By means of nuclear magnetic resonance carbon spectroscopy and phosphorus spectroscopy structure identification. And combined with physical and chemical index analysis. A quality assessment method applicable to the water treatment agent PBTCA was proposed.
PBTCA has excellent resistance to calcium carbonate scale and zinc stabilization performance. It has the advantages of stable structure, low phosphorus content, low toxicity, light pollution to the environment and good compatibility with other agents. PBTCA has been widely used in industrial circulating cooling water systems.
At present, there are more than a dozen domestic manufacturers of PBTCA water treatment chemicals. Customers come from various industries. Since the government has not yet developed national standards or industry standards for PBTCA. This makes the product quality assessment without rules to follow.
Manufacturers and users generally use the HEDP analytical test method for PBTCA quality assessment. This is very unscientific. In this paper, we have compiled the research results of PBTCA quality assessment methods. In this paper, we have compiled the research results of PBTCA quality assessment methods and conducted experimental exploration for the development of PBTCA national standards.
1. Test
1.1 Test Instrument
Bruker AM-300 NMR, 722 spectrophotometers, constant temperature blast oven (40℃~300℃), pH meter, densitometer and general laboratory instruments.
1.2 Samples and Reagents
Eighteen domestic PBTCA samples had an effective concentration of 40%.
Four imported PBTCA samples had an effective concentration of 50%.
Dioxane, 85% H3PO4, ammonium persulfate, ammonium molybdate, ascorbic acid, antimony potassium tartrate and general laboratory reagents.
1.3 Test Method
1.3.1 Nuclear magnetic resonance carbon spectroscopy
The stock solution was transferred directly into a special sample tube for NMR determination. Then a capillary sealed with heavy water is added for field locking. The qualitative measurement was carried out under the condition of broadband decoupling and pulse interval of 3 seconds. Dioxane was used as a reference (δ0=67.8×10-6, external standard).
1.3.2 Nuclear magnetic resonance phosphorus spectrum
Direct determination with stock solution, using heavy water to lock field. Quantitative determination was performed under inverse gated decoupling with a pulse interval of 30 seconds. 85% phosphoric acid was used as a reference (δ0=0 external standard).
The PBTCA content (molar percentage in P atoms) is calculated using equation (1).
Where, AX is the peak area of PBTCA(δ=20.3×10-6), and AT is the sum of all peak integrated areas in the phosphorus spectrum.
1.3.3 Physical and chemical indexes
See reference for conventional physical and chemical indexes involved in the analysis and testing methods literature.
2. Results and Discussion
2.1 Nuclear magnetic resonance carbon spectroscopy
The properties of organic compounds are closely related to their structures. The good performance of PBTCA in inhibiting carbon scale and stabilizing zinc. Because of its special molecular structure (Figure 1).
Infrared spectroscopy and nuclear magnetic resonance (NMR) are the most common and effective means to study the structure and functional groups of organic compounds.
For the structural identification of PBTCA, NMR carbon spectroscopy is more often used because of the high number of carbon atoms in the molecule. The 13C NMR spectrum of PBTCA is shown in Figure 2.
The characteristic chemical shifts of each peak in the 13C spectrum are as follows:
～27×10-6,～31×10-6,～37×10-6 Methylene.
~50×10-6 bimodal peaks with P-adjacent quaternary carbon.
175×10-6,～179×10-6 Carboxyl group.
From the characteristic chemical shift data in the 13C NMR spectra of the samples. It is possible to confirm whether it is PBTCA from the molecular structure.
2.2 Nuclear magnetic resonance phosphorus spectroscopy
During the synthesis of PBTCA, due to the complexity and incompleteness of the organic synthesis reaction, unreacted raw materials and by-products are mixed in the product. Their scale inhibition and dispersion properties are far from PBTCA. Their content should be strictly controlled.
Using 31P NMR analysis, information on P atoms in the sample can be obtained. The purity of PBTCA in the sample can be indirectly known by calculating the proportion of the P-atom signal on PBTCA to the total P-atom signal, which is generally required to be greater than 80%.
The analytical results of the actual samples are shown in Table 1.
| Sample | 31P-MNR (molar percent), % | Total P(PO43-), % | Solid content, % | pH |
|---|---|---|---|---|
| 1 | 90% | 15.5 | 43.4 | <2 |
| 2 | 17% | 16.0 | 42.6 | <2 |
| 3 | 53% | 16.3 | 44.0 | <2 |
From Table 1, it can be found that: there is no difference between these three samples in terms of physical and chemical indexes alone.
However, the results of 31P-NMR spectra calculations are very different. The value of sample 1 is considerably higher than the other two samples. In fact, sample 1 was used much better than samples 2 and 3.
It can be seen that the analysis of conventional physicochemical indicators, such as total phosphorus, solid content, pH value, etc., does not give information about the structure of the compounds. Thus, it is not possible to distinguish PBTCA from other organophosphine impurities.
2.3 Physical and chemical indexes
The effective concentration of commercial PBTCA water treatment chemicals using the NMR method is still difficult to determine. It can not completely replace the physical and chemical index analysis. The following lists the experimental results of physicochemical index analysis of two specifications of qualified PBTCA (Table 2).
| Items | PBTCA 1 | PBTCA 2 |
|---|---|---|
| Concentration, % | 40 | 50 |
| Total phosphorus content, % | 14.0 Min. | 18.0 Min. |
| Solid content, % | 40 Min. | 50 Min. |
| H3PO3, % | 0.2 Max. | 0.2 Max. |
| 1% aqueous solution, % | 2.0 Max. | 1.5~1.8 |
3. Conclusions
Research shows that the quality assessment of PBTCA needs to combine structural identification methods such as NMR carbon and phosphorus spectra with physical and chemical indexes. Such as nuclear magnetic resonance carbon spectroscopy, phosphorus spectroscopy and physical and chemical index analysis combined.
This will be more effective in checking whether the product quality is good. Prevent inferior products from entering the industrial circulating cooling water system. In order to ensure the safe production of production units.