It has been a long time since my last substantial post, for which I apologize. My onkly excuse is that I have been extremely busy with classwork, research, and several side project (which I will write about at some later date). As far as my research goes, I do have some updates to report regarding the Si extraction and analysis procedure.
1. Traditionally I have been using ~0.075 g of soil coupled with 100 ml 0.2M NaOH. I have been experimenting with the same amount of soil, but a much larger volume of NaOH (250 or 500 ml). This allows me to take a much larger aliquot, which lessens problems due to pipetting errors. Interestingly, I have been finding higher values of Si when I use a larger volume of NaOH (see graph). There may be several reasons for this: a) there may be residual Si on the inside of the bottles, the total amount of which may be magnified simply because of the larger aliquot taken; b) the larger aliquot (and NaOH volume) may be causing some weird absorption probem. I have observed in the past that NaOH, when mixed with molybdate before it is totally neutralized by acid, can raise absorption values by ~0.01. This is substantial.
The larger aliquot extracts probably require their own unique standard curve. If the NaOH is causing increased absorbance, or if there is residual Si, then it should be possible to correct for this in the standard curve in two ways. First, by premixing the Si standard with 0.2M NaOH, it will mimic conditions present in the bottle. When an aliquot of the NaOH/Si is added to the reaction flask (and the molybdate and acid reagents), it will be added in exactly the same fashion as the extractions. (By the way, I am now adding the molybdate reagent before I add the Si aliquot (see below). This could be critical, as the NaOH may not be neutralized completely by he acid (especially a larger aliquot). Second, water used to make the NaOH/Si standard should be first stored in one of the reaction bottles, to account for residual Si. Using these two steps, I should be able to determine if the problems outlined above are the real culprits.
2. The amount of 0.5M H2SO4 added to the flask matters. In the past when I was taking small aliquots (in the range of 0.25 ml) and adding 5 ml of H2SO4, this may have been problematic. The key is to keep the pH below 1.5 in the flask, which 5 ml of H2SO4 does, but the ionic strength may have been too great. An ionic strength of 0.5 and above may cause problems with the molybdate reaction. To account for this, I have now adjusted the acid volume in the flask relative to the Si aliquot volume. For example, an Si aliquot of 0.25 ml receives 3.9 ml H2SO4, while an Si aliquot of 1 ml receives 5 ml H2SO4, and so on. In this way, the pH and ionic strength remain low.
3. The mixing order of the reagents matter. In the past I have added the acid to the flask, followed by the Si aliquot, and then the molybdate reagent. This is not a problem for the molybdate, but it is problematic for the Si aliquot. Something happens to the Dissolved Si when it is added to only acid. My guess is that it polymerizes with some other compound present (perhaps the NaOH?). When this happens, it will not be able to combine with the molybdate. As I mentioned earlier, I suspect that unneitralized NaOH can raise the absorbance. Thus, the NaOH may be reacting directly with the molybdate. To account for these problems, I have begun adding the molybdate reagent to the flask before the Si aliquot. This has greatly reduced scatter problems. One would think that adding the Si aliquot before the molybdate would be preferrable, as the NaOH would be neutralized. I have found that this isn't the case.
More soon...
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