CIC is being determined in soil and clay fraction. First we wash the floor with sodium acetate three times, then three times with alcohol and finally we washed with ammonium acetate to quantify the amount of sodium.
Dear Ronni, I don.t have have data directly responding your question. However, it is known, that ammonium acetate dissolves soil carbonates (especially calcium carbonate) even at pH 7. This may affect results of CEC (Cation Exchange Capacity in English, or CIC - Capacidade de Intercambio Cationico in Spanish) in two opposite ways:
1) Underestimation of CEC measured directly, for example by measurement of ammonium ions with Kjehdahl method (in the version of the method when after use ammonium acetate and alcohol, it is washed then by KCl or NaCl, and then determined) or by measurement of Na ions (version of the method, you are describing, when ammonium ions from acetate were washed/removed by NaCl, and the Na ions are removed with other substance and determined by atomic absorption). See the paper attached for details of action of ammonium acetate. In few words, the carbonates are being dissolved by ammonium acetate, increasing the Ca concentration in solution - these Ca ions compete with ammonium ions for the places (negative charges) in soil;
2) Overestimation of CEC, when it is determined undirectly, based on the sum of cations washed by ammonium acetate. In this case, as ammonium acetate dissolves calcium carbonates (see the paper attached), the concentration of Ca in extract increases, and it is treated as exchangeable Ca. This causes increase of Ca exchangeable determined and thus overestimates the sum of bases and, consequently, whole CEC. It is especially important in calcareous soil. However, I think it occurs even in soil with pH of about 6,5 (in water), with very small carbonate content (undetectable by 10%HCl). I had great problem with this both in my Master and PhD thesis.
You should also remember, that USDA laboratory procedures use ammonium acetate at pH 5 to remove carbonates from the soil before determination of soil texture using pipette method!
Now I can more directly answer your question I think the lower pH of ammonium acetate applied, the CEC determined directly will be smaller, but the CEC measured from sum of cations willl be greater, than CEC determined by ammonium acetate at pH 7.
It would be interesting to verify, if I am right.
Saludos para el Peru!
The CEC method Ms Ronni following is sodium acetate method,not ammonium acetate method. It is recommend method for both noncalcaerious and calcaerious soils..In this method soil cation exchange sites are saturated with sodium, excess Na is removed with ispropyl alcohol and the exchangeable Na is replaced with ammonium acetate. Then the Na is estimated with AAS or by flame photometer.Ms.Renni may tell if there is any problem with this method.
Since soils have different pH and different clay mineral.There is permanent and pH dependent charge on major soil clay constituents. The oxide minerals and OM carry manly the pH dependent charges while other !:1 and 2:1 and 2:1:1 have relatively small amounts of pH dependent charge. Hence by varying the pH of NaoAC we will be introducing errors in our results. To avoid this, neutral pH has been found perfectly alright particularly in arid land inorganic alkaline soils. Dr. Michał Stępień has elaborated the mechanisms nicely for decrease or increase in CEC of soils by using extractant having different pH.
Soils are made saturated with Na ion by using Na-acetate of pH 8.2. Then exchangeable Na ions are extracted with NH4-acetate of pH 7. This is the protocol to determine CEC as per USDA for saline and alkaline soils. Change of pH of NH4 -acetate is not advised for calcareous saline and alkali soils.If the pH of NH4-acetate is changed it is expected the CEC value may vary,which may not be real value.
Drs Ghafoor and Dr.Pal,the description given Ms Ronni matches with sodium acetate method of Bower et.al 1952. The same method is available in US salinity laboratory hand book no.60.It also available in many other sources. As for my experience this method works well in neutral to alkaline soils.At pH 8.2 the calcium carbonate in calcareous soils may not interfere with this method. As pointed out by Dr.Pal ,CEC estimation by ammonium acetate method is at pH 7.0 only,not at different pHs.It is well known that in calcareous soils the ammonium acetate dissolves calcium carbonate and leads to over estimation of exchangeable Ca.Recently limed soils may create more problems than agged carbonates. Upto soil pH 7.5 small amounts of lime in soil may not pose problem in CEC estimation.In acid soils ammonium acetate method overestimates the CEC as pH dependent sites are also included in estimation and a separate method is needed for acid soils.In heavy metal polluted soils sodium or ammonium acetate may not be able to replace exchangeable di or trivalent ions .So the CEC is under estimated.In heavy clay soils especially in sodic soils the stirring/shaking and centrifugation with sodium acetate may give some problem.
On predicting the pH of pure sodium acetate aq. sol.; see: https://www.researchgate.net/post/Can_we_keep_the_pH_of_3M_Sodium_Acetate_around_7_instead_45-5_while_using_this_in_DNA_extraction_from_whole_blood
On predicting the pH of pure ammonium acetate aq. sol.; see: https://www.researchgate.net/post/Ammonium-acetate-additive-causing-pH-change
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