Analytical Chemistry Trinidad & 

Tobago Lab Resources

EDTA Buffers and Indicators

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Buffers for edta metals analysis

Buffers and indicators for metals determined by EDTA

pH buffers and the corresponding complexometric indicators used in EDTA determinations of water hardness
and some other common divalent metals. The ISE buffers TISAB and SAOB are also given below.

pHBuffer compositionUseIndicator
1067.5g NHCl + 570 ml (0.88) ammonia diluted to i liter.Ca; Mg;
Water hardness
Eriochrome or Solochrome
10100 ml 1M NHCl + 500 ml 1M ammonia. Most divalent heavy metals
Pyrocatechol violet
5.0100 ml 0.1M acetic acid + 200 ml 0.1M sodium acetatePb; Zn;
Xylenol orange
2.0500 ml 0.1M sodium acetate + 52.5 ml 1M HCl diluted to 1 literBi;
Xylenol orange

Buffer for Determination of iron:

Buffer pH 5.0
Dissolve 82.03g (1M) of sodium acetate in about 800mls of water, and add glacial acetic acid to give a pH of 5.0 (use of a pH meter is recommended) and dilute to 1 liter.

Special ISE Buffers

Fluoride, Sulphide, Silver.

TISAB II (Total ionic strength adjuster)
Used for low level F (-), < 0.4 ppm (2 x 10-5M)
Salt required is CDTA:trans 1,2, diaminocyclohexane N,N,N1,N1-tetra acetic acid, also known as cyclohexylene dinitro tetra acetic acid, or cyclohexylenediamine tetra acetic acid):
C6H10{N(CH2CO2H)2}2.xH2O: FW = 364.34
Dissolve 4 g CDTA, together with 57 mls glacial acetic acid and 58 g NaCl in about 500 mls distilled water, and using a pH meter, adjust to between 5 and 5.5 pH by adding 5M NaOH (200 g/l) and dilute to 1 liter with distilled water (about 130 mls NaOH).
Dissolve 300 g sodium citrate.2H2O (FW=294.10), 22 g CDTA, and 60 g NaCl in 1 liter water.
Use 10% buffer per volume standard and sample. (add 5 ml TISAB III to 50 ml volumes of standard and sample).
Complexes more than 100 ppm iron or aluminium in presence of 1 ppm F(-)
Dissolve 84 ml conc. HCl, 242 g TRIS (hydroxymethyl aminomethane, and 239 g sodium tartrate (FW=230.08), in about 500 mls water, cool, and transfer to 1 liter volumetric flask and make up to mark.
Use a 1 :1 volume to standard and sample.

An alternative preparation for TISAB.

Dissolve 58.44g NaCl, 61.50g sodium acetate, 0.29g sodium citrate and 15 ml acetic acid in 1 liter distilled water.

SAOB (Sulphide Anti-oxidant Buffer.)

(Salt required is ascorbic acid FW = 176.12; 0.2M in 2M NaOH). Dissolve 36 g ascorbic acid and 80 g NaOH in about 600 mls distilled water, add 67g disodium EDTA, stir, and make up to 1 liter. Note: Discard and make fresh solution when color changes to brown.
The ascorbic acid retards air oxidation of species being measured, makes the solution basic for measurement, and adjusts the ionic strength. Use a 1 : 1 ratio of sample and standard to SAOB.

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