Color distinction towards end point is very important using two indicators like ammonium purpureate and Erichrome black- T. You need truly a set eye to be precise  with such titrimetric methods.

Hello Hohai 

I hope this may help you. 

H Ciesielski, T Sterckeman, M Santerne, Jp Willery. A comparison between three methods forthe determination of cation exchange capacity and exchangeable cations in soils. Agronomie,EDP Sciences, 1997, 17 (1), pp.9-16.

https://hal.archives-ouvertes.fr/hal-00885821/document

Hadi, J., Tournassat, C., & Lerouge, C. (2016). Pitfalls in using the hexaamminecobalt method for cation exchange capacity measurements on clay minerals and clay-rocks: Redox interferences between the cationic dye and the sample. Applied Clay Science, 119, 393-400. https://www.researchgate.net/publication/276899356_Pitfalls_in_using_the_hexaamminecobalt_method_for_cation_exchange_capacity_measurements_on_clay_minerals_and_clay-rocks_Redox_interferences_between_the_cationic_dye_and_the_sample

Regards

Article Pitfalls in using the hexaamminecobalt method for cation exc...

Sometimes the end point is delayed due to interference of heavy metals. A solution of 2% NaCN may be added to overcome it.

Color distinction towards end point is very important using two indicators like ammonium purpureate and Erichrome black- T. You need truly a set eye to be precise  with such titrimetric methods.

Thank you so much Dr José António M Macedo, Dr  Nilay Borah and Dr Anoop Kumar Srivastava, it was very helpful

 Mr.Mohammed,it is a good practical question and answered well by colleagues.You did not give any information regarding your soils especially pH and calcium carbonate content.If you use neutral normal ammonium acetate method for estimation of CEC and exchangeable cation estimation,the exchangeable calcium will be over -estimated because of dissolution of calcium carbonate in ammonium acetate .It was clearly shown in the first paper attached by Dr.Jose.If the soil contains magnesium carbonate,then Mg will also be overestimated.In K fixing soils,NH4 ion can replace part of the fixed K leading to high K results.So in calcareous soils and high K fixing soils ammonium acetate  extraction may not be used for estimating exchangeable cations.

All scientists,

I understood that ammonium ion will not replace fixed K. Moreover ammonium acetate will not dissolve corbonates. Hence addition of Mg from Mag carbonates and calcium carbonates does not arise. It is only the portion of water soluble cations that can add to exchangeable cations.  However, estimation of CEC in calcareous and Mg carbonate rich soils  may give higher values of exchangeable cations. In my view, these carbonates also act as clays and give sites for adsorption of cations  along with some moisture.  Hence we get higher values of exchangeable cations.

Pl comment

I agree with you Dr Rajakuamr , ammonium ions  though ammonium acetate solution will not be able to replace the non-exchangeable/fixed-K . This is the reason neutral ammonium actete solution is used extracting water soluble and exchangeable-K. There is every possibility , such carbonates could be an active site for adsorption of nutrients. And , there is every possibility , such carbonates could also be asource of nutrients like Fe, Mn, Zn  upon dissolution . Let me share my experience  on  smectite soils having calcium carbonate nodules of various dimensions (  Age of such precipitation is perhaps more important , in terms of exposure to how many cycles of wetting -drying ) . When such caicium carbonate nodules were analysed for concentration of these nutrients, to your surprise, the concentration of such nutrients was far higher than their concentration soil devoid of such nodules. I fully endorse your excellent observations.

Dr.Rajakumar ,Please read the paper attached by Dr.Jose(A comparison between three methods...) and pay some attention figures 5 and 6  in the paper and pertinent explanation.

Dear all, here is another paper, which rather supports the idea of carbonates dissolution by ammonium acetate.

Well me must remember, the suppose dissolution of carbonates may act in two ways:

1) lowering CEC determined directly (for example by subsequent removing of ammonium ions from acetate absorbed in soil in subsequent washing with KCl, NaCl etc) as it was recommended in previous versions of WRB (1998 and 2006). This is the case described in the paper I attach;

2) increasing CEC if it is determined also with ammonium acetate method, but indirectly, by the sum of basic cations and exchangeable acidity or exchangeable Al (as it is recommended now in WRB 2014).

Certainly, some colloidal forms of carbonates may be considered clays. However, does it mean that they have negative charge and may interchange cations in similar way as, for example, illite?

May be it would be useful to make a simple experiment (or find it in libraries or internet, because probably something like this was done) - 1) take determined portion of calcium carbonate, 2) treat and incubate it with determined quantity of ammonium acetate (at pH 7, but I would also try at pH 8,2 which is a value tipical for hydrolysis of calcite), 3) remove (decantate?) ammonium acetate, than may be wash the carbonate with water or alcohol and dry (at temperature not higher than 40 Centigrades), 4) check the mass of carbonate after washing and drying and compare it with inital mass?

Dr.Stepein,appreciate good explanation ,appropriate reference and a good suggestion for verification.The following reference can also be seen.

 Chapter 9 Recommended methods for determining soil cation exchange capacity  by Donald S.Ross and Quirine Ketterings s3.amazonaws.com/udextension/lawngarden/files/2012/10/CHAP9.pdf

Colleagues, I wish to add one more reference for the increase in solution  K released from fixation sites.' In contrast to K fixed in the interlayers of vermiculite,a portion of the K in illite and certain soils is more readily replaced by NH4 ion  than by Ca or Mg ions.K fixed by illite and weathered micas in soil clays occupies edge-interlayer sites that are highly specific for K and other similar cations.Because of the similarity of K and NH4 ions and the small diffusion distance from solution to these sites,NH4 ions are effective in replacing K ions'.

Selective sorption and fixation of cations by clay minerals:a review by B.L.Sawhney Clays and clay minerals , vol 20,pp.93-100.(1972) pdf available online.

Sir,

Very good discussion. The references given by Subba Rao sir, and explanation given by Anoop sir are seems to be perfect.  I agree with their. views. My point is, on hydration of lime content in soil, the nano lime particles along with water of hydration  act as sites for adsorption of Ca and other cations.  If there is a dissolution of lime it should give out the bubbles when amonium acetate is added to soils. Hence, lime soils by virtue of lime content we correlate with higher CEC. But it is the case adsorption of cations.  I have also analysed the Red soils with some lime content at lower horizons. I got more Ca there !. Normally red soils have less exchangeable cations (Ca) compared to black soils.  

So think of adsoption instead of dissolution.

Sir,

Coming to the 'Na acetate', it has  very week adsorption power compared to all other cations.  It sits on negative charge site which are available on hydrated cations of  exchange sites. If it was efficient in removing cations, it would have used for extraction of these. But we use amonium acetate as amonium ion is equal to potassium ion which can also replace Ca and Mg on continuous agitation..

To solve the problem of CEC estimation, we should go for addition of individual cations after their determination by ammonium acetate method.

Nice feebacks Dr Rajakumar . A good argument   about the presence of distinguishable amount of lime through effervescence upon addition of ammonium acetate in lime -rich soils. But , effervescence might not be so much visibly seen with neutral pH.  The point of discussion is,  whether or not ,  some part of  lime acts as  clay  particles with negative charge ( Michal was point  blank in his expression ) to facilitate  an additional adsorption sites for  the cations  and later replaced/desorbed  with ammonium ions ? Addition of individual cations for CEC estimation using ammonium acetate extractant  often over-estimates the actual CEC?

the total of Amonium acetate extractable Ca, Mg, Na and K do not reflect the CEC at all in calcareous soils. Because this method remove soluble salts (which are not exchangeable), exchangeable cations and pH induced solubilization of Ca and Mg from carbonate minerals.  In calcareous soils the pH change in the 7.8-8.4 range. As these soils extracted with molar amonium acetate at pH 7, which is a sort of buffer, an excess amount of Ca and Mg are solubilized from carbonates or other minerals. In this case the as the original pH 8, Ca and Mg concentration in the amonium acetate extract can increase up to 100 fold depending on the solubilization reaction of Ca and Mg carbonates due to pH change in the soil.  Therefore the excessive Ca, Mg are common in calcareous soils as a result of soluble salts and pH induced solubilization of carbonate minerals ove CEC.

Uygur sir said some of the soluble portion of lime may come into ammonium acaetate solution.

To solve the problem of CEC estimation in such soils, we should go for substraction of water soluble fraction from sum of individual cations after their determination by ammonium acetate method.  Then we will get the actual CEC

Dr Rajakumar , endorsing your comments , let me add  further , this is exactly we do in salt affected soils . We remove first the soluble salts , then estimate CEC , simply to ward off the over-estimation of exchangeable cations on account of soluble salts.  

We have earlier discussed CEC estimation in calcareous,saline and sodic soils in researchgate.It was suggested that the sodium acetate method of Bower et al.(1952 )a better method for estimating CEC and exchangeable Ca and Mg in saline and sodic soils.

www.researchgate.net/post/what_is_the_effect_ on_the_CIC_to_be_determined_at_different_pHs_ammonium_acetate_?

Really thanks for all, it was very helpful

Dear Colleagues

Wonderful discussion, Added to above discussion, I am enclosing one research paper on this aspects.

Adjusting the pH of the NH4C2H3O2 pH to 8.5 suppresses the dissolution of carbonates and provides a more accurate measure of exchangeable Ca. Concurrent extraction of K, Mg, and Na was unaffected by increasing the pH from 7.0 to 8.5 based on the soils evaluated. Utilizing an extractant pH of 8.5 may provide an estimate of cation exchange capacity (CEC) based on sum of cations for alkaline soils.

REFERENCES: Doll, E.C. and R. E. Lucas. 1973. Testing soil for potassium,

Those interested at this discussion topic may possibly want to check: https://www.researchgate.net/post/How_to_calculate_cation_exchange_capacity_of_clay_soil