of words has already won. So, do you understand the truth, there are many kinds of instruments that will be used? That is the cow! Then again, analytical chemistry lies in analysis, and with theoretical knowledge, it is the truth to dare to try with different analytical instruments. After reading this article, Xiaobian asks again: How to detect the composition of a bottle of unknown solution? Do you already have your own answer?