Dear Indranil,

If you wanted to replace EDTA from Zn-EDTA complex (or masking Zn for complexometric determination of other metals), sulfur-coordinating ligands can be used, e.g. cystein (Talanta 1961, 8, 653), beta-aminoethylmercaptan (Talanta 1963, 10, 1041), or some other. I could help more, if you told me what you would like to do.

Please do not forget the stability of Zn-EDTA is strongly pH dependent. Sometimes the change of pH is enough to liberate enough Zn-ions to precipitate it with an appropriate anion. Selection of "anion" can be performed easily with a simple calculation taking into consideration the ZnEDTA stability constant, the pH, and the solubility product of the insoluble Zn-salt.

If you want to masking the Zn with an other ligand, the stabilities of the Zn-EDTA and the Zn-new ligand complex formed at the given pH have to be compared, taking into consideration the sum of Zn-ion, EDTA, and the new ligand concentrations , of course.

In determination of % Zn as Zn-EDTA in fertiliser sample I had to use KCN as masking agent. But it is very difficult to procure this fatal chemical in our country. I would therefore like to alter this method by replacing KCN with some chemical. I tried to digest the sample (as i found one referenced paper) with conc. H2SO4. I got black solution and after determining the % Zn in AAS i found all the standard and substandard samples having 12% Zn though it is far from reality. I tried with Diethyl dithio carbamate replacing KCN but I didn't got any satisfactory result. If possible please help with some replacement.

Dear Indranil,

Since you have AAS, I suggest doing a separation of ZnEDTA complex from the other EDTA complexes (there are various chromatographic methods for it) and determine the Zn-content of the pure Zn-EDTA contg. phase. The problem is, if you remove Zn from its EDTA complex with an other complex forming and ppt. forming agent, a lot of other elements which disturbs the Zn-determination by AAS migh be liberated.

Therefore, a simple column, a chromatographic separation,a classical AAS seems to be the best solution for you: I am just sending two papers about chromatographic conditons to separate ZnEDTA from fertilizer samples.

Here is the other paper. I will thinking about a classical method to remove Zn from fertilizer samples and send you a new message later.

Dear Indranil,

I have found a book (but written in hungarian language), thus I am trying to translate you the important parts. It seems to be very long, but after makign teh reagent solutions, the method is not as long.

First of all, the water-soluble zinc determination in fertilizers can be made in the following way (with photometry):

Determination the all Zn-content:

2 g of sample powder is melted with 10 g of K2CO3/Na2CO3 mixture in a Pt-crucible, the melt is dissolved in a mixt. of 100 ml of water and 10 ml of cc.HCl, then 10 ml of cc. HCl is added, and the solution is evapd. to dryness. The residue was taken in 20 ml of 10 % HCl, the mixt. is filtered, and the solution is put into a measuring flask and filling up until 50 ml. The ditizon method (see below) or AAS can be used to measure the Zn-content.

15 g of finely divided fertilizer sample is put into glass and stirred in 150 ml of 2 M HCl for 1 h. The solution has to be filtered, the liquid is evaporated into 20 ml. One ml of 20 % aq. H2O2 is added (to destroy the organic components) and the solution is evaporated almost to dryness. The residue is taken in 10 ml of hot water and put into a measuring flask and filled with water until 50 ml.

10 ml from this solution is put into separating funnel (250 ml) together with 20 ml of water, 50 ml of ammonium citrate buffer, and 3 ml of carbamate solution. The pH has to be between 8.5 and 8.8 (the pH can be adjusted with cc. NH4OH until 8.6). 10 ml of CCl4 contg. ditizon is added, and the mixture has to be shaken for 5-6 mins.

After separating the phases, the excess of ditizon is removed by extraction of the CCl4 phase with 25 ml of 0.01 M NH4OH. The CCl4 solution is filled up to 50 ml in a measuring flask and measured the Zn-content by photometry (535 nm). Since you have AAS, you can do the measurement with AAS as well, of course.

The materials can be made in the following way:

ditizon solution: 0.01 g ditizon is dissolved in 50 ml of CCl4. The solution is filtered, the clear filtrate is mixed with 50 ml of 3 % NH4OH in a shaking funnel. The org. part is discarded, the purified orange color aq. phase is washed 3 times with 10-10 ml of CCl4 (shaking and separating), the aq. phase is acidified with 10 ml of 15 % HCl. The color changes to be green. This soln. is extracted with 50 ml of CCl4, and this is the ditizon reagent which can be stored in dark glass under 1 % H2SO4 soln.

CArbamate soln.

0.2 g of sodium diethyldithiocarbamate is disoslved in 100 ml fo water and kept in dark glass.

citrate buffer:

90 g of ammonium citrate is dissolved in 1000 ml of water, the pH is adjusted to 8.5 by NH4OH.

I hope you can use this method, it is standardized and used in fertilizer factories in Hungary.

I am extremely thankful to you for this help. Presently I have no muffle furnace and I had to wait for it. Please mention the temperature for melting Zn-EDTA in furnace. Will 10 g of K2CO3/Na2CO3 mixture breaks the Zn-Edta complex? I received two papers but unfortunately we don't have any access to these instruments. So I am unable to test these methods.

Please let me know about the 2nd paragraph (15 g of finely divided fertilizer sample is put into glass and stirred in 150 ml) whether I can measure the Zn content in Zn-EDTA fertiliser by this method in AAS.

I have no Chromatographic arrangement. But let me knoqw if I can separate Zn from Zn-Edta and then ppt it. For interference of other elements if I use Thiourea plus L-ascorbic acid etc. and then measure it in AAS will it help.

Dear Indranil,

The 10 g of KNaCO3 melt (potassium-sodium carbonate) is enough to destroy the Zn-EDtA complex in the given amount of fertilizer (2 g), because at the high temperature the organic parts decompose completely. The sodium carbonate/potassium carbonate mixture melts at lower temperature than the pure compounds, but the melting point depends on the composition. I suggest using (it is the typical) 1:1 molar ratio, the melting point will be around 700 °C (as you can see in the attached paper, on Fig. 2).

IF you use this method to recover zinc, you can measure the all zinc content in the fertilizer, with any method, e.g. AAS, or after adjustment the pH with a simple complexometry with EDTA. In the last case it is important to know what interfering metal ions are present in order to do masking.

No need any instrument, all the ligands are destroyed in this method, and Zn will be in the form of ZnCl2 in the analyte. If you do not want to speciate the Zn-forms I suggest using this method.

About the another method (15 g of finely...)

This method relates to remove the zinc by adding HCl, then extraction with ditizon (because dithizon-complex can be measured by photometry in CCl4 solution directly). This is more complicated and slower than the previous method, because the purification is made with extraction with CCl4/H2O, and removing of excess of ditizone is made with extraction at another pH.

About two papers: The papers contains not only the measuring methdos with not available instruments, but the method for separating of ZnEDTA from the other EDTA complexes are present in the fertilizer (e..g iron) by chromatography. The chromatographic conditions should be checked in these papers.

It means, a glass column with a tap is enough if you buy the sorbent (gave in the papers) and eluents. After some practising, you can separate the all of the elements from the ZnEDTA, and measure the Zn content directly e.g. with AAS, because nothing will interfere with that.

I am sorry, I have forgotten to attach the paper with the melting diagram. Here it is...

I heartily thank you for guiding me. I will try these methods and let you know the results. However I had to wait for the furnace to be ready. then only I can perform these steps. I have an idea of an alternative please comment on this process.

If I use use 1 g of Zn-EDTA fert sample and digest for 6 hrs qwith conc. Sulphuric acid. Then an aliquot from this digest is treated with hydrogen peroxide to destroy all the organic portion and then from this solution if I measure Zn in AAS what will be the probable outcome.

Dear Indranil,

In principle this idea seems to be correct, in acidic conditions the H2O2 (because of formation of Caro's acid which is a very strong oxidantfrom the H2O2 and H2SO4) can destroy the organic part. I would left a day to digest the sample together with H2O2 and H2SO4. Be careful with H2O2-H2SO4 mixt., because the presence of Cu and Fe in the fertilizer sample can catalyze the decomposition of H2O2, and a vigourous gas (O2) evolution might cause explosion-like gas evolution. If you use Caro's acid, always have to use glass and gloves.

However, first a test analysis should be done with a known amount of pure ZnEDTA (it can be prepared from equimol. amt. of a Zn-salt and Na2EDTA) because it can happen that some sulphonated organic parts will not be destroyed completely. It is not very probable but should be disclosed the possibility completely.

Thank you. I will definitely try this and let you know.