Dear colleagues,
from a practical point of view, how do I accurately measure sediment pH of more or less compacted, anoxic lake sediments in the field? Imagine there is a freshly recovered and opened core in front of you.
The problems that I imagine I have to deal with are:
1. that I may only have a very small amount of material available, say maximum 2 g or ml, maybe even less
2. that water content of the sediment will vary (maybe this is important when preparing a suspension?)
3. that handling steps should be minimezed due to anoxic sediment becoming oxidised quickly
4. that due to the small amount of of sediment available, the amount of suspension is also limited, so I might have to use 2 ml Eppendorf tubes, if I am lucky I can use 15 ml falcons, to obtain enough suspension for the pH-Meter to be submerged into and measure. Standard lab pH-Meters only barely fit 15 ml falcon tubes but not 2 ml Eppendorf tubes. Any advice about the hardware?
Using needle-like microsensors to measure directly in the sediment core is unfortunately not feasilbe.
Thank you for your comments!
In addition: I read that CaCl2-solution may beused for agricultural soils and KCL-solution for more acid forest soils to counterbalance the dilution effect when preparing suspensions. Only in highly weathered soil like we may find in the tropics you would use distilled water. I have not read anything particular about sediments, which to make matters worse might be strongly laminated and show different sedimentation regimes/influences.
Hi Petra,
You could try using a minisensor like a H138 pH meter from Hach (minisensor pocket series), which are able to measure pH in a single drop of solution (that you could extract from your sesiment core with an hypodermic needle attached to a siringe (both items easy to obtain). So no real need to prepare suspensions and the modification of the sample is minimal. We have used this method in the past for our lake sediment cores in the field and incubation columns in the lab and it worked pretty well. Accuracy is probably not as good as with the standard pHmeters, but they are still more than OK to establish vertical gradients of pH in your sediments (if they exist) with a reasonable degree of precision.
Cheers.
I would also go for a Microsensor. Unisense has sensors in a hypodermic needle which are rather robust do that you can push them into the sediment. Taking a small subsample always bears the risk of getting it oxidized and thus too low pH readings
We have had success with using spear tip electrodes for measuring saturated soil pH (e.g. like this one, https://sensorex.com/product/ph2200-spear-tip-ph-sensor/ ). These connect to standard portable pH meters. If you are really worried about oxidation changing the pH then you could do this in a portable nitrogen glove box.
Thank you everybody for your suggestions and shared experience. I appreciate it and will give a little feedback, once we have been in the field.
Klaus-Holger Knorr I have known about the microsensors from Unisense, they have a great Field Multimeter (also for Redox-, Sulfide-, Oxygenmeasurements, ...), but it comes at a considerable cost and as we may always have small rocks or pebbles in the samples I am not sure if the delicate needle-like sensor might be damaged. So I think this is not feasible for us. Especially as this is only one of so many variables we have or will measure. Javier Sánchez-España I have not thought at all about the possibility of using a hypodermic needle to extract pore water from the sediment, very interesting. Have you tried this with sediments which contain some clay, or with sediments which are so compacted/with such low water content, that they are not fully binding (i don't know if this is the right word), like, they form cracks after recovery? What I wonder about with this method is, when using such a small amount of sample to measure pH, will it not be instantly changed upon contact with air/oxygen when it originated from anoxic sediment? Also, I am not entirely sure, but I had a feeling, that for soils and sediments it might make a difference if you only measure in pore water or in the pore water/gas/solids mix which is the sample (maybe due to excange processes and such).
For my purpose Luke Mosley suggestion seems to fit rather well formy purpose and I will discuss this with my group. When you measured your samples, were there small rocks included? Or was it rather sand fraction and smaller? Obviously I can't look into the sediment, but do you think it would damage the electrode if it is carefully pushed into the sediment and it touches on a pebble with it's tip?
I think oxidation is only a problem, if I have to extract a subsample from the sediment to create a suspension. In previous campaigns we have noticed, that even after ~3 h of sampling, only the topmost first mm of the opend core oxidated. If you carefully remove this, the sediment was still anoxic, judging by its color. Luke Mosley Would you expect the measurement to be flawed, when the sample is not fully saturated?
Thanks again and best wishes
Petra
Hi Petra, we did not have a lot of rocks in our samples, mainly soft muds and sands. The glass tips are reasonably robust so should not be damaged if you insert gently, even if you hit a rock. You could also try these solid state type electrodes (see https://process.honeywell.com/us/en/products/process-instruments/analytical-instruments-and-sensors/ph-orp/durafet-ph-sensor andArticle Testing the Honeywell Durafet (R) for seawater pH applications
). Yes I think measuring soon after collection and inserting into sediment deeper than surface layer should be fine. Cheers, Luke
Klaus-Holger Knorr I have just taken a closer look also on the needle-sensors you recommended but found that Unisense does NOT recommend to use these sensors in sediment. So I wanted to ask, if despite that, have used them yourself successfully, and if you have used them in rather compacted and comparably dry sediments?
Hello Petra.
Following the suggestion of Javier Sánchez-España , you might check the paper we recently published:
Article Glycerol amendment enhances biosulfidogenesis in Acid Mine D...
We used hypodermic needles to extract a small portion of the sample from the incubation column to follow the pH and ORP evolution. For the latter I used a conventional YSI Pro1020 probe common in limnology and the results for pH were very close (on the order of 0.1, very rarely 0.2, which is basically the error claimed by Hach for H138). Regarding to the oxidation, I believe that it is not that fast to seriously affect the result, but you can run a trial measurements in a glovebox and under normal conditions, leaving a drop for 5-10 minutes and observing the differences in the pH.
What you should be careful about is not to create borrows in the sediment, which will end up connecting different layers or even make all the way up to the water column. For that the extracted amount should be as small as possible and ideally you should have different access points for extraction (i.e. make more than 1 port for each layer). Also, the smallest amount of samples is important for low permeability sediment, so that you don´t drain all of the interpore water out.
Regarding to the syringes, I would recommend the finest for the water part, and 0.8X40mm (21Gx1.5) for the sediment. Exceptionally, if the clayish sediment is becoming too coarse and you have serious clogging, you might consider getting 1.2x40mm syringes (18Gx1.5"). Using these syringes, you actually get the mix of water + sediment, which is a fair approximation of what is going on in these layers. Unfortunately, the gas would ex-solve very fast and you may qualitatively detect it (e.g. H2S) smell, but not really measure it´s effect on the pH.
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