US 2890177 A
Description (OCR text may contain errors)
United States Patent tion Company, Incorporated, New York, N.Y., a corporation of New York No Drawing. Application December 26, 1956 Serial No. 630,473
9 Claims. or. 252-408) This invention relate to chemical indicator solutions responsive to the presence of carbon dioxide in respiratory gases.
There has been a great need' for a simple and inexpensive device for use inoxygen therapy and in anesthetic administration which is effective for detecting the presence of carbon dioxide at relatively low concentrations. The need for such detection arises from the fact that it is necessary to remove the carbon: dioxide from a patients respiratory gases when these gases are confined and recycled to the patient. Conventional anesthetic administering apparatus and oxygen therapy devices employ an adsorbent charge such as soda lime which removes the carbon dioxide content of these gases by contact. In time, however, the adsorbent becomes depleted and a breakthrough of carbon dioxide occurs following which the content thereof in the breathing circuit may rapidly rise to an undesirable and possibly dangerous level. It is highly advantageous, therefore, that such detection devices be capable ofresponse at relatively low concentrations of carbon dioxide.
Chemical indicators have been used for such purposes. Such indicators are in the form of a solution having a characteristic color so long as the air stream or respiratory gases contain less than a' small concentration of carbon dioxide corresponding to the normal level present in the breathing circuit. When the concentration reaches a higher level, depending upon the sensitivity of the indicator, a color change occurs which thereby gives an indication of the existence of higher concentrations of the carbon dioxide. On such indicator solution, known as theDraper solution which has been used, comprises an aqueous solution of calcium carbonate as a chemical buffer and from cresol purple as a dye. This solution has been found fairly effective and has been the most generally accepted carbon dioxide indicator used in the medical field. Generally, however, the known chemical indicators are effective for reliably indicating the presence of carbon dioxide in a breathing circuit only above about 1 /2-%. Advances in techniques relative to the administration of anesthetics, particularly, now make it extremely desirable to permit the detection of carbon dioxide at levels of Well below 1% and preferably of about /2%. However, none of the known chemical indicators afford this degree of sensitivity. In addition, the indicators presently in use frequently provide poor contrasts in the color characteristics between normal and responsive conditions which makes it difficult to determine whether the carbon dioxide concentration has risen. Also many of the solutions are characteristicly unstable during storage or in use' and must be replaced frequently in order to insure reliable readings.
' It is, therefore, an object of the present invention to provide" a chemical indicator solution for the detection of carbon dioxide in respiratory gases which is responsive about /z% or less.
It is a further object of the present invention to provide such a chemical indicator in which a sharply contrasting change in color occurs in the responsive range of the indicator.
It is another object of this invention to provide an indicator solution for the detection of carbon dioxide in respiratory gases which produces a reversible, sharply defined color change when the carbon dioxide content reaches about /2 and which is relatively stable during storage and while in use.
In accordance with the present invention, a highly effective, sensitive carbon dioxide indicator comprises an aqueous solution of an alkali metal oxalate and a pH sensitive color dye. The most suitable and readily available oxalate salt is sodium oxalate. The oxalate salt may be used with various-indicator dyes by adjusting its concentration so that the pH change of the solution corresponding to the change from a condition of equilibrium with air .to the condition of equilibrium with about /2 CO falls within the pH transition interval of the dye. Generally, color indicators, which are responsive in the pH range of 6.6 to 5.8, may be used advantageously and include, for example, brom cresol purple, broin thymol blue, indo-oxine, p-nitro phenol, chlorphenol red, beta naphthol, brom thymol blue-brorn cresol purple, and brom cresol green-chlorphenol red. Molar concentrations of aboutt).0002 to 0.01 of the oxalate salt are eifective with these indicator dyes. Higher concentrations of the oxalae salt tend to retard the responsiveness of the indicator with an undesirable resultant loss in sensitivity. For this reason, it is preferred to utilize the indicators which may be employed with the oxalate salt in lower concentrations. Particularly advantageous results have been obtained, for example, by the use of sodium oxalate with brom cresol purple as an indicator. With this system, molar concentrations of from 0.001 to 0.01 of the oxalate may be used substantially without significant loss of sensitivity, although optimum resultsare obtained with about a 0.005 molar concentration. With this oxalatebrom cresol purple system its normal purple color corresponding to the atmospheric level of carbon dioxide concentration, undergoes an easily discernible change to a bluish green when the carbon dioxide concentration increases to about 0.3%, and to a light green at about 0.5%. -It thus affords an extremely sensitive means of detection for carbon dioxide concentrations well below those concentrations, which it has been possible to detect heretofore by other chemical indicators. As the concentration of carbon dioxide increases substantially above /2%, the contrast in color of the solution becomes greater, so that at levels of about 1%, where heretofore the change in color with other indicators has been barely discernible, the change may be visibly determined with ease.
It is highly advantageous, in order to render the indicator solution described herein unaffected by prolonged storage or by exposure to respiratory gases during continuous use, to combine with it a small amount of a nontoxic, non-ionic, water soluble anti-oxidation stabilizing agent. The stabilizer prevents the degradation of the indicator and maintains it in highly sensitive condition. A stabilizing agent, effective for this purpose, is glycerine. The concentration of the stabilizer does not appear to be critical. However, glycerine in concentrations of about five percent by volume stabilize the solution to the extent that it may be stored indefinitely and used continuously in conventional anesthetic circuits for many days without requiring replenishing or replacement.
An effective indicator solution prepared in accordance with the present invention may be used by contacting therewith a sample of a gas to be tested. This may be accomplished, for example, by a simple, bulb aspirator connected through suitable conduits to a breathing circuit, preferably at a point downstream of the carbon dioxide absorbent means. Such apparatus, for example, may be as shown in the Draper, United States Patent No. 2,136,226, in which the indicator solution is held in a transparent glass vial. By causing the gas sample to bubble through the indicator solution, a change from its normal color indicates the presence of carbon dioxide above the minimum concentration at which the solution is responsive. Thus, in the case of a sodium-oxalate-brom cresol purple solution of the concentration above described, the maintenance of the normal purple color of the solution will show that the carbon dioxide in the system is below 0.3%. Upon a change in color of the solution to light green, for example, it will be known that the concentration of the carbon dioxide in the system has increased to about 0.5%. Such an occurrence will serve to give a timely indication to the operator of the presence of such carbon dioxide concentrations, so that suitable precautions, such as the replacement of the absorbent charge, may be taken. As the concentration rises to about 1%, the indicator will turn to yellow and remain this color for concentrations over this value. When the concentration of the carbon dioxide in the system returns to its normal level, the indicator solution will revert to its original color. The color variations will change, of course, for different indicator dyes and each system must be calibrated accordingly to determine the minimum concentration of carbon dioxide which produces a visible response. The brom cresol purple dye is especially suitable because of the contrast in color which occurs at the minimum response level.
Generally, the concentration of the indicator dye is not critical. However, the dye should be added in amounts within the limits of visual ability to detect the changes in carbon dioxide concentration with the changes in the color of the solution corresponding thereto. The optimum amount of dye corresponds to the amount giving a shade or depth of color whose change may be observed most readily.
It will be understood that the present invention is not intended to be limited to any particular theory or explanation of its mode of operation. However, it is believed that a greater understanding of the invention may be had by a consideration of the equilibrium ionization of carbon dioxide in water, upon which the response of the indicator solutions depends, as expressed, by the following equation:
When carbon dioxide is present in a sample gas contacted with the aqueous solution, equilibrium of the above ionization is reached at which the solution has a corresponding pH value depending upon the concentration of the carbon dioxide. It may be seen that the pH of the solution will vary depending upon the concentration of the carbon dioxide. The pH sensitive dyes are employed as a means of indicating the change in pH of the solutions. The pH of the solution without the addition of a buffer, however, is not a reliable basis for determining the carbon dioxide concentration, because the solution in this instance is extremely sensitive to transient conditions and to minor, or trace, contaminants, such as minute amounts of soda lime that are frequently found in the gases of an anesthetic circuit. The butter agents, such as those heretofore used, donate additional HCO ions sufficient to stabilize the solution against these transient conditions. When provided with small amounts of these buffers, however, the pH increments corresponding to the lower concentrations of carbon dioxide lie outside of the pH transition intervals of the indicator dyes that would afford a suitable color contrast and a pH value capable of afiecting these dyes is reached only after a substantial increase in carbon dioxide has occurred. Thus as previously mentioned, the best detection heretofore possible has been above about 1 /2%. The concentration of the buffer agent cannot be employed to adjust the pH of the solution, so that it will more closely approximate the transition intervals of the usable dyes, because the concentration of the HCO; ions is then changed to a level rendering the solution highly insensitive to changes in carbon dioxide concentration. The indicator would then create no detectable signal. The present invention, however, involves the unique discovery that an alkali metal oxalate solution, as hereinabove described, affords changes in the pH of the solution in the low range of carbon dioxide concentrations of as little as /z% and less, which fall within the pH transition intervals of the usable pH sensitive dyes. As a result, a clearly visible change in color is produced enabling detection of carbon dioxide concentration of a much lower magnitude by means of a chemical indicator. The exact manner in which the oxalate salt achieves this result is not clearly understood, although it is apparent that it aflords a mixed oxalatebicarbonate system in which the pH range is shifted so that a great sensitivity of the pH sensitive dyes is afforded at the lower carbon dioxide concentrations.
A preferred indicator solution, in accordance with the present invention, is prepared by dissolving from 0.6 to 0.7 gram (0.005 mol) of pure reagent grade sodium oxalate in one liter of distilled water to which is added 50 ml. of U.S.P. glycerine and 15 ml. of an 0.04% brom cresol purple dye solution. The dye may be prepared by dissolving 0.1 gram of finely powdered brom cresol purple in 18.5 cc. of 0.01 N NaOH diluted to 250 cc. volume with distilled water.
While the present solution has been described primarily as a visual indicator for carbon dioxide, it will be understod that it may be used in connection with spectrophotometric instruments which utilize the change in transmission characteristics of the solution. Thus, the solution provided in accordance with this invention will give extremely high spectrophotometric sensitivity over the range of 0.03 to 0.5% carbon dioxide in which a transmission difference of about 22.5% occurs.
The invention is not limited to the specific embodiment herein described, but may be used in other ways with the departure from its scope as defined in the following claims.
1. An indicator for detecting the presence of carbon dioxide in respiratory gases at concentrations above that in air comprising an aqueous solution of an alkali metal oxalate and a pH psensitive colored dye responsive to produce a discernible color change in the pH range of about 6.6 to 5.8, said solution being characterized by undergoing a discernible change in color when the carbon dioxide concentration increases to the level of response of said indicator.
2. An indicator for detecting carbon dioxide in respiratory gases comprising an aqueous solution of an alkali metal oxalate having a molar concentration between 0.0002 to 0.01 and a pH sensitive dye having a pH transition interval responsive to produce a discernible color change in the pH range of 6.6 to 5.8.
3. An indicator for detecting carbon dioxide in respiratory gases comprising an aqueous solution of sodium oxalate having a molar concentration of between 0.0002 and 0.01 and a pH sensitive dye having a pH transition interval responsive to produce a discernible color change in the pH range of 6.6 to 5.8.
4. An indicator for detecting carbon dioxide comprising an aqueous solution of an alkali metal oxalate and brom cresol purple as a pH sensitive, color indicator dye.
5. An indicator for detecting carbon dioxide comprising an aqueous solution of sodium oxalate and brom cresol purple as a pH sensitive, color indicator dye.
6. An indicator for detecting carbon dioxide comprising 0.001 to 0.01 molar concentration of an alkali metal 5 6 oxalate in aqueous solution and brorn cresol purple as 9. An indicator according to claim 8 containing glycapH sensitive, color indicator dye. erme as a stabilizer.
7. A chemical indicator for carbon dioxide comprising an aqueous solution of sodium oxalate having a molar concentration of about 0.005 and brom cresol purple as a 5 References Cited in the file of this patent UNITED STATES PATENTS PH sensitive 2,136,236 Draper Nov. 8, 1938 8. An indicator according to claim 7 wherein the equiv- 2,601,173 Seltz June 171 1952 alent of about 15 ml. of an 0.04% solution of said brorn OTHER REFERENCES cresol purple dye is employed for each liter of said 10 Kolthoff et aL: Acid Base Indicators, The MacMfllan aqueous sodium oxalate solution. 00., New York, 1937, page 265.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,890,177 June 9, 1959 Tho'burn Kilmer It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.
Column 1, line 1, for "relate" read m relates line 4% for "On such" read One such line 48, for "from" read brom coluzim :2 lines 43 and .44, for "with. the" read he without line 5G, for "psensi tive" read sensitive o Signed and sealed this 23rd day of. February 19630 (SEAL) Attest: KARL H, AXLINE, ROBERT C. WATSON Commissioner of Patents Attesting Officer