WO1995033203A1

WO1995033203A1 - Method for quantitative analysis of total cyanide

Info

Publication number: WO1995033203A1
Application number: PCT/US1994/006041
Authority: WO
Inventors: Richard J Berman
Original assignee: Berman, Richard, J.
Priority date: 1994-05-27
Filing date: 1994-05-27
Publication date: 1995-12-07

Abstract

An improved method for the quantitative analysis of total cyanide in effluent from industrial processes. Surfactant-free tubing is used in the ultraviolet digestor to avoid the use of wetting agents that interfere with test results. Titanium (IV) oxide is added to the test reagents, allowing use of the preferred low wattage, long wave length ultraviolet light, achieving improved precision and speed, without inaccuracy resulting from interference from thiocyanate.

Description

DESCRIPTION

METHOD FOR QUANTITATIVE ANALYSIS OF TOTAL CYANIDE

TECHNICAL FIELD The invention relates to the field of methods to measure cyanide, a toxic substance found in the effluent water from a wide range of industrial processes.

BACKGROUND ART Cyanide is a toxic substance which is found in the effluent from a wide range of industrial processes. Companies that have cyanide in their effluent water are regulated on the upper limit of cyanide they may discharge periodically, such as each day and over the course of four days. Most regulations are based on the measurement of total cyanide. However, there are several distinct disadvantages with the current standard method for the measurement of total cyanide: existing methods re uire a minimum of 1 and 1/2 hours to complete, a 1 hour distillation step, the use of hazardous chemical reagents. Further, existing methods result in hazardous chemical wastes. Additionally, existing methods are subject to many interferences which introduce inaccuracy in the test measurements. These problems are well known to both the involved industries and regulators. However, it is recognized that there is a need to test for low levels (2 parts per billion) of total cyanide. The problems with the standard method are such that total cyanide testing is not frequently done because existing methods are difficult, slow, imprecise and costly. Two sources of these problems are (1) the surfactant used in conventional automated methods to wet the interior of the tubing or conduits through which the sample flows, and (2) the creation of thiocyanate by the ultraviolet light digestor.

In regard to the first problem, the surfactant, under existing automated methods, in order to achieve the low detection limits required (2 parts-per-billion) , gas-segmented continuous-flow analysis is used. Gas segmentation prevents sample dispersion (dilution) as it flows through the analyzer conduits. The residence time for total cyanide can be as long as 20 minutes. Normally, when operating in a gas-segmented flow mode with microconduits (0.8-1.0 mm internal diameter), it has been necessary to add a surfactant to wet the inner conduit walls to allow the gas bubbles to flow smoothly, with the disadvantage that the surfactant would wet the surface of the hydrophσbic gas diffusion membrane, an essential component of an analyzer. If the surface is wetted, the membrane passes ions which could act as interferents in the detection of cyanide in the ampero etric detector. Certain types of tubing have been well known over the years to be unsuitable for gas-segmented flow; i.e., bubbles normally shred as they flow through Teflon PTFE of TEFZEL tubing. Until the present invention, no suitable surfactant-free polymeric tubing or conduit has been successfully used. Glass has been used, but its fragility is a disadvantage for this application.

In regard to the second problem, the presence of thiocyanate, there has been a goal for a long time to measure total cyanide by on-line UV digestion without inclusion of thiocyanate. Kelada, N.J. J. Water Pollution Central Federation. 1989, 61, 350-6. The problem in the past has been that the UV lamp that has been used is a short wavelength UV lamp (254 nm) . This breaks down the metal-cyanide bonds to release cyanide, but it also breaks down the thiocyanate (SCN) bond to release cyanide and/or sulfide. These are measured as cyanide and since thiocyanate is not included in the definition of total cyanide, the result is biased high.

Several groups have reported ways to get around this problem, principally by using longer wavelength UV light. Kelada, N.J. J. Water Pollution Central Federation. 1989, 61, 350-6; Meeussen, J.C.L.; Keizer, M.G., Lukassen, W.D. Analyst, 1992, 117. 1009-12. This light is capable of breaking down the stronger metal-cyano complexes, but it does not break the sulfur-cyanide (S-CN) bond. However, since the intensity of light at the longer wavelengths is weaker than with the short UV lamps, it is not as effective at breaking down the stronger metal-cyano complexes. For example, a 4 Watt long wavelength lamp only gives approximately 40% recovery for a 1 mg/L of potassium hexacyanoferrate (III) in the analyzer compared to >98% recovery with a 4 Watt short wavelength lamp. While other groups have demonstrated the ability to get high recoveries of the strong metal-cyano complexes without thiocyanate, they have had two major disadvantages:

(a) Long Residence Times: In order to increase the exposure to the UV light, other systems require the use of long coils wrapped around the UV lamp. One system uses a coil with a volume of 13 mL. This results in a residence time of approximately 15 minutes in the UV digestor alone.

(b) Powerful UV Lamps: In order to compensate for the weakness of lamp intensity at the longer wavelengths of light, one system uses a 300 Watt lamp and discriminates wavelengths using a coil material that filters out the short wavelength light. This type of system requires a lot of space, needs ventilation, and also has a slow residence time.

DISCLOSURE OF THE INVENTION

The objects of this invention are to provide a method for the quantitative analysis of total cyanide which does not require the use of a surfactant in the tubing to, within, and from the ultraviolet light digestor, so that low levels of cyanide may be measured and interferents are avoided in the gas diffusion step and the accuracy of the measurement of total cyanide is enhanced. Use of tubing such as Teflon FEP achieves these objectives.

It is a further object of this invention to provide such a method with enhanced accuracy of measurement by avoiding creating thiocyanate, which, while not included in the definition of total cyanide, is measured as cyanide, resulting in an upward bias in, and an inaccuracy in the measurement of, total cyanide. Adding Titanium (IV) oxide (Titanium dioxide) to the carrier reagent in an ultraviolet light digestor achieves this object, in less time and with the use of a lower wattage ultraviolet lamp, than has previously been reported. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings can be briefly described as follows. Figure 1 illustrates the front panel of, and a schematic of the flow of fluid through, a typical cyanide analyzer and shows the surfactant-free (Teflon FEP) tubing (1 and 2) in the sections of conduit carrying the fluid leading into and coming from the digestor. Figures 2 and 3 are diagrams of the digestor showing the said tubing (3) comprising a coil within the digestor.

BEST MODE FOR CARRYING OUT THE INVENTION An improved method has been developed which measures total cyanide by gas-segmented continuous-flow analysis, using but improving on a conventional automated analyzer such as the Perstorp Analytical Model 3500 or the Bran & Lubbe Traacs 800. In order to achieve the low detection limits required for an effective method for the quantitative analysis of low levels of total cyanide (2 parts-per-billion) , gas-segmented continuous-flow analysis is used. Gas segmentation prevents sample dispersion (dilution) as it flows through the analyzer conduits.

A sample is randomly selected from the industrial effluent to be tested. The sample is loaded into a discreet sample loop in an injection valve and is injected into a carrier stream. This technique is known as flow injection analysis. The stream is then acidified by addition of an acid to form hydrocyanic acid (HCN) . Sulfuric acid is used, prepared by use of 55.5 ml of concentrated εulfuric acid and 1% hypophosporous acid (40 ml of a 50% solution) , diluted to 1 liter using deionized water. Air is then added to segment the stream. (If free cyanide is to be measured, the flow then directly proceeds to a gas diffusion separator. Here, HCN diffuses across a hydrophobic membrane and is collected in an alkaline receiving stream.) In the inventive method to measure total cyanide, the flow enters an ultraviolet (UV) light digestor where the cyano-metal complexes are photodecomposed to HCN. Tubing or conduit to, from and within the ultraviolet lamp digestor is used, made from a material such as Teflon FEP, which supports a good bubble pattern without the need for a surfactant. Such tubing is used for both the mixing coils and for the coil that is wrapped around the ultraviolet lamp.

Further, in the inventive method, 0.005 g/1 titanium (IV) oxide (-325 mesh, Aldrich Chemical Company, Milwaukee, Wisconsin) in deionized water is added to the carrier reagent to aid in increasing the photolysis of the cyano-metal complexes in the digestor. The base reagent is 0.1 normal sodium hydroxide (4 gram 1) sodium hydroxide. When titanium (IV) oxide is added the recovery increases to 85% compared to approximately 40 percent in conventional methods. When an 8 Watt lamp is used, the recovery improvement when using 0.005 g/L titanium (IV) oxide is from 70% in conventional methods to 97-101% adding the method of this invention.

The addition of titanium (IV) oxide to one of the reagents results in the following advantages:

(a) Faster analysis time (8 minutes versus 20) .

(b) Mild UV conditions (8 Watt lamp) .

The mechanism for how titanium (IV) oxide behaves in this system is not fully understood. Titanium (IV) oxide is a well known oxidizing agent and through creation of hydroxyl radicals it may assist in the hydrolysis of the metal-cyano complexes. It may also form coordination complexes with the metals, resulting in easier hydrolysis of the metal-cyano bonds.

Titanium (IV) oxide has been used to destroy cyanide all the way to nitrate and carbon dioxide. However, under the conditions of the inventive method there is no oxidation of cyanide.

Experimental results have shown that the recovery for potassium cyanide is the same whether the UV light is on or off and either with or without titanium (IV) oxide. This is the first time titanium (IV) oxide has been used to break down metal-cyano complexes with the purpose of measuring the released cyanide rather than the purpose of totally destroying the cyanide. In this system, titanium (IV) oxide is aiding in an analytical system. One reason why cyanide is not being oxidized further in this case is the presence of a reducing acid in the acid reagent (hypophoεphorous acid) .

The use of titanium (IV) oxide works for total cyanide in this system because there is a gas diffusion step after digestion. This allows the sample stream containing titanium (IV) oxide to flow to waste. Direct detection without a separation step would result in problems if titanium (IV) dioxide passed through the photometric or amperometric detectors.

As in conventional methods the presence of hypophosphoric acid in the acid reagent acts to prevent over oxidation of the released cyanide to cyanate and carbon dioxide. After UV digestion, the flow enters the gas diffusion separator as was described for free cyanide. The alkaline receiving stream then proceeds to a detector where cyanide is measured. Both amperometric and photometric detectors can be used. As in conventional methods, in order to detect photometrically, the receiving stream must be merged with a series of colorimetric reagents which form a colored product which is then detected by absorbance. Amperometric detection offers the advantages of non-hazardous reagents and results in non- hazardous waste.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

CLAIMS What I claim is:

1. An improved process for the analysis of total cyanide in effluent, comprising: a. Loading a sample of said effluent into a discreet sample loop; b. Injecting said sample into a carrier stream; c. Acidifying said carrier stream by addition of an acid to form hydrocyanic acid; d. Segmenting said stream by addition of air; e. Directing the flow of said segmented stream into an ultraviolet light digestor using a long wavelength ultraviolet lamp with a peak at 312 nm; f. Adding titanium (IV) oxide to the carrier reagent; g. Photodecomposing cyano-metal complexes in said segmented stream to hydrocyanic acid in an ultraviolet light digestor; h. Diffusing, in a gas diffusion separator, said acid across a hydrophobic membrane; i. Collecting said acid in an alkaline receiving stream; and j . Detecting and measuring cyanide directly by amperometry or photometrically by use of colorimetric agents forming a colored product detected by absorbance.

2. A process as in Claim 1, further comprising the step of using tubing, for which no surfactant is required, to move the said sample to, within, and from said digestor, in order to avoid wetting the surface of said membrane.

3. An improved process for the analysis of total cyanide in effluent, comprising: a. Loading a sample of said effluent into a discreet sample loop; b. Injecting said sample into a carrier stream; c. Acidifying said carrier stream by addition of an acid to form hydrocyanic acid; d. Segmenting said stream by addition of air; e. Directing the flow of said segmented stream into an ultraviolet light digestor using a long wavelength ultraviolet lamp with a peak at 312 nm; f. Photodecomposing cyano-metal complexes in said segmented stream to hydrocyanic acid in an ultraviolet light digestor; g. Using tubing, for which no surfactant is required, to move said sample to, within, and from said digestor, in order to avoid wetting the surface of said membrane; h. Collecting said acid in an alkaline receiving stream; and i. Detecting and measuring cyanide directly by amperometry or photometrically by use of colorimetric agents forming a colored product detected by absorbance.