Stabilised formazin composition, method of preparation and diluent for formazin composition

Abstract

A stabilised aqueous formazin composition comprises 100 parts by weight water; up to 0.03 parts by weight formazin polymer; 0.4 to 5 parts by weight hexamethylenetetramine; and a buffer composition, wherein the buffer composition is adjusted to maintain a pH of the formazin solution in a range between 8.4 and 10. The stabilised formazin composition has a turbidity of 400 NTU or less. A diluent for preparing the stabilised formazin composition from a formazin stock composition is provided, wherein the diluent comprises hexamethylenetetramine in a concentration of between 4 and 50 g/l and a buffer composition adjusted to a pH of the diluent in a range of from 8.4 to 10.

Claims

1. A stabilised aqueous formazin composition having a turbidity of 400 NTU or less, said composition comprising: 100 parts by weight water; up to 0.03 parts by weight formazin polymer; 0.4 to 5 parts by weight hexamethylenetetramine; and a buffer composition, wherein the buffer composition is adjusted to a pH of the formazin composition in a range of from 8.4 to 10.

2. The composition of claim 1, wherein said hexamethylenetetramine is present in an amount of 0.4 to 4 parts by weight.

3. The composition of claim 1 wherein the buffer composition is selected from the group consisting of ammonium buffers, phosphate buffers and borate buffers.

4. The composition of claim 3 wherein the buffer composition comprises a dihydrogen phosphate salt and sodium hydroxide.

5. The composition of claim 3 wherein the buffer composition comprises a borate salt and hydrogen chloride.

6. The composition of claim 1 wherein the buffer composition is present in a concentration effective to provide a buffer capacity of at least 1 percent.

7. The composition of claim 1 wherein the formazin composition has a buffer capacity of 1 to 15 percent.

8. The composition of claim 1 wherein the formazin composition has a turbidity of 100 NTU or less, and comprises up to about 0.0075 parts by weight of the formazin polymer.

9. A method for preparing a stabilised aqueous formazin composition having a turbidity of 400 NTU or less and a pH in the range of from 8.4 to 10, comprising the steps of: providing an aqueous diluent comprising hexamethylenetetramine in a concentration of between 4 and 50 g/l and a buffer composition adjusted to a pH of the diluent in a range of from 8.4 to 10; and mixing the diluent to an aqueous formazin stock composition having a turbidity of greater than 400 NTU, to obtain the stabilised aqueous formazin composition having an NTU of 400 or less.

10. The method of claim 9, wherein the diluent is free of sulfate ions.

11. The method of claim 9 wherein the buffer composition is selected from the group consisting of ammonium buffers, phosphate buffers and borate buffers.

12. The method of claim 11 wherein the buffer composition comprises a dihydrogen phosphate salt and an alkali metal hydroxide.

13. The method of claim 11 wherein the buffer composition comprises a borate salt and hydrogen chloride.

14. The method of claim 9 wherein the diluent has a buffer capacity of 1 to 15 percent.

15. The method of claim 9 wherein the formazin composition is exposed to UV radiation for disinfection.

16. A diluent solution comprising hexamethylenetetramine in a concentration of between 4 and 50 g/l and a buffer composition adjusted to a pH of the diluent solution in a range of from 8.4 to 10.

17. The composition of claim 1 where the buffer composition is adjusted to the pH of the formazin composition in the range of from 8.4 to 9.

18. The composition of claim 2, wherein the hexamethylenetetramine is present in the amount of 0.4 to 3 parts by weight.

19. The process of claim 9 wherein the buffer composition is adjusted to the pH of the diluent in the range of from 8.4 to 9.

20. The diluent solution of claim 16 wherein the buffer composition is adjusted to the pH of the diluent solution in the range of from 8.4 to 9.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The invention is described in more detail hereinafter with reference to preferred embodiments which are understood as non-limiting examples illustrating the invention.

(2) Diluted formazin calibration standards can be made by diluting a concentrated 4000 NTU formazin stock solution made according to ISO 7027:1999 to ensure accuracy. 4000 NTU formazin stock solutions are also commercially available. In all cases, the reagents used to make formazin (hexamethylenetetramine and hydrazine sulfate), are primary standard grade reagents (assayed>99%).

(3) The reagent water used in all testing is tap water passed through ion exchange columns and then through a reverse osmosis filter. The reagent water has a turbidity of 0.025 NTU or less. The reagent water is also referred to as ultra-low turbidity water.

(4) Twenty-five millilitres of the concentrated 4000 NTU formazin stock composition are diluted to 250 millilitres using an aqueous diluent comprising hexamethylenetetramine to obtain a 400 NTU stabilised formazin composition. The diluent is an aqueous solution containing 4 g/l hexamethylenetetramine and a buffer composition comprising sodium tetraborate and hydrogen chloride. The pH of the diluent is adjusted to a pH of between 8.4 and 9. The molar concentration of the buffer in the diluent may vary in a range of from 0.01 mol/l to 0.5 mold.

(5) The diluent may be filtered to low turbidity through a 0.2 m filter before use. After filtering, the diluent shall have a turbidity of less than 0.06 NTU, or it should be re-filtered. Lower turbidity standards can be obtained by volumetric linear dilution of the 4000 NTU formazin stock solution with filtered diluent. For example, a 40 NTU formazin standard is obtained by diluting 2.0 ml of the 4000 NTU formazin stock solution to 200 ml, and a 4 NTU formazin standard can be obtained by diluting 1.0 ml of the 4000 NTU formazin stock solution to 1000 ml. Stabilised formazin standards remain stable for longer than one year, even at low turbidity levels of less than 0.5 NTU.

EXAMPLE 1

(6) A formazin stock composition (4000 NTU) is prepared according to ISO 7027 having the following ingredients in the amounts shown. Unless stated otherwise, all amounts in the examples are given in weight percent.

(7) TABLE-US-00001 Formazin polymer 0.21573% Formaldehyde 0.11539% Ammonium sulfate 0.50800% Hexamine 4.73100% Water 94.49300%

EXAMPLE 2

(8) A diluent solution is prepared by adding 20 g of analytically pure hexamine to 1 l of ultra-low turbidity water. A buffer composition comprising potassium dihydrogen phosphate in an amount of 13.6 g and 10.0 N sodium hydroxide in an amount of 9.22 ml is added to the hexamine solution and adjusted to a pH of 8.4. The solution is filtered through a 0.2 m filter, and disinfected by exposure to UV light at a wavelength of between 240 to 280 nm and at a UV dose of between 2 and 60 mJ/cm.sup.2. The solution is then diluted to 2 l using ultra-low turbidity water to obtain a diluent having an hexamine concentration of 10 g/l.

EXAMPLE 3

(9) A stabilised formazin composition (400 NTU) is prepared by means of a linear 1:10 dilution of the formazin stock composition of Example 1 (4000 NTU) with the diluent solution of Example 2.

(10) The pH of the composition is adjusted to 8.4. The storage stability of the formazin composition at 25 C. is longer than 12 months.

EXAMPLE 4

(11) A stabilised formazin composition (40 NTU) is prepared by a 1:10 dilution of the 400 NTU stabilised composition of Example 3 using the diluent solution of Example 2.

(12) The pH of the composition is adjusted to 8.4. The storage stability of the formazin composition is longer than one year.

EXAMPLE 5

(13) A diluent solution containing 10 g/l hexamine was prepared according to Example 2. The pH of the diluent solution was adjusted to 8.5. A stabilised formazin solution was prepared by linear dilution of a 4000 NTU stock solution of Example 1 to the desired NTU value varying from 200 NTU to 0.32 NTU.

(14) The stabilized formazin solutions were stored in glass vials in the dark at 85 C. for a predetermined period of time. After that time, they were allowed to cool down to room temperature (about 20 C.) prior to measurements. Turbidity measurements were taken from the solutions after mixing by inversion using a commercial turbiditmeter calibrated on fresh prepared formazin standards. Experimental data shows that one day of storage at 85 C. correlates to 4.25 days of stability at 20 C.

(15) The results of the above accelerated stability tests are given in Table 1 wherein Time (Days) refers to calculated time at room temperature.

(16) TABLE-US-00002 TABLE 1 Accelerated Stability - 10 g/l hexamine in phosphate buffer Time (Days; Standard Value (NTU) calculated) 200 20.5 1.05 0.32 2 202.833 20.203 1.041 0.349 9 201.361 20.136 1.033 0.350 28 201.306 19.967 1.013 0.352 59 201.556 19.936 1.007 0.345 66 201.111 19.942 1.005 0.344 84 200.917 19.958 1.015 0.351 126 201.861 20.297 1.036 0.360 427 199.389 20.194 1.064 0.374 Time (Days; calculated) % Chg 200 % Chg 20.5 % Chg 1.05 % Chg 0.32 2 0.00 0.00 0.00 0.00 9 0.73 0.33 0.84 0.25 28 0.75 1.17 2.75 0.77 59 0.63 1.32 3.33 1.19 66 0.85 1.29 3.48 1.58 84 0.94 1.21 2.54 0.49 126 0.48 0.47 0.50 3.15 427 1.70 0.04 2.17 7.21

(17) The standard formazin solutions are considered stable if the change in the NTU values is less than 5 percent for all standard solutions greater than 1 NTU, or if the net change is less than 0.05 NTU for all standard solutions of 1 NTU or less.

EXAMPLE 5

(18) A buffer composition was prepared by dissolving 4.765 g of sodium tetraborate decahydrate in 150 ml of filtered (0.02 m), deionized water and adding 11.6 ml of 1.0 N hydrogen chloride (HCl). 4 g of analytically pure (99.95%) hexamine was added to the buffer composition, and the solution was diluted to 1 litre. The pH of the solution was adjusted to between 8.5 and 9.0, thus obtaining a diluent solution having an hexamine concentration of 4 g/l. Prior to use, the diluent solution was filtered through a 0.2 m filter, and disinfected by exposure to UV light at a wavelength of between 240 to 280 nm and at a UV dose of between 2 and 60 mJ/cm.sup.2.

(19) The diluent solution was used to prepare a stabilised formazin solution by linear dilution of a 4000 NTU stock solution of Example 1 to obtain formazin standard solutions having the desired NTU value varying from 200 NTU to 0.44 NTU.

(20) Accelerated stability tests were carried out with the formazin standard solutions as described in Example 4. The results of the accelerated stability tests are given in Table 2.

(21) TABLE-US-00003 TABLE 2 Accelerated Stability - 4 g/l hexamine in borate buffer Time (Days; Standard Value (NTU) calculated) 200 20.10 1.02 0.77 0.44 1 202.33 20.16 1.01 0.77 0.43 5 201.69 20.08 1.02 0.44 17 202.64 19.97 1.03 0.43 20 0.77 26 201.17 20.02 1.02 0.44 34 0.76 41 202.72 19.99 1.03 0.44 68 200.53 19.93 1.01 0.43 61 0.76 83 201.78 19.90 1.01 0.43 76 0.76 115 193.69 19.51 1.00 0.42 108 0.74 128 0.74 135 195.47 19.54 1.00 0.42 150 194.81 19.45 0.99 0.41 145 0.74 239 193.44 19.26 0.97 0.72 0.40 280 193.42 19.03 0.94 0.71 0.40 293 193.94 19.08 0.97 0.72 0.41 313 193.69 19.00 0.96 0.71 0.40 355 192.53 19.34 0.98 0.73 0.41 375 194.17 19.34 0.98 0.73 0.41 677 192.78 19.12 0.96 0.71 0.40 Standard Value (NTU) 200 20.10 1.02 0.77 0.44 % Chg % Chg % Chg % Chg % Chg Time (Days) 200 20.1 1.02 0.77 0.44 1 0.000 0.000 0.000 0.000 0.000 5 0.316 0.372 0.859 0.991 17 0.151 0.951 1.365 1.267 20 0.004 34 0.134 41 0.192 0.841 1.557 0.576 68 0.892 1.116 0.198 1.708 61 1.215 83 0.275 1.295 0.129 1.068 76 1.223 115 4.270 3.237 1.386 3.761 108 3.142 128 3.559 135 3.391 3.045 1.444 3.883 150 3.720 3.528 1.875 4.740 145 3.370 239 4.393 4.437 3.657 6.204 6.979 280 4.407 5.581 6.924 6.900 7.350 293 4.146 5.333 4.360 6.082 5.815 313 4.270 5.760 5.590 7.131 8.827 355 4.846 4.051 3.468 4.750 5.444 375 4.036 4.065 3.124 5.089 4.817 677 4.723 5.167 4.676 7.566 7.503

(22) The standard formazin solutions are considered stable if the change in the NTU values is less than 5 percent for all standard solutions greater than 1 NTU, or if the net change is less than 0.05 NTU for all standard solutions of 1 NTU or less.