Method for treating indicator fields, indicator field and test device comprising such an indicator field

Abstract

A method for treating a plurality of indicator fields for determining the pH-value of an aqueous solution is provided. Each of the indicator fields contains at least one indicator dye, which is fixed in a non-bleeding manner, in particular on a layer including cellulose or regenerated cellulose, and changes its color within a pH-turning-range. The indicator fields having buffer capacities which differ from one another. All the indicator fields are impregnated from the outside with an impregnation which influences the buffer capacities of the indicator fields in such a way that the deviations are levelled.

Claims

1. A method for treating a plurality of indicator fields for determining pH value of an aqueous solution, the method comprising the steps of: a) providing the plurality of indicator fields having differing buffer capacities from each other, each indicator field includes at least one indicator dye fixed on a substrate, the dye capable of changing its color within a pH turning range; and b) applying an agent to each indicator field, wherein the agent is applied at a concentration which equalizes the differing buffer capacities of the indicator fields.

2. The method of claim 1, wherein the indicator fields originate from different production batches.

3. The method of claim 1, wherein the agent is an acid or a base.

4. The method of claim 3, wherein the acid or the base has a concentration of 0.001 molar.

5. The method of claim 4, wherein the base is lye.

6. The method of claim 1, further comprising the step of fastening the indicator fields to a carrier after applying the agent to each indicator field.

7. The method of claim 6, wherein the carrier comprises plastic.

8. The method of claim 7, wherein the carrier is strip-shaped and longer than the indicator fields.

9. The method of claim 1, wherein the substrate comprises cellulose or regenerated cellulose.

10. An indicator field comprising a) a plurality of indicator dyes, each fixed in a non-bleeding manner on a substrate, wherein the plurality of indicator dyes change color within a pH-turning range and the plurality of indicator dyes have differing buffer capacities from each other; and b) an agent applied to each of the indicator dyes, wherein the agent is applied at a concentration which equalizes the differing buffer capacities of the indicator dyes.

11. The indicator field of claim 10, wherein the substrate comprises cellulose or a regenerated cellulose.

12. The indicator field of claim 10, wherein the agent is an acid or a base.

13. The indicator field of claim 12, wherein the acid or base has a concentration of 0.001 molar.

14. The indicator field of claim 13, wherein the base is lye.

15. A test device comprising a carrier and the indicator field of claim 10 fastened to the carrier.

16. The test device of claim 15, wherein the carrier comprises plastic.

17. The test device of claim 15, wherein the carrier is strip-shaped and longer than the indicator field.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 a perspective schematically view of test device, and

(2) FIG. 2 a perspective schematically view of a test device according to an embodiment of the present invention which results, when an indicator field of the test device shown in FIG. 1 is treated with the method according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(3) Test device 1 is used to determine the pH-value of an aqueous solution. It comprises presently a strip-shaped carrier 2 which is made of plastic or consists of plastic. An indicator field 3 is provided on the upper side of the carrier 2 and contains at least one indicator dye which is fixed in a non-bleeding manner, in particular on a layer of the indicator field 3 comprising cellulose or regenerated cellulose. The indicator dye has the property of changing its color within a predetermined pH-turning-range. In other words, a color change occurs when the indicator dye is immersed in an aqueous solution, the pH-value of which is within the pH-turning-range. The indicator dye has an acid/base function and therefore has a certain acid/base strength or buffer capacity. In order to effect said color-change or color-turn of the indicator dye when immersed in an aqueous solution, the aqueous solution must also have a certain acid/base strength in order to overcome this buffer capacity. The buffer capacity should be constant for basically identical indicator fields 3 in order to archive uniform and thus repeatable measurement results with all indicator fields 3. However, this is not the case in practice, since the buffer capacity is subjected to production-related fluctuations, which in particular are due to smallest fluctuations in the indicator dye quantity and/or to fluctuations in the pH-value of the material of indicator field 3. Large fluctuations in buffer capacity can be observed in particular when comparing indicator fields 3, which are basically identical and originate from different production batches, with each other.

(4) Within the framework of experiments, it has now been established that production-related fluctuations in the buffer capacity of indicator fields 3 can be leveled after their production by impregnating indicator fields 3 from the outside with an impregnation 4 that influences the buffer capacity of indicator fields 3, as shown in FIG. 2. Diluted acids, in particular 0.001 molar acids, have proved to be particularly suitable if the pH-turning-range of the indicator dye is in the acid pH-range and/or in the neutral pH-range and/or in the lower alkaline pH-range. Diluted lyes, in particular 0.001 molar lyes, are suitable for impregnation if the pH-turning-range is in the medium and/or upper alkaline pH-range.

(5) In the following, series of experiments will be described which will show the effect of an impregnation according to the invention.

(6) In a first step, a total of a six color-fixed indicator fields 3 were selected, more precisely two indicator fields 3 from the same production batch from each of a group of in total three groups that differ from each other in terms of the pH-turning-range and are subsequently labeled with the indices 3A, 3B and 3C, see table 1. The indicator fields 3A and 3B have a color turning in the pH-range 4.0 to 9.0 and 4.5 to 8.5, respectively. The color turning of the indicator field 3C lies in the alkaline pH-range 7.5 to 9.5.

(7) TABLE-US-00001 TABLE 1 entry pH-turning-range color change 3A 4.0-9.0 orange - blue green 3B 4.5-8.5 orange - green blue 3C 7.5-9.5 orange - violet

(8) The listed indicator fields 3A, 3B and 3C were exposed to a gaseous acetic acid and ammonia respectively in a second step. The indicator fields 3A, 3B and 3C exposed to a acetic acid are subsequently marked with index 1, the indicator fields 3A, 3B and 3C exposed to ammonia with index 2. These treatments with acetic acid and ammonia changing the buffer capacities inherent in the indicator fields 3 simulated an acid production batch 3A.sub.1, 3B.sub.1 and 3C.sub.1 and an alkaline production batch 3A.sub.2, 3B.sub.2 and 3C.sub.2 of indicator fields 3. The respective colors of the dry indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2 are listed in table 2.

(9) TABLE-US-00002 TABLE 2 3A.sub.1 3A.sub.2 3B.sub.1 3B.sub.2 3C.sub.1 3C.sub.2 acetic ammonia acetic ammonia acetic ammonia entry acid (g) (g) acid (g) (g) acid (g) (g) dry indicator yellow- turquoise orange- turquoise yellow yellow- paper orange red orange DI-water yellow- turquoise red- dark- yellow yellow- orange orange green orange 0.001M yellow- Orange orange green- — — hydrochloric orange red acid 0.001M sodium — — — — yellow- red- hydroxide red violet titrisol pH 3 yellow- green orange- orange- — — (1:10 dil.) orange red red titrisol pH 11 — — — — red- violet (1:10 dil.) violet

(10) In a third step the manipulated indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2 where each sealed on a carrier 2 and immersed in defined test solutions. The different colors of the respective indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2 are still recognizable. When the indicator fields 3A.sub.1 and 3A.sub.2 are immersed in weakly buffered solutions such as 0.001 molar hydrochloric acid or a 1:10 diluted Titrisol pH 3 solution, different colors can also be recognized, namely yellow-orange and orange or yellow-orange and green. After immersion in weakly buffered solutions such as 0.001 molar hydrochloric acid, the indicator fields 3B.sub.1 and 3B.sub.2 show an orange or green-red color reaction. After immersion in 1:10 diluted titrisol pH 3 solution, an orange-red color reaction can be observed in both indicator fields 3B.sub.1 and 3B.sub.2. After immersion in weakly buffered solutions such as 0.001 molar sodium hydroxide solution or 1:10 diluted titrisol pH 11 solution, the indicator fields 3C.sub.1 and 3C.sub.2 show different colors, namely yellow-red and red-violet or red-violet and violet respectively.

(11) To level the buffer capacities inherent in the indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2, they were treated as follows according to an embodiment of the present invention:

(12) The indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2 manipulated with gaseous acetic acid and gaseous ammonia where provided with an impregnation 4, see FIG. 2. For the indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1 and 3B.sub.2 diluted acid was used as impregnation, presently 0.001 molar acid. As impregnation 4 for the indicator elements 1C.sub.1 and 1C.sub.2, however, diluted lye was used, presently a 0.001 molar diluted lye.

(13) The successful leveling of the buffer capacities of the impregnated indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2 is shown in table 3 in a similar to the same color reaction when the impregnated indicator fields 3A.sub.1, 3A.sub.2, 3B.sub.1, 3B.sub.2, 3C.sub.1 and 3C.sub.2 are immersed in the weakly buffered test solutions indicated there, which correspond to the test solutions according to table 2.

(14) TABLE-US-00003 TABLE 3 3A.sub.1 3A.sub.2 3B.sub.1 3B.sub.2 3C.sub.1 3C.sub.2 acetic ammonia acetic ammonia acetic ammonia entry acid (g) (g) acid (g) (g) acid (g) (g) dry indicator yellow- green orange- green yellow- light red paper orange red orange DI-water yellow- green red- dark- yellow- red orange orange green orange 0.001M yellow- yellow- orange- orange- — — hydrochloric orange orange red red acid 0.001M sodium — — — — red- violet hydroxide violet titrisol pH 3 yellow- yellow- orange- orange- — — (1:10 dil.) orange orange red red titrisol pH 11 — — — — violet violet (1:10 dil.)

(15) As a result, the reimpregnation of indicator fields 3 according to the invention achieve leveling of production-related fluctuations of the buffer capacities inherent in indicator fields 3, which leads to constant or at least approximately constant measurement results across production batches. In this way, the reliability of pH-measurements can be significantly increased.

(16) At this point, it should be noted that indicator fields 3 are preferably provided with the impregnation 4 during the production of indicator fields 3, i.e. before the indicator fields are attached to a carrier 2 to produce a test device 1.