Boron forms refractory oxides, carbides, nitrides which can not be broken into atoms in a small residence time in Flame. So the analysis by AAS should be done in high residence time Graphite Furnace. But boron has a tendency to form refractory carbides on reaction with the surface of the graphite tube which decreases the life of the tube, decreases sensitivity of the analysis and will increase the memory effect. So the surface of the tube should be protected from reaction with boron of the analyte. This can be effected by addition of modifiers. Modifiers can act in two different ways. Either it will form a coating over graphite or it will react with boron before boron reacts with graphite furnace. We have used both the types of modifiers simultaneously to get better reproducibility. The modifiers used are Zirconium Nitrate and Nickel Nitrate.
a) Zirconium Nitrate forms very stable carbide on the surface of the graphite tube and reduces the B – C interaction [Zr – C bond strength is 561 kJ mole-1, whereas B – C bond strength is 448 kJ mole-1].
b) Boron is easily vaporized as Nickel Boride thus decreasing the tendency of Boron to react with Graphite tube surface.
Recovery Study with CRM Solutions was done and the results are tabulated below:
| Sl. No. | Conc. In StandardSolution used (mg/L) | Obtained conc. Afteranalysis in AAS/GF (mg/L) | % Recovery |
| 1. | 10.0 | 9.95 ± 0.05 | 99.5 |
| 2. | 5.0 | 4.97 ± 0.04 | 99.4 |
| 3. | 2.0 | 1.95 ± 0.05 | 97.5 |
| 4. | 1.0 | 0.95 ± 0.05 | 95.0 |
As the recovery is very good, the method was used for analysis of water samples and the values were compared with the values obtained by APHA 21st Edition method of Spectrophotometric method for determination of boron by Carminic acid – Conc. H2SO4. The results were found to be in conformity (Table below).
| Sl. No. | Conc. Obtained by Carminic Acid – Spectrophotometry (mg/L) | Conc. Obtained AAS / GF using modifiers (mg/L) | % Recovery |
| 1. | 3.42 ± 0.05 | 3.35 ± 0.05 | 97.95 |
| 2. | 2.70 ± 0.03 | 2.65 ± 0.06 | 98.15 |
| 3. | 1.92 ± 0.03 | 1.90 ± 0.04 | 98.96 |
Introduction of the above method in the Analysis of Boron in Ferroalloys
The method was introduced in case of analysis of Ferro-alloys where boron is a tramp element and its presence is detrimental in some cases. But the problems in analysis boron in Ferro alloys are:
-
Dissolution of boron in Ferro alloys
-
Matrix effect due to presence of large quantity of other ions in the solution which have spectral interference.
As Boron is oxophil, it will dissolve in acids in a strongly oxidizing condition or bases in strong oxidizing condition (Na2O2). In case of Ferromanganese and Silicomanganese alloys, samples were dissolve in conc. HCl – conc. HNO3 followed by H2SO4 fuming. In case of High Carbon Ferrochromium, samples were fused with Na2O2 in a Nickel crucible and the fused mass was dissolved in HCl. In both the cases, the solutions were filtered to remove solid siliceous impurities.
Matrix effect was minimized by Standard Addition Method. The quantitative determination of Boron in ore samples has been done using standard addition method by AAS with GTA. The simple calibration procedure may result in serious errors if the boron standards contain only pure boron salt dissolved in water. The boron signal obtained from the sample may be enhanced or reduced by the presence of other components. This so-called matrix effect would be eliminated if the standards had the same bulk composition as the unknown sample solution. However, in many cases matrix matching is virtually impossible.
The technique of standard additions is widely applied to spectroscopic and electrochemical methods in order to overcome matrix effects.
Sample preparation: 0.5g sample (100 mesh size) was taken in a Teflon beaker. 20 ml aquaregia was added, mixed well and digestion was carried out till the volume reduced to 2 – 3 ml. Then 20 ml conc. H2SO4was added, again digestion was carried out till white fumes evolved. The resultant solution was cooled, filtered and volume was made upto 250 ml.
Quantification: Equal volume of the samples (25 ml) was taken and different known amounts of boron solution (1, 2, 3 & 4 ml of 5 ppm boron solution) were added to each sample solution except one. Then 0.15g Nickel nitrate and 0.15g of Zirconyl nitrate were added to each solution as modifier, mixed well and the final volume were made upto 50 ml.
The absorbance was measured for every solution using AAS with graphite tube atomizer (GTA) and plotted on a graph with the dependent variable on the y-axis and the amount of boron added on the x-axis. Extrapolation of the straight line thus obtained to the point where the x-axis is cut (i.e. when y = 0) provides a measure of boron in the sample.
Two synthetic standards were prepared in the laboratory and were analyzed (Data given below). Values were in conformity.
After the procurement of a CRM standard (SL 01 – 04, certified value 51.0 ppm) the standard was analyzed and the value was found to be almost the same as declared value.
After the introduction of the standard addition method, the method was found to be accurate and was utilized for analysis of a large number of samples submitted (more than 100). Some typical analyses are given below. It shows that the method is accurate even in extremely low concentration also. Moreover two of the samples analyzed here were utilized for calibrating the ICP – OES for boron analysis.
