Analysis Of Association Species In Nitrate Solution Structure

The study of solution structure includes many aspects.It's very difficult to study the structure of a single pure aqueous solution,not to mention the specific binary,ternary and other diverse electrolyte solutions.As a common inorganic metal salt,nitrate is widely found in seawater,lake water and salt lake brine.It has an indispensable role in industry and agriculture.It is also one of the atmospheric pollutants with strong water absorption and can directly affect atmospheric deposition and atmospheric precipitation cause serious damage to the environment.Ion association is common in electrolyte solutions.There are four types of ions in solution:free ion(FIP),shared ion pair(SIP),contact ion pair(CIP)and complex ion pair(Complex),the substance is also present in these four forms in the nitrate solution.Raman spectroscopy combined with Gaussian multimodal fitting to study the association species in nitrate solution can quantitatively analyze the association species.Theoretical configuration calculation based on density functional theory,combined with Raman spectroscopy experiments on microscopic analysis of nitrate solution.The clusters were explored to provide valuable experimental data and theoretical models for the pollution control and scientific research of nitrates.The main research contents and conclusions are as follows:1.Analysis of the effect on humidity on the association species of nitrate solution.Using the self-designed relative humidity control device and sample cell,connected to the Raman spectrometer,the localized part of the micro-nose of the small droplets of the nitrate can be obtained from the maximum humidity to the appearance of the crystal.The type fit shows that the in-plane symmetric stretching vibration peaks of the nitrate(calcium,sodium,magnesium)droplets are blue-shifted with the decrease of humidity,except of the small potassium nitrate droplets.The content distribution and variation of four ion pairs in nitrate(calcium,sodium and magnesium)droplets under different humidity were obtained by Gaussian multimodal fitting method.With the decrease of humidity,the FIP and SIP in the three nitrate droplets decreased,CIP and Complex increased,but the CIP in the small droplets of calcium nitrate increased first and then decreased,and the CIP of the small droplets of magnesium nitrateincreased.The small droplets of sodium nitrate are small,and it is presumed that the CIP is not easily formed in the calcium salt and the magnesium salt.2.Analysis of the effect of series brine on the association species of sodium nitrate and magnesium nitrate solution.The Raman spectra of Salt-water molar ratio(WSR)under the same conditions were collected by capillary method.The relative content distribution of four ion pairs at different brine ratios was obtained by Gaussian multi-peak fitting method.Combined with the humidity experiment of nitrate droplets,the salt water molar ratio of the nitrate solution was changed from 4 to 320,supersaturated and the species before the crystallization.The trend of the four ion pairs in the sodium nitrate solution can be divided into three concentration stages: 80?WSR?320,30?WSR<80,6?WSR<30,and the concentration in the magnesium nitrate solution is 46?WSR?166,26?WSR<46,12?WSR<26,the concentration required to convert the shared ion pair into the contact ion pair in the magnesium nitrate solution is WSR=46,and the sodium nitrate is WSR=80.It is indicated that there are many free ion pairs and shared ion pairs in the magnesium nitrate solution,and the concentration required to convert into contact ion pairs is large.At the same water-salt ratio,the wave number of the symmetric stretching vibration peak of nitrate is from small to large,sodium nitrate < magnesium nitrate < calcium nitrate.3.According to the DFT configuration optimization calculation,combined with the Raman spectroscopy experiment,the structural information of the association species that may exist in the magnesium nitrate and sodium nitrate solution is obtained.It is found that the SIP and the CIP appearing in the magnesium nitrate solution are anion-cation-anion triple ion clusters,the complex ion pair is a shared multi-ion cluster,and the SIP and the CIP appearing in the sodium nitrate solution are anion-cation-anion triple ion clusters,and the complex ion pair may be a contact-type multi-ion cluster composed of three nitrate ions and one sodium ion.