WO2024165161A1

WO2024165161A1 - Method for operating a dishwasher with a measurement unit for measuring a ph-value of the washing liquor

Info

Publication number: WO2024165161A1
Application number: PCT/EP2023/053203
Authority: WO
Inventors: Clemens Herlitzius; Ingo Gau; Silke Walz
Original assignee: V-Zug AG
Current assignee: V-Zug AG
Priority date: 2023-02-09
Filing date: 2023-02-09
Publication date: 2024-08-15
Anticipated expiration: 2025-08-09
Also published as: EP4661729A1; AU2023428673A1; CN120676897A

Abstract

The invention relates to a method for operating a dishwasher. The dishwasher comprises a tub (1) for loading dishes, a detergent reservoir (11) for dispensing detergent and a circulating system for circulating the washing liquor. After a washing process has started, a first amount of a dishwashing detergent is added to the washing liquor. A main measurement is performed for determining a pH-value of the washing liquor. An additional amount of the dishwashing detergent may be added to the washing liquor, wherein said amount depends on the pH-value of the washing liquor determined by the main measurement.

Description

Method for operating a dishwasher with a measurement unit for measuring a pH-value of the washing liquor

Technical Field

The invention relates to a dishwasher and to a method for operating the dishwasher . The dishwasher comprises a tub for loading dishes , a detergent reservoir for dispensing a dishwashing detergent and a circulating system for circulating the washing liquor . The invention further relates to a dishwashing detergent for use in the method and for use in the dishwasher .

Background Art

Nowadays , dishwashing detergent is often provided as a tab . The user can insert the tab into a receptacle usually located in the inside of the dishwasher door . Alternatively, the user can fill the receptacle with detergent in the form of a powder or with liquid or gel detergent .

The tab has a predefined si ze . For every washing process , the same amount of detergent is inserted into the dishwasher independent of the number of dishes arranged in the tub and independent of the soiling of the dishes . I f liquid detergent or detergent in form of powder is used, the user normally fills up the receptacle to a maximum marking to be sure that enough detergent is available in the washing process .

In most cases , a smaller amount of dishwashing detergent would be suf ficient or the same result could be achieved i f the amounts of some components of the detergent were reduced .

Definitions

Unless otherwise stated, the following definitions shall apply in this speci fication :

As used herein, the term " a" , " an" , " the" and similar terms used in the context of the present invention ( especially in the context of the claims ) are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context .

As used herein, the terms " including" , " containing" and " comprising" are used herein in their open-ended, nonlimiting sense . It is understood that the various embodiments , preferences and ranges may be combined at will . Thus , for instance a solution comprising a compound A may include other compounds besides A. However, the term "comprising" also covers , as a particular embodiment thereof , the more restrictive meanings of "consisting essentially of" and "consisting of , so that for instance "a solution comprising A, B and optionally C" may also ( essentially) consist of A and B, or ( essentially) consist of A, B and C . As used herein, the transitional phrase "consisting es- sentially of" ( and grammatical variants ) is to be interpreted as encompassing the recited materials or steps and those that do not materially af fect the basic and novel characteristic ( s ) of the claimed invention . Thus , the term "consisting essentially of" should not be interpreted as equivalent of "comprising" .

As used herein, the term " about" means that the amount or value in question may be the speci fic value designated or some other value in its neighborhood . Generally, the term " about" denoting a certain value is intended to denote a range within ± 5 % of the value . As one example , the phrase " about 100" denotes a range of 100 ± 5 , i . e . the range from 95 to 105 . Preferably, the range denoted by the term " about" denotes a range within ± 3 % of the value , more preferably ± 1 % . Generally, when the term " about" is used, it can be expected that similar results or ef fects according to the invention can be obtained within a range of ±5 % of the indicated value .

Disclosure of the Invention

The problem to be solved by the present invention is to provide a method for operating a dishwasher with an optimi zed consumption of dishwashing detergent .

This problem is solved by the method for operating a dishwasher according to claim 1 . The dishwasher comprises

- a tub for loading dishes ; in particular at least two baskets are arranged in the tub and can be loaded with dishes to be washed .

- a detergent reservoir for dispensing a dishwashing detergent . In particular, the detergent reservoir can be a tank adapted to dispense liquid detergent , solid detergent or gel detergent , in particular wherein the solid detergent is in powder form or in pellet form . - a circulating system for circulating the washing liquor . In particular, the washing liquor is collected at the bottom of the dishwasher, pumped to spraying arms arranged in the tub and sprayed on the dishes in the tub .

The method comprises the following steps , in particular with the following order : a ) starting a first washing process , in particular wherein the washing process is started by the user ; b ) adding a first amount of a dishwashing detergent to the washing liquor ; in particular, the first amount dispensed might be the same amount at every washing process . In particular, it might be a di f ferent amount i f a di f ferent washing program ( speed program, standard program, intensive program etc . ) is performed but the same amount of detergent might be dispensed for every washing process of the same washing program . The amount could be adj usted over time depending on the user ' s behaviour . The first amount is chosen such that in most cases it is a suf ficient amount for the whole washing process . In a few cases , where the pH-value of the washing liquors decreases during the washing process below a certain pH-target value , the first amount of dishwashing detergent might not be suf ficient for the whole washing process . c ) performing a main measurement for determining a pH-value of the washing liquor . In particular, the main measurement is performed after a certain operating time has elapsed but the washing process has not yet terminated . d) adding an additional amount of a dishwashing detergent or of an alkalinity source to the washing liquor, i f the pH-value of the washing liquor determined by the main measurement is below a pH-target value . The dishwasher will not always perform this step because in most cases the measured pH-value will be above the pH- target value and there is no need for adding an additional amount of dishwashing detergent . But in cases where the measured pH-value is below the pH-target value , additional dishwashing detergent or alkalinity source is dispensed into the washing liquor since an alkaline washing liquor is required in order to achieve a good washing result . The alkalinity source may be selected from an alkali metal carbonate as described herein, an alkali metal hydroxide as described herein, an alkali metal silicate as described herein and mixtures thereof , and may contain from about 0 . 001 wt-% to about 0 . 5 wt-% of the herein described pH-sensitive dye .

Alternatively, at least a part of , in particular the complete , washing liquor is replaced by a new washing liquor, in particular comprising fresh water, i f the pH-value of the washing liquor determined by the main measurement is below a predefined pH-target value . I f the determined pH-value is above the predefined pH-target value , the washing liquor is not replaced and the washing process is continued .

In particular the main measurement is performed during an initial water rinse step and the washing liquor is replaced during an end phase of the initial water rinse step or during a start phase of a directly following washing step . e ) terminating the first washing process .

This means that all the steps a ) , b ) , c ) , d) , e ) are performed during the first washing process , i . e . during one single washing process . During the single washing process , the dishwasher is not unloaded .

The measurement and the possibility of an additional amount of dishwashing detergent or alkalinity source have the advantage that the first amount has not to be overdosed . Only a small first amount is added during the start phase of the washing process ( i . e . at step b ) of the washing process ) . I f the washing process is completed with only the small first amount of detergent , then no additional amount is required and no detergent is wasted .

Advantageously, the method comprises a further step f ) conducted between steps b ) and c ) . A second amount of a dishwashing detergent is added to the washing liquor before the main measurement is performed . In particular, the dishwashing detergent added at step f ) comprises a pH-sensitive dye which colors the washing liquor depending on the pH-value of the washing liquor . The pH- value of the washing liquor can be measured with a colour sensor by measuring a parameter dependent on transmission or reflection in the visible electromagnetic spectral range of at least two , in particular at least three wavelengths of the washing liquor . This measurement method is cost-ef fective since colour sensors are inexpensive . Even i f the first amount of dishwashing detergent already comprises a pH-sensitive dye , adding a second amount refreshes the colour of the washing liquor and results in a more precise pH-measurement .

In particular, the second amount is predefined and independent of any measurement . The second amount is only added to refresh the colour of the washing liquor and not for substantially increasing the pH-value of the washing liquor .

Advantageously, the second amount is added to the washing liquor not earlier than one minute , preferably not earlier than 30 seconds , more preferably not earlier than 15 seconds before step c ) . This ensures that the colour of the washing liquor does not fade before the main measurement which would falsi fy the measured pH- value .

In a preferred embodiment , the second amount is added to the washing liquor not later than hal f a minute , preferable not later than 15 seconds , before step c ) . A certain minimal time span between adding the second amount and the main measurement is required to ensure that the pH-sensitive dye is mixed with the entire washing liquor by circulating it through the circulating system . I f the main measurement is performed directly after adding the second amount , the washing liquor would not have a homogenous color and the pH-measurement would be unprecise .

In particular, the additional amount depends on a di f ference between a predefined pH-target value and the pH-value of the washing liquor determined at step c ) . After having measured the pH-value of the washing liquor, it is determined i f , and i f yes , which additional amount of dishwashing detergent or alkalinity source is added . For example , i f the predefined pH-target value is 11 and the measured pH-value is 7 , dishwashing detergent or alkalinity source has to be added to the washing liquor for increasing the pH-value from 7 to 11 . The amount of detergent or alkalinity source to be dispensed depends on the composition of the detergent/alkalinity source and total volume of the washing liquor . I f the composition of the detergent/alkalinity source is not known, a conservative high amount of detergent is added to the washing liquor .

In particular, the predefined pH-target value depends on the temperature of the washing liquor .

In particular, the predefined pH-target value depends on the washing program . For example , a washing program for washing glasses should have a lower pH-target value ( e . g . pH = 10 ) than an intensive washing program for washing kitchen equipment . A washing program for washing glasses with a lower pH-value avoids or reduces glass corrosion .

Thus , the pH-target value can be predefined separately for every washing program and independent of the temperature of the washing liquor . Alternatively, the pH-target value can be predefined only dependent on the temperature of the washing liquor or dependent on both, the washing program and the temperature . Advantageously, the method comprises a further step g) conducted between step a ) and f ) , in particular conducted between step a ) and b ) or between step b ) and f ) . A premeasurement for determining a pH-value of the washing liquor is performed, preferably the premeasurement is performed not earlier than five minutes , preferably not earlier than two minutes , preferably not earlier than one minute , before step f ) . In particular the additional amount of detergent depends on the pH-value of the washing liquor determined by the premeasurement and on the pH-value of the washing liquor determined at step c ) .

In particular, the premeasurement and the main measurement are performed with the same measurement unit , in particular with the colour sensor .

The premeasurement measures the colour of the washing liquor before the second amount is added . The washing liquor may be coloured by dyes contained by the soil of the dishes , which would falsi fy the main measurement . In order to avoid this , a premeasurement is performed and the measurement result is used to correct the main measurement . This is possible , since the colour spectra of the added detergent and dye are known .

In one preferred embodiment , at step d) a dishwashing detergent is used . Preferably, the composition of the dishwashing detergent used at step b ) is identical to the composition of the detergent used at step d) and/or f ) . It reduces costs i f only one single detergent reservoir with one single dishwashing detergent is arranged in the dishwasher . The first , the second and the additional amount are dispensed from the same detergent reservoir .

Another problem to be solved by the present invention is to provide a dishwasher with an optimi zed consumption of dishwashing detergent .

This problem is solved by the dishwasher according to claim 15 . The dishwasher comprises a tub for loading the dishes, a detergent reservoir for dispensing a dishwashing detergent into a washing liquor and a circulating system for circulating the washing liquor. The dishwasher is adapted to perform the method as described above .

A further aspect according to the present invention is directed to a dishwashing detergent for use in the dishwasher and the method for operating the dishwasher claimed and described herein, wherein said detergent comprises: from about 10 wt-% to about 20 wt-%, preferably from about 10 wt-% to about 15 wt-% of an alkalinity source selected from an alkali metal carbonate, an alkali metal hydroxide, an alkali metal silicate and mixtures therof; from about 0.001 wt-% to about 0.5 wt-%, preferably from about 0.01 wt-% to about 0.1 wt-%, of a pH-sensitive dye; and one or more functional additives, preferably selected from a water softener (e.g. builders or complexing agents) , a surfactant, an anti-redeposition agent, an enzyme (e.g. protease, amylase) , a bleaching agent (e.g. sodium peroxycarbonate) , a bleach activator, a corrosion inhibitor (e.g. zinc acetate) and a perfume.

The inventive dishwashing detergent may be a liquid, a gel or a solid, preferably a gel or a solid, more preferably a solid. The solid may be a powder, a tablet or a pellet.

A typical cleaning process in a dishwasher to remove food soil often includes an initial water rinse step to remove the water soluble soil, a washing step with an aqueous solution containing a dishwashing detergent, and a final fresh water rinse step. The dishwasher and the dishwashing detergent according to the present invention enable accurate dosage of the dishwashing detergent in response to the amount of acidic soils present on the dishes during the washing step.

Standard dishwashing detergents usually contain about 40 wt-% to about 50 wt-% of an alkalinity source selected from an alkali metal carbonate, an alkali metal hydroxide and an alkali metal silicate. Advantageously, the amount of the alkalinity source present in the dishwashing detergent claimed and described herein is significantly lower than in the currently available dishwashing detergents. Thus, the present dishwashing detergent is more environmentally friendly and has lower manufacturing costs than the currently available dishwashing detergents and does not impair the quality of the dishwashing .

As used herein, the term "alkali metal carbonate" includes alkali metal carbonates (e.g. sodium carbonate, potassium carbonate, bicarbonates (e.g. sodium bicarbonate, potassium bicarbonate) , and sesquicarbonates (e.g. sodium sesquicarbonate, potassium sesquicarbonate) .

As well known to the skilled person the term "alkali metal hydroxide" includes sodium hydroxide, potassium hydroxide, and lithium hydroxide.

As used herein, the term "alkali metal silicate" includes alkali metal silicates (e.g. sodium silicate, potassium silicate) , alkali metal disilicates (e.g. sodium disilicate, potassium disilicate) , and alkali metal metasilicates (e.g. sodium metasilicate, potassium metasilicate) .

Preferably, the alkalinity source is a mixture of an alkali metal carbonate and an alkali metal silicate .

The pH-sensitive dye is preferably selected from malachite green, thymol blue, water blue (disodium hydrogen aminomethyl [ [4- [ ( sulphonatophenyl ) amino] phenyl] [4- [ (sulphonatophenyl) imino] cyclohexa-2, 5-dien- 1-ylidene ] methyl ] benzenesulphonate ) , neutral red (3- amino-7-dimethylamino-2-methylphenazine hydrochloride) , phenolphthalein, thymolphtalein, alizarin ( 1 , 2-dihydroxy- anthraquinone) , alizarin yellow R (sodium 2-hydroxy-5- [ (E )- ( 4-nitrophenyl ) diazenyl ] benzoate ) , alizarin red S (3, 4-dihydroxy-9, 10-dioxo-9, 10-dihydroanthracene-2-sul- fonic acid) , indigo carmine, bromothymol blue, Nile blue A, hematoxylin, and 2-naphtol orange. An especially preferred pH-sensitive dye is alizarin red S ( 3 , 4-dihydroxy- 9, 10-dioxo-9, 10-dihydroanthracene-2-sulfonic acid) .

The pH-sensitive dye preferably exhibits a color change between a pH of about 8 and about 12, more preferably between about 9 and about 11, and most preferably at a pH between about 9 and about 10.5.

The dishwashing detergent further contains one or more functional additives. Said functional additives provide desired properties and functionalities to the detergent. As used herein, the term "functional additive" includes a material that when dispersed or dissolved in an aqueous solution provides a beneficial property in a particular use. Suitable functional additives for dishwashing detergents are known in the field and may be used in the context of this invention accordingly.

The one or more functional additives are preferably selected from a water softener, a surfactant, an anti-redeposition agent, an enzyme, a bleaching agent, a bleach activator, a corrosion inhibitor and a perfume.

In a preferred embodiment according to the present invention, the detergent comprises:

• at least about 45 wt-%, preferably at least about 50 wt-%, more preferably at least 60 wt-% of a water softener; and/or

• at least about 10 wt-%, preferably at least about 12.5 wt-%, more preferably at least about 15.0 wt-%, of a bleaching agent; and/or

• at least about 3 wt-%, preferably at least about 4 wt-%, more preferably at least about 5 wt-% of a surfactant; and/or

• at least about 2 wt-%, preferably at least about 3 wt-%, more preferably at least 4 wt-%, of an enzyme.

Dishwashing detergents containing at least 45 wt-%, preferably at least about 50 wt-%, more preferably at least 60 wt-% of a water softener are particularly useful when the water used in the water liquor is very hard (e.g. superior to 12 °dH; 1 °dH corresponds to 17.8 ppm alkaline-earth ions) . When the water used for dishwashing is soft or medium hard or an ion-exchange column is used in the dishwasher, it is cost-efficient and environmentally friendly to decrease the wt-% of the water softener in the dishwashing detergents (e.g. by a factor 2, or by a factor 3, or by a factor 4) and increase the wt-% of the other functional additives contained by said detergent.

Other advantageous embodiments are listed in the dependent claims as well as in the description below.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent from the following detailed description thereof. Such description makes reference to the annexed drawings, wherein :

Fig. 1 shows a dishwasher according to the invention; and

Fig. 2 shows a timeline of one single dishwasher process.

Mode for Carrying Out the Invention

Fig. 1 shows a household dishwasher with a tub 1. Three baskets are arranged in tub 1, a lower basket 2a, an upper basket 2b, and a cutlery basket 2c. The baskets can be loaded with dishes. Different spraying means 3 are arranged in tub 1, lower spraying mean 3a and middle spraying means 3b in form of spraying arms and upper spraying means 3c for spraying washing liquor on dishes. A circulating system with a circulation pump 4 circulates the washing liquor from a bottom area 5 of tub 1 to the spraying means 3 via a circulation channel. A heating device 6 is arranged in the circulation channel to heat the washing liquor to a certain temperature .

The dishwasher further comprises a draining pump 7 to drain washing liquor from the bottom area 5 of tub 1 and a fresh water inlet 8 to feed the dishwasher with fresh water .

The di f ferent actors of the dishwasher, in particular pump 4 and 7 and the heating device 6 are controlled by the control device 9 . Input and output means 10 are connected with the control device 9 via which the user can insert commands to the dishwasher and the dishwasher can output information to the user . The control device 9 comprises di f ferent washing programs for operating the dishwasher . It comprises a speed program, a standard program and an intensive program .

The dishwasher further comprises a detergent reservoir 11 for dispensing dishwashing detergent into tub 1 . Detergent reservoir 11 can be filled with liquid or powder detergent . A measurement unit 12 is arranged in the bottom area 5 . The measurement unit 12 is a colour sensor, in particular a multichannel spectral colour sensor, for example AMS AS7341 - I l-Channel Spectral Colour Sensor, which is an optical device for measuring a parameter dependent on transmission or reflection in the visible electromagnetic spectral range of the washing liquor at eleven di f ferent wavelengths . The light transmitted or reflected is generated by a white LED with a broadband light spectrum .

Alternatively, the multi-colour detection is possible by di f ferent colour-LEDs and a broadband light sensor .

The method for operating the dishwasher is now described . Fig . 2 shows a timeline of one single dishwasher process . At time 0 , the user starts a standard dishwasher program and the dishwasher process starts with an initial water rinse step . Fresh water or waste water from the final fresh water rinse step of the previous washing process is fed into tub 1 via fresh water inlet 8 and the pump 4 starts circulating the fresh water through the circulating system. The spraying means 3a, 3b, 3c spray the fresh water on the dishes arranged in the baskets 2a, 2b and 2c and the water soluble soil is removed.

After five minutes, at time 1, circulating water is drained by the drain pump 7 and new fresh water is fed into tub 1. The washing step starts. The fresh water, hereinafter referred to as the washing liquor, is circulated in the circulating system and heated by the heating device 6 up to a certain temperature, e.g. 50°C.

At time 2, which is three minutes after washing liquor was fed into tub 1, 10 g of powder detergent is dispensed from the detergent reservoir 11 into the bottom area 5. This is the first amount of powder detergent comprising a small amount (e.g. from about 0.001 wt- % to about 0.5 wt-%) of alizarin red S as a pH-sensitive dye. It is mixed into the washing liquor and the dishes are washed for a while.

Ten minutes after dispensing the first amount of powder detergent, a sequence of several actions start. These actions are named in Fig. 2 by the reference numbers 3, 4, 5 and 6. At time 3, the measurement unit 12 measures the color of the washing liquor. This is a premeasurement .

At time 4, a second amount of powder detergent is dispensed from the detergent reservoir 11 into the bottom area 5. Again, the powder detergent comprises a small amount of alizarin red S (e.g. from about 0.001 wt-% to about 0.5 wt-%) as a pH-sensitive dye. The second amount is about 3 g powder detergent. At every washing process, the same amount of powder detergent is dispensed at time 4 and the amount is independent of the premeasurement. The addition of the second amount is only intended to refresh the coloring of the washing liquor.

At time 5, 30 seconds after time 4, a main measurement is performed by the measurement unit 12. Both, the premeasurement and the main measurement generate a signal at multiple di f ferent wavelengths . The di fference of the signals between the premeasurement and the main measurement is calculated and the pH-value is determined by a least square fitting of the di f ference values . The fit function is known from spectral calibration measurements of the components of the detergent .

At time 6 , directly after having determined the pH-value of the washing liquor by the main measurement , an additional amount of powder detergent or alkalinity source is dispensed into the washing liquor i f the pH-value of the washing liquor is too low . For example , i f a pH-target value of 11 is required for an ef fective washing, and the determined pH-value is 7 , an additional amount is dispensed, for example an additional amount of 10 g . The larger the di f ference between the pH-target value and the determined pH-value is , the more additional detergent has to be dispensed into the washing liquor .

The washing process continues and at time 7 , the washing liquor is drained by the drain pump 7 . New fresh water is fed into tub 1 and heated by the heating device 4 to perform a final fresh water rinse step . The colour sensor can also be used to determine the turbidity of the rinse water . Dependent on the turbidity, the rinsing step can be repeated .

At time 8 , the circulating water is drained again by the drain pump 8 . The drying process starts . Tub 1 is cooled down so that humid air condenses at the cold walls of tub 1 .

At time 9 , the washing process is terminated .

Remark :

A first measurement for determining a pH- value could already be performed during the initial water rinse step . The amount of dishwashing detergent added during a start phase of the main washing step could depend on the determined pH-value . I f a high pH-value is determined during the initial water rinse step, only a small amount of dishwashing detergent could be added during the start phase of the main washing step and i f a low pH-value is determined, a larger amount could be added to the washing liquor .

Claims

1. A method for operating a dishwasher, wherein the dishwasher comprises

- a tub (1) for loading dishes,

- a detergent reservoir (11) for dispensing a dishwashing detergent,

- a circulating system for circulating the washing liquor, wherein the method comprises the following steps a) starting a first washing process, b) adding a first amount of a dishwashing detergent to the washing liquor, c) performing a main measurement for determining a pH-value of the washing liquor, d) - adding an additional amount of a dishwashing detergent or of an alkalinity source to the washing liquor, if the pH-value of the washing liquor determined by the main measurement is below a predefined pH- target value, or

- replacing at least a part of the washing liquor by new washing liquor, in particular comprising fresh water, if the pH-value of the washing liquor determined by the main measurement is below a predefined pH-target value, and e) terminating the first washing process.

2. The method according to claim 1, further comprising step f) conducted between steps b) and c) f) adding a second amount of a dishwashing detergent to the washing liquor.

3. The method according to claim 2, wherein the dishwashing detergent added at step f) comprises a pH-sensitive dye, and preferably the pH-value of the washing liquor is determined by measuring a colour of the washing liquor coloured by the pH-sensitive dye.

4. The method according to claim 2 or 3, wherein the second amount is predefined and independent of any measurement.

5. The method according to any one of claims

2 to 4, wherein the second amount is added to the washing liquor not earlier than one minute, preferably not earlier than 30 seconds, more preferably not earlier than 15 seconds, before step c) .

6. The method according to any one of the claims 2 to 5, wherein the second amount is added to the washing liquor not later than 30 seconds, preferably not later than 15 seconds, before step c) .

7. The method according to any one of the preceding claims, wherein the additional amount depends on a difference between the predefined pH-target value and the pH-value of the washing liquor determined at step c) .

8. The method according to any one of the preceding claims, wherein the predefined pH-target value depends on a washing program or depends on a temperature of the washing liquor.

9. The method according to claim 2 and according to one of the preceding claims, further comprising step g) conducted between step a) and f ) , in particular conducted between step a) and b) or between step b) and f ) , g) performing a premeasurement of the washing liquor, wherein preferably the premeasurement is performed not earlier than five minutes , preferably not earlier than two minutes , more preferably not earlier than one minute , before step f ) .

10 . The method according to claim 9 , wherein the additional amount depends on the premeasurement .

11 . The method according to claim 9 or 10 , wherein the additional amount depends on a di f ference or on a quotient between the premeasurement and the main measurement .

12 . The method according to any one of the preceding claims , wherein the dishwashing detergent added at step b ) and the dishwashing detergent added at step d) have an identical composition .

13 . The method according to claim 2 and according to one of the preceding claims , wherein the dishwashing detergent added at step b ) , the dishwashing detergent added at step f ) and the dishwashing detergent added at step d) have an identical composition .

14 . The method according to any of the preceding claims , wherein the washing process further comprises an initial water rinse step and a first measurement for determining a pH-value is performed during the initial water rinse step, in particular at the latest two minutes , in particular at the latest one minute , after the start of the washing process .

15 . The method according to any of the preceding claims , wherein step c ) is performed during an initial water rinse step and the washing liquor is replaced according to step d) during an end phase of the initial water rinse step or during a start phase of a directly following washing step.

16. Dishwasher for washing dishes comprising

- a tub (1) for loading the dishes,

- a detergent reservoir (11) for dispensing a dishwashing detergent into a washing liquor,

- a circulating system for circulating the washing liquor,

- a measurement unit (12) for determining a pH-value of the washing liquor, wherein the dish washer is adapted to perform a method according to any one of the preceding claims.

17. Dishwasher according to claim 16, wherein the measurement unit (12) is an optical device for measuring a parameter dependent on transmission or reflection in the visible electromagnetic spectral range at at least two, in particular at least three wavelengths.

18. A dishwashing detergent for use in the method according to any one of the claims 1 to 15, or for use in the dishwasher according to claim 16 or 17, wherein said detergent comprises: from about 10 wt-% to about 20 wt-%, preferably from about 10 wt-% to about 15 wt-% of an alkalinity source selected from an alkali metal carbonate, an alkali metal hydroxide, an alkali metal silicate, and mixtures thereof; from about 0.001 wt-% to about 0.5 wt-%, preferably from about 0.01 wt-% to about 0.1 wt-%, of a pH-sensitive dye ; and one or more functional additives, preferably selected from a water softener, a surfactant, an anti-redeposition agent, an enzyme, a bleaching agent, a bleach activator, a corrosion inhibitor and a perfume, wherein the wt-% are based on the total weight of the detergent.

19. The detergent according to claim 18, wherein the detergent is a gel or a solid, preferably a solid .

20. The detergent according to claim 18 or

19, wherein the pH-sensitive dye is selected from malachite green, thymol blue, water blue, neutral red, phenolphthalein, thymolphtalein, alizarin, alizarin yellow R, alizarin red S, indigo carmine, bromothymol blue, Nile blue A, hematoxylin, and 2-naphtol orange.

21. The detergent according to any one of claims 18 to 20, wherein the detergent comprises: